Search

JWST, despite not being designed to observe astrophysical phenomena that vary on rapid time scales, can be an unparalleled tool for such studies. If timing systematics can be controlled, JWST will be able to open up the sub-second infrared timescale regime. Rapid time-domain studies, such as lag measurements in accreting compact objects and Solar System stellar occultations, require both precise inter-frame timing and knowing when a time series begins to an absolute accuracy significantly below 1s. In this work we present two long-duration observations of the deeply eclipsing double white dwarf system ZTF J153932.16+502738.8, which we use as a natural timing calibrator to measure the absolute timing accuracy of JWST's clock. From our two epochs, we measure an average clock accuracy of $0.12\pm0.06$s, implying that JWST can be used for sub-second time-resolution studies down to the $\sim100$ms level, a factor $\sim5$ improvement upon the pre-launch clock accuracy requirement. We also find an asymmetric eclipse profile in the F322W2 band, which we suggest has a physical origin., Comment: 16 pages, 13 figures, accepted for publication in AJ

Long-period radio transients are an emerging class of extreme astrophysical events of which only three are known. These objects emit highly polarised, coherent pulses of typically a few tens of seconds duration and minutes to hour-long periods. While magnetic white dwarfs and magnetars, either isolated or in binary systems, have been invoked to explain these objects, a consensus has not emerged. Here we report on the discovery of ASKAP J193505.1+214841.0 (henceforth ASKAPJ1935+2148) with a period of 53.8 minutes exhibiting three distinct emission states - a bright pulse state with highly linearly polarised pulses with widths of 10-50 seconds; a weak pulse state which is about 26 times fainter than the bright state with highly circularly polarised pulses of widths of approximately 370 milliseconds; and a quiescent or quenched state with no pulses. The first two states have been observed to progressively evolve over the course of 8 months with the quenched state interspersed between them suggesting physical changes in the region producing the emission. A constraint on the radius of the source for the observed period rules out a magnetic white dwarf origin. Unlike other long-period sources, ASKAPJ1935+2148 is the first to exhibit drastic variations in emission modes reminiscent of neutron stars. However, its radio properties challenge our current understanding of neutron star emission and evolution., Comment: Published in Nature Astronomy

High-energy neutrinos could be produced in the interaction of charged cosmic rays with matter or radiation surrounding astrophysical sources. To look for transient sources associated with neutrino emission, a follow-up program of neutrino alerts has been operating within the ANTARES Collaboration since 2009. This program, named TAToO, has triggered robotic optical telescopes (MASTER, TAROT, ROTSE and the SVOM ground based telescopes) immediately after the detection of any relevant neutrino candidate and scheduled several observations in the weeks following the detection. A subset of ANTARES events with highest probabilities of being of cosmic origin has also been followed by the Swift and the INTEGRAL satellites, the Murchison Widefield Array radio telescope and the H.E.S.S. high-energy gamma-ray telescope. The results of twelve years of observations are reported. No optical counterpart has been significantly associated with an ANTARES candidate neutrino signal during image analysis. Constraints on transient neutrino emission have been set. In September 2015, ANTARES issued a neutrino alert and during the follow-up, a potential transient counterpart was identified by Swift and MASTER. A multi-wavelength follow-up campaign has allowed to identify the nature of this source and has proven its fortuitous association with the neutrino. The return of experience is particularly important for the design of the alert system of KM3NeT, the next generation neutrino telescope in the Mediterranean Sea., Comment: 27 pages, 14 figures, submitted to JCAP

The Green Bank North Celestial Cap survey is one of the largest and most sensitive searches for pulsars and transient radio objects. Observations for the survey have finished; priorities have shifted toward long-term monitoring of its discoveries. In this study, we have developed a pipeline to handle large datasets of archival observations and connect them to recent, high-cadence observations taken using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. This pipeline handles data for 128 pulsars and has produced measurements of spin, positional, and orbital parameters that connect data over observation gaps as large as 2000 days. We have also measured glitches in the timing residuals for five of the pulsars included and proper motion for 19 sources (13 new). We include updates to orbital parameters for 19 pulsars, including 9 previously unpublished binaries. For two of these binaries, we provide updated measurements of post-Keplerian binary parameters, which result in much more precise estimates of the total masses of both systems. For PSR J0509+3801, the much improved measurement of the Einstein delay yields much improved mass measurements for the pulsar and its companion, 1.399(6)\Msun and 1.412(6)\Msun, respectively. For this system, we have also obtained a measurement of the orbital decay due to the emission of gravitational waves: $\dot{P}_{\rm B} = -1.37(7)\times10^{-12}$, which is in agreement with the rate predicted by general relativity for these masses., Comment: accepted for publication in The Astrophysical Journal

We present and evaluate the prospects for detecting coherent radio counterparts to gravitational wave (GW) events using Murchison Widefield Array (MWA) triggered observations. The MWA rapid-response system, combined with its buffering mode ($\sim4$ minutes negative latency), enables us to catch any radio signals produced from seconds prior to hours after a binary neutron star (BNS) merger. The large field of view of the MWA ($\sim1000\,\text{deg}^2$ at 120\,MHz) and its location under the high sensitivity sky region of the LIGO-Virgo-KAGRA (LVK) detector network, forecast a high chance of being on-target for a GW event. We consider three observing configurations for the MWA to follow up GW BNS merger events, including a single dipole per tile, the full array, and four sub-arrays. We then perform a population synthesis of BNS systems to predict the radio detectable fraction of GW events using these configurations. We find that the configuration with four sub-arrays is the best compromise between sky coverage and sensitivity as it is capable of placing meaningful constraints on the radio emission from 12.6\% of GW BNS detections. Based on the timescales of four BNS merger coherent radio emission models, we propose an observing strategy that involves triggering the buffering mode to target coherent signals emitted prior to, during or shortly following the merger, which is then followed by continued recording for up to three hours to target later time post-merger emission. We expect MWA to trigger on $\sim5\text{--}22$ BNS merger events during the LVK O4 observing run, which could potentially result in two detections of predicted coherent emission., Comment: Accepted for publication in PASA

The Australian, Chinese, European, Indian, and North American pulsar timing array (PTA) collaborations recently reported, at varying levels, evidence for the presence of a nanohertz gravitational wave background (GWB). Given that each PTA made different choices in modeling their data, we perform a comparison of the GWB and individual pulsar noise parameters across the results reported from the PTAs that constitute the International Pulsar Timing Array (IPTA). We show that despite making different modeling choices, there is no significant difference in the GWB parameters that are measured by the different PTAs, agreeing within $1\sigma$. The pulsar noise parameters are also consistent between different PTAs for the majority of the pulsars included in these analyses. We bridge the differences in modeling choices by adopting a standardized noise model for all pulsars and PTAs, finding that under this model there is a reduction in the tension in the pulsar noise parameters. As part of this reanalysis, we "extended" each PTA's data set by adding extra pulsars that were not timed by that PTA. Under these extensions, we find better constraints on the GWB amplitude and a higher signal-to-noise ratio for the Hellings and Downs correlations. These extensions serve as a prelude to the benefits offered by a full combination of data across all pulsars in the IPTA, i.e., the IPTA's Data Release 3, which will involve not just adding in additional pulsars, but also including data from all three PTAs where any given pulsar is timed by more than as single PTA., Comment: 21 pages, 9 figures, submitted to ApJ

The vast majority of extragalactic compact continuum radio sources are associated with star formation or jets from (super)massive black holes and, as such, are more likely to be found in association with starburst galaxies or early-type galaxies. Two new populations of radio sources were recently identified: (a) compact and persistent sources (PRSs) associated with fast radio bursts (FRBs) in dwarf galaxies and (b) compact sources in dwarf galaxies that could belong to the long-sought population of intermediate-mass black holes. Despite the interesting aspects of these newly found sources, the current sample size is small, limiting scrutiny of the underlying population. Here, we present a search for compact radio sources coincident with dwarf galaxies. We search the LOFAR Two-meter Sky Survey (LoTSS), the most sensitive low-frequency (144 MHz central frequency) large-area survey for optically thin synchrotron emission to date. Exploiting the high spatial resolution (6 arcsec) and low astrometric uncertainty (~0.2 arcsec) of the LoTSS, we match its compact sources to the compiled sample of dwarf galaxies in the Census of the Local Universe, an H alpha survey with the Palomar Observatory 48 inch Samuel Oschin Telescope. We identify 29 over-luminous compact radio sources, evaluate the probability of chance alignment within the sample, investigate the potential nature of these sources, and evaluate their volumetric density and volumetric rate. While optical line-ratio diagnostics on the nebular lines from the host galaxies support a star-formation origin rather than an AGN origin, future high-angular-resolution radio data are necessary to ascertain the origin of the radio sources. We discuss planned strategies to differentiate between candidate FRB hosts and intermediate-mass black holes., Comment: Accepted to A&A

The International Pulsar Timing Array 2nd data release is the combination of datasets from worldwide collaborations. In this study, we search for continuous waves: gravitational wave signals produced by individual supermassive black hole binaries in the local universe. We consider binaries on circular orbits and neglect the evolution of orbital frequency over the observational span. We find no evidence for such signals and set sky averaged 95% upper limits on their amplitude h 95 . The most sensitive frequency is 10nHz with h 95 = 9.1 10-15 . We achieved the best upper limit to date at low and high frequencies of the PTA band thanks to improved effective cadence of observations. In our analysis, we have taken into account the recently discovered common red noise process, which has an impact at low frequencies. We also find that the peculiar noise features present in some pulsars data must be taken into account to reduce the false alarm. We show that using custom noise models is essential in searching for continuous gravitational wave signals and setting the upper limit.

In Paper I, we presented an overview of the Southern-sky MWA Rapid Two-metre (SMART) survey, including the survey design and search pipeline. While the combination of MWA's large field-of-view and the voltage capture system brings a survey speed of ~450 square degrees per hour, the survey progression relies on the availability of compact configuration of the Phase II array. Over the past few years, by taking advantage of multiple windows of opportunity when the compact configuration was available, we have advanced the survey to 75% completion. To date, about 10% of the data collected thus far have been processed for a first-pass search, where 10 minutes of observation is processed for dispersion measures out to 250 ${\rm pc\,cm^{-3}}$, to realise a shallow survey for long-period pulsars. The ongoing analysis has led to two new pulsar discoveries, as well as an independent discovery and a rediscovery of a previously incorrectly characterised pulsar, all from ~3% of the data for which candidate scrutiny is completed. Here we describe the strategies for further detailed follow-up including improved sky localisation and convergence to timing solution, and illustrate them using example pulsar discoveries. The processing has also led to re-detection of 120 pulsars in the SMART observing band, bringing the total number of pulsars detected to date with the MWA to 180, and these are used to assess the search sensitivity of current processing pipelines. The planned second-pass (deep survey) processing is expected to yield a three-fold increase in sensitivity for long-period pulsars, and a substantial improvement to millisecond pulsars by adopting optimal de-dispersion plans. The SMART survey will complement the highly successful Parkes High Time Resolution Universe survey at 1.2-1.5 GHz, and inform future large survey efforts such as those planned with the low-frequency Square Kilometre Array., Comment: 22 pages, 9 figures, 7 tables, Accepted for publication in PASA

We present an overview of the Southern-sky MWA Rapid Two-metre (SMART) pulsar survey that exploits the MWA's large field of view and voltage capture system to survey the sky south of 30 degree in declination for pulsars and fast transients in the 140-170 MHz band. The survey is enabled by the advent of the Phase II MWA's compact configuration, which offers an enormous efficiency in beam-forming and processing costs, thereby making an all-sky survey of this magnitude tractable with the MWA. Even with the long dwell times of the survey (4800 s), data collection can be completed in < 100 hours of telescope time, while still retaining the ability to reach a limiting sensitivity of ~2-3 mJy. Each observation is processed to generate ~5000-8000 tied-array beams that tessellate the full ~610 square degree field of view, which are then processed to search for pulsars. The voltage-capture recording allows a multitude of post hoc processing options including the reprocessing of data for higher time resolution. Due to the substantial computational cost in pulsar searches at low frequencies, processing is undertaken in multiple passes: in the first pass, a shallow survey is performed, where 10 minutes of each observation is processed, reaching about one-third of the full search sensitivity. Here we present the system overview and initial results. Further details including first pulsar discoveries and a census of low-frequency detections are presented in a companion paper. Future plans include deeper searches to reach the full sensitivity and acceleration searches to target binary and millisecond pulsars. Simulation analysis forecasts ~300 new pulsars upon the completion of full processing. The SMART survey will also generate a complete digital record of the low-frequency sky, which will serve as a valuable reference for future pulsar searches planned with the low-frequency Square Kilometre Array., Comment: 22 pages, 12 figures, 2 tables, Accepted for publication in PASA

We present a low-frequency (170\textendash200\,MHz) search for prompt radio emission associated with the long GRB 210419A using the rapid-response mode of the Murchison Widefield Array (MWA), triggering observations with the Voltage Capture System (VCS) for the first time. The MWA began observing GRB 210419A within 89\,s of its detection by \textit{Swift}, enabling us to capture any dispersion delayed signal emitted by this GRB for a typical range of redshifts. We conducted a standard single pulse search with a temporal and spectral resolution of $100\,\upmu$s and 10\,kHz over a broad range of dispersion measures from 1 to $5000\,\text{pc}\,\text{cm}^{-3}$, but none were detected. However, fluence upper limits of $77\text{--}224$\,Jy\,ms derived over a pulse width of $0.5\text{--}10$\,ms and a redshift of $0.6

We searched for an isotropic stochastic gravitational wave background in the second data release of the International Pulsar Timing Array, a global collaboration synthesizing decadal-length pulsar-timing campaigns in North America, Europe, and Australia. In our reference search for a power law strain spectrum of the form $h_c = A(f/1\,\mathrm{yr}^{-1})^{\alpha}$, we found strong evidence for a spectrally-similar low-frequency stochastic process of amplitude $A = 3.8^{+6.3}_{-2.5}\times10^{-15}$ and spectral index $\alpha = -0.5 \pm 0.5$, where the uncertainties represent 95\% credible regions, using information from the auto- and cross-correlation terms between the pulsars in the array. For a spectral index of $\alpha = -2/3$, as expected from a population of inspiralling supermassive black hole binaries, the recovered amplitude is $A = 2.8^{+1.2}_{-0.8}\times10^{-15}$. Nonetheless, no significant evidence of the Hellings-Downs correlations that would indicate a gravitational-wave origin was found. We also analyzed the constituent data from the individual pulsar timing arrays in a consistent way, and clearly demonstrate that the combined international data set is more sensitive. Furthermore, we demonstrate that this combined data set produces comparable constraints to recent single-array data sets which have more data than the constituent parts of the combination. Future international data releases will deliver increased sensitivity to gravitational wave radiation, and significantly increase the detection probability., Comment: 17 pages, 12 figures, accepted in MNRAS

Here we present a low frequency (170-200MHz) search for coherent radio emission associated with nine short GRBs detected by the Swift and/or Fermi satellites using the Murchison Widefield Array (MWA) rapid-response observing mode. The MWA began observing these events within 30 to 60s of their high-energy detection, enabling us to capture any dispersion delayed signals emitted by short GRBs for a typical range of redshifts. We conducted transient searches at the GRB positions on timescales of 5s, 30s and 2min, resulting in the most constraining flux density limits on any associated transient of 0.42, 0.29, and 0.084Jy, respectively. We also searched for dispersed signals at a temporal and spectral resolution of 0.5s and 1.28MHz but none were detected. However, the fluence limit of 80-100Jy ms derived for GRB 190627A is the most stringent to date for a short GRB. We compared the fluence and persistent emission limits to short GRB coherent emission models, placing constraints on key parameters including the radio emission efficiency of the nearly merged neutron stars ($\lesssim10^{-4}$), the fraction of magnetic energy in the GRB jet ($\lesssim2\times10^{-4}$), and the radio emission efficiency of the magnetar remnant ($\lesssim10^{-3}$). Comparing the limits derived for our full GRB sample to the same emission models, we demonstrate that our 30-min flux density limits were sensitive enough to theoretically detect the persistent radio emission from magnetar remnants up to a redshift of $z\sim0.6$. Our non-detection of this emission could imply that some GRBs in the sample were not genuinely short or did not result from a binary neutron star merger, the GRBs were at high redshifts, these mergers formed atypical magnetars, the radiation beams of the magnetar remnants were pointing away from Earth, or the majority did not form magnetars but rather collapse directly into black holes., Comment: Accepted for publication in PASA

We present results from a search for the radio counterpart to the possible neutron star-black hole merger GW190814 with the Australian Square Kilometre Array Pathfinder. We have carried out 10 epochs of observation spanning 2-655 days post-merger at a frequency of 944 MHz. Each observation covered 30 deg$^2$, equivalent to 87% of the event localisation. We conducted an untargeted search for radio transients in the field, as well as a targeted search for transients associated with known galaxies. We find one radio transient, ASKAP J005022.3-230349, but conclude that it is unlikely to be associated with the merger. We use our observations to place constraints on the inclination angle of the merger and the density of the surrounding environment by comparing our non-detection to model predictions for radio emission from compact binary coalescences. This survey is also the most comprehensive widefield search (in terms of sensitivity and both areal and temporal coverage) for radio transients to-date and we calculate the radio transient surface density at 944 MHz.

We present a broadband radio study of the transient jets ejected from the black hole candidate X-ray binary MAXI J1535-571, which underwent a prolonged outburst beginning on 2 September 2017. We monitored MAXI J1535-571 with the Murchison Widefield Array (MWA) at frequencies from 119 to 186 MHz over six epochs from 20 September to 14 October 2017. The source was quasi-simultaneously observed over the frequency range 0.84-19 GHz by UTMOST (the upgraded Molonglo Observatory Synthesis Telescope), the Australian Square Kilometre Array Pathfinder, the Australia Telescope Compact Array (ATCA), and the Australian Long Baseline Array (LBA). Using the LBA observations from 23 September 2017, we measured the source size to be $34\pm1$ mas. During the brightest radio flare on 21 September 2017, the source was detected down to 119 MHz by the MWA, and the radio spectrum indicates a turnover between 250 and 500 MHz, which is most likely due to synchrotron self-absorption (SSA). By fitting the radio spectrum with a SSA model and using the LBA size measurement, we determined various physical parameters of the jet knot (identified in ATCA data), including the jet opening angle (= $4.5\pm1.2^{\circ}$) and the magnetic field strength (= $104^{+80}_{-78}$ mG). Our fitted magnetic field strength agrees reasonably well with that inferred from the standard equipartition approach, suggesting the jet knot to be close to equipartition. Our study highlights the capabilities of the Australian suite of radio telescopes to jointly probe radio jets in black hole X-ray binaries (BH-XRBs) via simultaneous observations over a broad frequency range, and with differing angular resolutions. This suite allows us to determine the physical properties of XRB jets. Finally, our study emphasizes the potential contributions that can be made by the low-frequency part of the Square Kilometre Array (SKA-Low) in the study of BH-XRBs., Comment: This article has been accepted for publication in PASA. This article has 16 pages, 7 figures and 1 table

The PSR J2222-0137 binary system is a unique laboratory for testing gravity theories. To fully exploit its potential for the tests, we aim to improve the measurements of its physical parameters: spin, orbital orientation, and post-Keplerian parameters which quantify the observed relativistic effects. We present improved analysis of archival VLBI data, using a coordinate convention in full agreement with that used in timing. We also obtain much improved polarimetry with FAST. We provide an analysis of significantly extended timing data taken with Effelsberg, Nancay, Lovell and Green Bank telescopes. From VLBI analysis we obtain a new estimate of the position angle of ascending node, Omega=189(19) deg, and a new position of the pulsar with more conservative uncertainty. The FAST polarimetry and in particular the detection of an interpulse, yield much improved estimate for the spin geometry of the pulsar, in particular an inclination of the spin axis of 84 deg. From the timing we obtain a new 1% test of general relativity (GR) from the agreement of the Shapiro delay and the advance rate of periastron. Assuming GR in a self-consistent analysis of all effects, we obtain much improved mass: 1.831(10) M_sun for the pulsar and 1.319(4) M_sun for the companion; the total mass, 3.150(14) M_sun confirms it as the most massive double degenerate binary known in the Galaxy. This analysis also yields the orbital orientation: the orbital inclination is 85.27(4) deg, indicating a close alignment between the spin of the pulsar and the orbital angular momentum; Omega = 188(6) deg, matching our VLBI result. We also obtain precise value of the orbital period derivative, 0.251(8)e-12 s s^-1, consistent with the expected variation of Doppler factor plus the orbital decay caused by emission of gravitational wave (GW) predicted by GR. This agreement introduces stringent constraint on the emission of dipolar GW., Comment: 17 pages, 8 figures, 7 tables, accepted for publication in A&A

We present analysis of multiple Chandra and XMM-Newton spectra, separated by 9-19 years, of four of the youngest central compact objects (CCOs) with ages < 2500 yr: CXOU J232327.9+584842 (Cassiopeia A), CXOU J160103.1-513353 (G330.2+1.0), 1WGA J1713.4-3949 (G347.3-0.5), and XMMU J172054.5-372652 (G350.1-0.3). By fitting these spectra with thermal models, we attempt to constrain each CCO's long-term cooling rate, composition, and magnetic field. For the CCO in Cassiopeia A, 14 measurements over 19 years indicate a decreasing temperature at a ten-year rate of 2.2+/-0.2 or 2.8+/-0.3 percent (1sigma error) for a constant or changing X-ray absorption, respectively. We obtain cooling rate upper limits of 17 percent for CXOU J160103.1-513353 and 6 percent for XMMU J172054.5-372652. For the oldest CCO, 1WGA J1713.4-3949, its temperature seems to have increased by 4+/-2 percent over a ten year period. Assuming each CCO's preferred distance and an emission area that is a large fraction of the total stellar surface, a non-magnetic carbon atmosphere spectrum is a good fit to spectra of all four CCOs. If distances are larger and emission areas are somewhat smaller, then equally good spectral fits are obtained using a hydrogen atmosphere with B <= 7x10^10 G or B >= 10^12 G for CXOU J160103.1-513353, B <= 10^10 G or B >= 10^12 G for XMMU J172054.5-372652, and non-magnetic hydrogen atmosphere for 1WGA J1713.4-3949. In a unified picture of CCO evolution, our results suggest most CCOs, and hence a sizable fraction of young neutron stars, have a surface magnetic field that is low early in their life but builds up over several thousand years., Comment: 15 pages, 9 figures; accepted for publication in MNRAS

Here we present stringent low-frequency 185MHz limits on coherent radio emission associated with a short gamma-ray burst (SGRB). Our observations of the short GRB 180805A were taken with the upgraded Murchison Widefield Array (MWA) rapid-response system, which triggered within 20s of receiving the transient alert from Swift, corresponding to 83.7s post-burst. The SGRB was observed for 30m, resulting in a 3sigma persistent flux density upper-limit of 40.2mJy/beam. Transient searches were conducted at the Swift position of this GRB on 0.5s, 5s, 30s, and 2m timescales, resulting in 3sigma limits of 570-1830, 270-630, 200-420, and 100-200mJy/beam, respectively. We also performed a dedispersion search for prompt signals at the position of the SGRB with a temporal and spectral resolution of 0.5s and 1.28MHz, resulting in a 6sigma fluence upper-limit range from 570Jyms at DM=3000pc/cm^3 (z~2.5) to 1750Jyms at DM=200pc/cm^3 (z~0.1). We compare the fluence prompt emission limit and the persistent upper-limit to SGRB coherent emission models assuming the merger resulted in a stable magnetar. Our observations were not sensitive enough to detect prompt emission associated with the alignment of magnetic fields of a binary neutron star just prior to the merger, from the interaction between the relativistic jet and the interstellar medium or persistent pulsar-like emission from the spin-down of the magnetar. However, in the case of a more powerful SGRB (a gamma-ray fluence an order of magnitude higher than GRB 180805A and/or a brighter X-ray counterpart), our MWA observations may be sensitive enough to detect coherent radio emission from the jet-ISM interaction and/or the magnetar remnant. Finally, we demonstrate that of all current low-frequency radio telescopes, only the MWA has the sensitivity and response times capable of probing prompt emission models associated with the initial SGRB merger event., Comment: 20 pages, 11 figures, accepted for publication in PASA

We report the discovery of the first new pulsar with the Murchison Widefield Array (MWA), PSR J0036$-$1033, a long-period (0.9 s) nonrecycled pulsar with a dispersion measure (DM) of 23.1 ${\rm pc\,cm^{-3}}$. It was found after processing only a small fraction ($\sim$1%) of data from an ongoing all-sky pulsar survey. Follow-up observations have been made with the MWA, the upgraded Giant Metrewave Radio Telescope (uGMRT), and the Parkes 64 m telescopes, spanning a frequency range from $\sim$150 MHz to 4 GHz. The pulsar is faint, with an estimated flux density ($S$) of $\sim$1 mJy at 400 MHz and a spectrum $S(\nu)\,\propto\,\nu^{-2.0 \pm 0.2}$, where $\nu$ is frequency. The DM-derived distance implies that it is also a low-luminosity source ($\sim$ 0.1 ${\rm mJy\,kpc^2}$ at 1400 MHz). The analysis of archival MWA observations reveals that the pulsar's mean flux density varies by up to a factor of $\sim$5-6 on timescales of several weeks to months. By combining MWA and uGMRT data, the pulsar position was determined to arcsecond precision. We also report on polarization properties detected in the MWA and Parkes bands. The pulsar's nondetection in previous pulsar and continuum imaging surveys, the observed high variability, and its detection in a small fraction of the survey data searched to date, all hint at a larger population of pulsars that await discovery in the southern hemisphere, with the MWA and the future low-frequency Square Kilometre Array., Comment: 12 pages, 6 figures, 2 tables, Accepted for publication in The ApJ Letters

Most neutron stars are expected to be born in supernovae, but only about half of supernova remnants (SNRs) are associated with a compact object. In many cases, a supernova progenitor may have resulted in a black hole. However, there are several possible reasons why true pulsar-SNR associations may have been missed in previous surveys: The pulsar's radio beam may not be oriented towards us; the pulsar may be too faint to be detectable; or there may be an offset in the pulsar position caused by a kick. Our goal is to find new pulsars in SNRs and explore their possible association with the remnant. The search and selection of the remnants presented in this paper was inspired by the non-detection of any X-ray bright compact objects in these remnants when previously studied. Five SNRs were searched for radio pulsars with the Green Bank Telescope at 820 MHz with multiple pointings to cover the full spatial extent of the remnants. A periodicity search plus an acceleration search up to 500 m/s^2 and a single pulse search were performed for each pointing in order to detect potential isolated binary pulsars and single pulses, respectively. No new pulsars were detected in the survey. However, we were able to re-detect a known pulsar, PSR J2047+5029, near SNR G89.0+4.7. We were unable to detect the radio-quiet gamma-ray pulsar PSR J2021+4026, but we do find a flux density limit of 0.08 mJy. Our flux density limits make our survey two to 16 times more sensitive than previous surveys, while also covering the whole spatial extent of the same remnants. We discuss potential explanations for the non-detection of a pulsar in the studied SNRs and conclude that sensitivity is still the most likely factor responsible for the lack of pulsars in some remnants., Comment: Accepted for publication in A&A on 14/01/2021

Cosmological Gamma-Ray Bursts (GRBs) are known to arise from distinct progenitor channels: short GRBs mostly from neutron star mergers and long GRBs from a rare type of core-collapse supernova (CCSN) called collapsars. Highly magnetized neutron stars called magnetars also generate energetic, short-duration gamma-ray transients called Magnetar Giant Flares (MGFs). Three have been observed from the Milky Way and its satellite galaxies and they have long been suspected to contribute a third class of extragalactic GRBs. We report the unambiguous identification of a distinct population of 4 local ($<$5 Mpc) short GRBs, adding GRB 070222 to previously discussed events. While identified solely based on alignment to nearby star-forming galaxies, their rise time and isotropic energy release are independently inconsistent with the larger short GRB population at $>$99.9% confidence. These properties, the host galaxies, and non-detection in gravitational waves all point to an extragalactic MGF origin. Despite the small sample, the inferred volumetric rates for events above $4\times10^{44}$ erg of $R_{MGF}=3.8_{-3.1}^{+4.0}\times10^5$ Gpc$^{-3}$ yr$^{-1}$ place MGFs as the dominant gamma-ray transient detected from extragalactic sources. As previously suggested, these rates imply that some magnetars produce multiple MGFs, providing a source of repeating GRBs. The rates and host galaxies favor common CCSN as key progenitors of magnetars., Comment: Accepted for publication in ApJL. Updated versions fix typos in the table and updates citations to published versions

We present a sensitive search with the Karl G. Jansky Very Large Array (VLA) for the radio counterpart of the gravitational wave candidate S191216ap, classified as a binary black hole merger, and suggested to be a possible multi-messenger event, based on the detection of a high energy neutrino and a TeV photon. We carried out a blind search at C band (4--8 GHz) over 0.3 deg$^2$ of the gamma-ray counterpart of S191216ap reported by the High-Altitude Water Cherenkov Observatory (HAWC). Our search, spanning three epochs over 130 days post-merger and having mean source-detection threshold of 75$\mu$Jy/beam (4$\sigma$), yielded 5 variable sources associated with AGN activity and no definitive counterpart of S191216ap. We find $<$2\% ($3.0\pm1.3$\%) of the persistent radio sources at 6 GHz to be variable on a timescale of $<$1 week (week--months), consistent with previous radio variability studies. Our 4$\sigma$ radio luminosity upper limit of $\sim$1.2$\times{10}^{28}$ erg s$^{-1}$ Hz$^{-1}$ on the afterglow of S191216ap, within the HAWC error region, is 5--10 times deeper than previous BBH radio afterglow searches. Comparing this upper limit with theoretical expectations given by \citeauthor{Perna2019} for putative jets launched by BBH mergers, for on-axis jets having energy $\simeq10^{49}$ erg, we can rule out jet opening angles $\lesssim$20 degrees (assuming that the counterpart lies within the 1$\sigma$ HAWC region that we observed)., Comment: 9 pages, 5 figures, 2 tables, submitted to ApJ

We report the first discovery of a fast radio burst (FRB), FRB 20200125A, by the Green Bank Northern Celestial Cap (GBNCC) Pulsar Survey conducted with the Green Bank Telescope at 350 MHz. FRB 20200125A was detected at a Galactic latitude of 58.43 degrees with a dispersion measure of 179 pc cm$^{-3}$, while electron density models predict a maximum Galactic contribution of 25 pc cm$^{-3}$ along this line of sight. Moreover, no apparent Galactic foreground sources of ionized gas that could account for the excess DM are visible in multi-wavelength surveys of this region. This argues that the source is extragalactic. The maximum redshift for the host galaxy is $z_{max}=0.17$, corresponding to a maximum comoving distance of approximately 750 Mpc. The measured peak flux density for FRB 20200125A is 0.37 Jy, and we measure a pulse width of 3.7 ms, consistent with the distribution of FRB widths observed at higher frequencies. Based on this detection and assuming an Euclidean flux density distribution of FRBs, we calculate an all-sky rate at 350 MHz of $3.4^{+15.4}_{-3.3} \times 10^3$ FRBs sky$^{-1}$ day$^{-1}$ above a peak flux density of 0.42 Jy for an unscattered pulse having an intrinsic width of 5 ms, consistent with rates reported at higher frequencies. Given the recent improvements in our single-pulse search pipeline, we also revisit the GBNCC survey sensitivity to various burst properties. Finally, we find no evidence of interstellar scattering in FRB 20200125A, adding to the growing evidence that some FRBs have circumburst environments where free-free absorption and scattering are not significant., Comment: 15 pages, 6 figures, Submitted to ApJ

We present a calibration component for the Murchison Widefield Array All-Sky Virtual Observatory (MWA ASVO) utilising a newly developed PostgreSQL database of calibration solutions. Since its inauguration in 2013, the MWA has recorded over thirty-four petabytes of data archived at the Pawsey Supercomputing Centre. According to the MWA Data Access policy, data become publicly available eighteen months after collection. Therefore, most of the archival data are now available to the public. Access to public data was provided in 2017 via the MWA ASVO interface, which allowed researchers worldwide to download MWA uncalibrated data in standard radio astronomy data formats (CASA measurement sets or UV FITS files). The addition of the MWA ASVO calibration feature opens a new, powerful avenue for researchers without a detailed knowledge of the MWA telescope and data processing to download calibrated visibility data and create images using standard radio-astronomy software packages. In order to populate the database with calibration solutions from the last six years we developed fully automated pipelines. A near-real-time pipeline has been used to process new calibration observations as soon as they are collected and upload calibration solutions to the database, which enables monitoring of the interferometric performance of the telescope. Based on this database we present an analysis of the stability of the MWA calibration solutions over long time intervals., Comment: 12 pages, 9 figures, Accepted for publication in PASA

Binaries harbouring millisecond pulsars enable a unique path to determine neutron star masses: radio pulsations reveal the motion of the neutron star, while that of the companion can be characterised through studies in the optical range. PSR J1012+5307 is a millisecond pulsar in a 14.5-h orbit with a helium-core white dwarf companion. In this work we present the analysis of an optical spectroscopic campaign, where the companion star absorption features reveal one of the lightest known white dwarfs. We determine a white dwarf radial velocity semi-amplitude of K_2 = 218.9 +- 2.2 km/s, which combined with that of the pulsar derived from the precise radio timing, yields a mass ratio of q=10.44+- 0.11. We also attempt to infer the white dwarf mass from observational constraints using new binary evolution models for extremely low-mass white dwarfs, but find that they cannot reproduce all observed parameters simultaneously. In particular, we cannot reconcile the radius predicted from binary evolution with the measurement from the photometric analysis (R_WD=0.047+-0.003 Rsun). Our limited understanding of extremely low-mass white dwarf evolution, which results from binary interaction, therefore comes as the main factor limiting the precision with which we can measure the mass of the white dwarf in this system. Our conservative white dwarf mass estimate of M_WD = 0.165 +- 0.015 Msun, along with the mass ratio enables us to infer a pulsar mass of M_NS = 1.72 +- 0.16 Msun. This value is clearly above the canonical 1.4 Msun, therefore adding PSR J1012+5307 to the growing list of massive millisecond pulsars., Comment: 13 pages, 7 figures. Accepted for publication in MNRAS

A search of the first Data Release of the VISTA Variables in the Via Lactea (VVV) Survey discovered the exceptionally red transient VVV-WIT-01 ($H-K_s=5.2$). It peaked before March 2010, then faded by $\sim$9.5 mag over the following two years. The 1.6--22 $\mu$m spectral energy distribution in March 2010 was well fit by a highly obscured black body with $T \sim 1000$ K and $A_{K_s} \sim 6.6$ mag. The source is projected against the Infrared Dark Cloud (IRDC) SDC G331.062$-$0.294. The chance projection probability is small for any single event ($p \approx 0.01$ to 0.02) which suggests a physical association, e.g. a collision between low mass protostars. However, black body emission at $T \sim 1000$ K is common in classical novae (especially CO novae) at the infrared peak in the light curve, due to condensation of dust $\sim$30--60 days after the explosion. Radio follow up with the Australia Telescope Compact Array (ATCA) detected a fading continuum source with properties consistent with a classical nova but probably inconsistent with colliding protostars. Considering all VVV transients that could have been projected against a catalogued IRDC raises the probability of a chance association to $p=0.13$ to 0.24. After weighing several options, it appears likely that VVV-WIT-01 was a classical nova event located behind an IRDC., Comment: 13 pages, 7 figures. Accepted by MNRAS

The regularity of pulsar emissions becomes apparent once we reference the pulses' times of arrivals to the inertial rest frame of the solar system. It follows that errors in the determination of Earth's position with respect to the solar-system barycenter can appear as a time-correlated bias in pulsar-timing residual time series, affecting the searches for low-frequency gravitational waves performed with pulsar timing arrays. Indeed, recent array datasets yield different gravitational-wave background upper limits and detection statistics when analyzed with different solar-system ephemerides. Crucially, the ephemerides do not generally provide usable error representations. In this article we describe the motivation, construction, and application of a physical model of solar-system ephemeris uncertainties, which focuses on the degrees of freedom (Jupiter's orbital elements) most relevant to gravitational-wave searches with pulsar timing arrays. This model, BayesEphem, was used to derive ephemeris-robust results in NANOGrav's 11-yr stochastic-background search, and it provides a foundation for future searches by NANOGrav and other consortia. The analysis and simulations reported here suggest that ephemeris modeling reduces the gravitational-wave sensitivity of the 11-yr dataset; and that this degeneracy will vanish with improved ephemerides and with the longer pulsar timing datasets that will become available in the near future., Comment: Fixed typo in author list. Code that supports all calculations and figures is available at github.com/nanograv/11yr_stochastic_analysis/tree/master/bayesephem

The compact configuration of Phase II of the Murchison Widefield Array (MWA) consists of both a redundant subarray and pseudo-random baselines, offering unique opportunities to perform sky-model and redundant interferometric calibration. The highly redundant hexagonal cores give improved power spectrum sensitivity. In this paper, we present the analysis of nearly 40 hours of data targeting one of the MWA's EoR fields observed in 2016. We use both improved analysis techniques presented in Barry et al. (2019) as well as several additional techniques developed for this work, including data quality control methods and interferometric calibration approaches. We show the EoR power spectrum limits at redshift 6.5, 6.8 and 7.1 based on our deep analysis on this 40-hour data set. These limits span a range in $k$ space of $0.18$ $h$ $\mathrm{Mpc^{-1}}$ $

The mergers of supermassive black hole binaries (SMBHBs) promise to be incredible sources of gravitational waves (GWs). While the oscillatory part of the merger gravitational waveform will be outside the frequency sensitivity range of pulsar timing arrays (PTAs), the non-oscillatory GW memory effect is detectable. Further, any burst of gravitational waves will produce GW memory, making memory a useful probe of unmodeled exotic sources and new physics. We searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GW memory. This dataset is sensitive to very low frequency GWs of $\sim3$ to $400$ nHz (periods of $\sim11$ yr $-$ $1$ mon). Finding no evidence for GWs, we placed limits on the strain amplitude of GW memory events during the observation period. We then used the strain upper limits to place limits on the rate of GW memory causing events. At a strain of $2.5\times10^{-14}$, corresponding to the median upper limit as a function of source sky position, we set a limit on the rate of GW memory events at $<0.4$ yr$^{-1}$. That strain corresponds to a SMBHB merger with reduced mass of $\eta M \sim 2\times10^{10}M_\odot$ and inclination of $\iota=\pi/3$ at a distance of 1 Gpc. As a test of our analysis, we analyzed the NANOGrav 9-year data set as well. This analysis found an anomolous signal, which does not appear in the 11-year data set. This signal is not a GW, and its origin remains unknown., Comment: 10 pages, 6 figures, submitted to ApJ

The Murchison Widefield Array (MWA) is an open access telescope dedicated to studying the low frequency (80$-$300 MHz) southern sky. Since beginning operations in mid 2013, the MWA has opened a new observational window in the southern hemisphere enabling many science areas. The driving science objectives of the original design were to observe 21\,cm radiation from the Epoch of Reionisation (EoR), explore the radio time domain, perform Galactic and extragalactic surveys, and monitor solar, heliospheric, and ionospheric phenomena. All together 60$+$ programs recorded 20,000 hours producing 146 papers to date. In 2016 the telescope underwent a major upgrade resulting in alternating compact and extended configurations. Other upgrades, including digital back-ends and a rapid-response triggering system, have been developed since the original array was commissioned. In this paper we review the major results from the prior operation of the MWA, and then discuss the new science paths enabled by the improved capabilities. We group these science opportunities by the four original science themes, but also include ideas for directions outside these categories., Comment: 38 pages, 12 figures, accepted for publication in PASA

The Murchison Widefield Array (MWA) is an electronically steered low frequency ($<300$\,MHz) radio interferometer, with a `slew' time less than 8seconds. Low frequency ($\sim 100$\,MHz) radio telescopes are ideally suited for rapid-response follow-up of transients due to their large field of view, the inverted spectrum of coherent emission, and the fact that the dispersion delay between a 1GHz and 100MHz pulse is on the order of $1-10$\,min for dispersion measures of $100-2000$\,pc/cm$^3$. The MWA has previously been used to provide fast follow up for transient events including gamma-ray bursts, fast radio bursts, and gravitational waves, using systems that respond to gamma-ray coordinates network (GCN) packet-based notifications. We describe a system for automatically triggering MWA observations of such events, based on VOEvent triggers, which is more flexible, capable, and accurate than previous systems. The system can respond to external multi-messenger triggers, which makes it well-suited to searching for prompt coherent radio emission from gamma-ray bursts, the study of fast radio bursts and gravitational waves, single pulse studies of pulsars, and rapid follow-up of high-energy superflares from flare stars. The new triggering system has the capability to trigger observations in both the regular correlator mode (limited to $\geq 0.5$\,s integrations) or using the Voltage Capture System (VCS, $0.1$\,ms integration) of the MWA, and represents a new mode of operation for the MWA. The upgraded standard correlator triggering capability has been in use since MWA observing semester 2018B (July-Dec 2018), and the VCS and buffered mode triggers will become available for observing in a future semester., Comment: 14 pages, 5 figures, accepted for publication in PASA

An ensemble of inspiraling supermassive black hole binaries should produce a stochastic background of very low frequency gravitational waves. This stochastic background is predicted to be a power law, with a spectral index of -2/3, and it should be detectable by a network of precisely timed millisecond pulsars, widely distributed on the sky. This paper reports a new "time slicing" analysis of the 11-year data release from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) using 34 millisecond pulsars. Methods to flag potential "false positive" signatures are developed, including techniques to identify responsible pulsars. Mitigation strategies are then presented. We demonstrate how an incorrect noise model can lead to spurious signals, and show how independently modeling noise across 30 Fourier components, spanning NANOGrav's frequency range, effectively diagnoses and absorbs the excess power in gravitational-wave searches. This results in a nominal, and expected, progression of our gravitational-wave statistics. Additionally we show that the first interstellar medium event in PSR J1713+0747 pollutes the common red noise process with low-spectral index noise, and use a tailored noise model to remove these effects., Comment: 14 pages, 13 figures, fixed typo in abstract of earlier version

One of the major challenges for pulsar timing array (PTA) experiments is the mitigation of the effects of the turbulent interstellar medium (ISM) from timing data. These can potentially lead to measurable delays and/or distortions in the pulse profiles and scale strongly with the inverse of the radio frequency. Low-frequency observations are therefore highly appealing for characterizing them. However, in order to achieve the necessary time resolution to resolve profile features of short-period millisecond pulsars, phase-coherent de-dispersion is essential, especially at frequencies below $300$ MHz. We present the lowest-frequency ($80$-$220$ MHz), coherently de-dispersed detections of one of the most promising pulsars for current and future PTAs, PSR J2241$-$5236, using our new beam-former software for the MWA's voltage capture system (VCS), which reconstructs the time series at a much higher time resolution of $\sim 1 \mu$s by re-synthesizing the recorded voltage data at $10$-kHz/$100$-$\mu$s native resolutions. Our data reveal a dual-precursor type feature in the pulse profile that is either faint or absent in high-frequency observations from Parkes. The resultant high-fidelity detections have enabled dispersion measure (DM) determinations with very high precision, of the order of $(2$-$6)\times10^{-6}$ $\rm pc\,cm^{-3}$, owing to the microsecond level timing achievable for this pulsar at the MWA's low frequencies. This underscores the usefulness of low-frequency observations for probing the ISM toward PTA pulsars and informing optimal observing strategies for PTA experiments., Comment: 09 pages, 4 figures, 2 tables

We present the DSA-2000: a world-leading radio survey telescope and multi-messenger discovery engine for the next decade. The array will be the first true radio camera, outputting science-ready image data over the 0.7 - 2 GHz frequency range with a spatial resolution of 3.5 arcsec. With 2000 x 5 m dishes, the DSA-2000 will have an equivalent point-source sensitivity to SKA1-mid, but with ten times the survey speed. The DSA-2000 is envisaged as an all-sky survey instrument complementary to the ngVLA, and as a counterpart to the LSST (optical), SPHEREx (near-infrared) and SRG/eROSITA (X-ray) all-sky surveys. Over a five-year prime phase, the DSA-2000 will image the entire sky above declination -30 degrees every four months, detecting > 1 unique billion radio sources in a combined full-Stokes sky map with 500 nJy/beam rms noise. This all-sky survey will be complemented by intermediate and deep surveys, as well as spectral and polarization image cubes. The array will be a cornerstone for multi-messenger science, serving as the principal instrument for the US pulsar timing array community, and by searching for radio afterglows of compact object mergers detected by LIGO and Virgo. The array will simultaneously detect and localize ~10,000 fast radio bursts each year, realizing their ultimate use as a cosmological tool. The DSA-2000 will be proposed to the NSF Mid-Scale Research Infrastructure-2 program with a view to first light in 2026, Comment: Submitted as a Project White Paper for the Astro2020 Decadal Survey (12 pages, 7 figures, 2 table)

The new field of gravitational wave astrophysics requires a growing pool of students and researchers with unique, interdisciplinary skill sets. It also offers an opportunity to build a diverse, inclusive astronomy community from the ground up. We describe the efforts used by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) NSF Physics Frontiers Center to foster such growth by involving students at all levels in low-frequency gravitational wave astrophysics with pulsar timing arrays (PTAs) and establishing collaboration policies that ensure broad participation by diverse groups. We describe and illustrate the impact of these techniques on our collaboration as a case study for other distributed collaborations., Comment: Submitted as a State of the Profession White Paper for the Astro2020 Decadal Survey (10 pages + cover and references, 6 figures)

A major goal for the astronomy and astrophysics communities is the pursuit of diversity, equity, and inclusion (DEI) in all ranks, from students through professional scientific researchers. Large scientific collaborations - increasingly a primary place for both professional interactions and research opportunities - can play an important role in the DEI effort. Multimessenger astronomy, a new and growing field, is based on the principle that working collaboratively produces synergies, enabling advances that would not be possible without cooperation. The nascent Multimessenger Diversity Network (MDN) is extending this collaborative approach to include DEI initiatives. After we review of the current state of DEI in astronomy and astrophysics, we describe the strategies the MDN is developing and disseminating to support and increase DEI in the fields over the coming decade: provide opportunities (real and virtual) to share DEI knowledge and resources, include DEI in collaboration-level activities, including external reviews, and develop and implement ways to recognize the DEI work of collaboration members., Comment: Submitted to the Astro2020 Decadal Survey APC White Paper call

We apply two methods to estimate the 21~cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum estimate to be undertaken on the multiple redundantly-spaced triangles of antenna tiles, as well as an estimate based on data gridded to the $uv$-plane. The direct and gridded bispectrum estimators are applied to 21 hours of high-band (167--197~MHz; $z$=6.2--7.5) data from the 2016 and 2017 observing seasons. Analytic predictions for the bispectrum bias and variance for point source foregrounds are derived. We compare the output of these approaches, the foreground contribution to the signal, and future prospects for measuring the bispectra with redundant and non-redundant arrays. We find that some triangle configurations yield bispectrum estimates that are consistent with the expected noise level after 10 hours, while equilateral configurations are strongly foreground-dominated. Careful choice of triangle configurations may be made to reduce foreground bias that hinders power spectrum estimators, and the 21~cm bispectrum may be accessible in less time than the 21~cm power spectrum for some wave modes, with detections in hundreds of hours., Comment: 19 pages, 10 figures, accepted for publication in PASA

We present timing solutions for four pulsars discovered in the Green Bank Northern Celestial Cap (GBNCC) survey. All four pulsars are isolated with spin periods between 0.26$\,$s and 1.84$\,$s. PSR J0038$-$2501 has a 0.26$\,$s period and a period derivative of ${7.6} \times {10}^{-19}\,{\rm s\,s}^{-1}$, which is unusually low for isolated pulsars with similar periods. This low period derivative may be simply an extreme value for an isolated pulsar or it could indicate an unusual evolution path for PSR J0038$-$2501, such as a disrupted recycled pulsar (DRP) from a binary system or an orphaned central compact object (CCO). Correcting the observed spin-down rate for the Shklovskii effect suggests that this pulsar may have an unusually low space velocity, which is consistent with expectations for DRPs. There is no X-ray emission detected from PSR J0038$-$2501 in an archival swift observation, which suggests that it is not a young orphaned CCO. The high dispersion measure of PSR J1949+3426 suggests a distance of 12.3$\,$kpc. This distance indicates that PSR J1949+3426 is among the most distant 7% of Galactic field pulsars, and is one of the most luminous pulsars., Comment: 7 pages, 5 figures

The detection of a neutron star merger by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo gravitational wave detectors and the subsequent detection of an electromagnetic counterpart has opened a new era of transient astronomy. With upgrades to the Advanced LIGO and Advanced Virgo detectors and new detectors coming online in Japan and India, neutron star mergers will be detected at a higher rate in the future, starting with the O3 observing run which will begin in early 2019. The detection of electromagnetic emission from these mergers provides vital information about merger parameters and allows independent measurement of the Hubble constant. The Australian Square Kilometre Array Pathfinder (ASKAP) is expected to become fully operational early 2019 and its 30 deg$^2$ field of view will enable us to rapidly survey large areas of sky. In this work we explore prospects for detecting both prompt and long-term radio emission from neutron star mergers with ASKAP and determine an observing strategy that optimises the use of telescope time. We investigate different strategies to tile the sky with telescope pointings in order to detect radio counterparts with limited observing time, using 475 simulated gravitational wave events. Our results show a significant improvement in observing efficiency when compared with a na\"ive strategy of covering the entire localisation above some confidence threshold, even when achieving the same total probability covered.

Observations indicate that nearly all galaxies contain supermassive black holes (SMBHs) at their centers. When galaxies merge, their component black holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays (PTAs). We have searched the recently-released North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits. As we did not find strong evidence for GWs in our data, we placed 95\% upper limits on the strength of GWs from such sources as a function of GW frequency and sky location. We placed a sky-averaged upper limit on the GW strain of $h_0 < 7.3(3) \times 10^{-15}$ at $f_\mathrm{gw}= 8$ nHz. We also developed a technique to determine the significance of a particular signal in each pulsar using ``dropout' parameters as a way of identifying spurious signals in measurements from individual pulsars. We used our upper limits on the GW strain to place lower limits on the distances to individual SMBHBs. At the most-sensitive sky location, we ruled out SMBHBs emitting GWs with $f_\mathrm{gw}= 8$ nHz within 120 Mpc for $\mathcal{M} = 10^9 \, M_\odot$, and within 5.5 Gpc for $\mathcal{M} = 10^{10} \, M_\odot$. We also determined that there are no SMBHBs with $\mathcal{M} > 1.6 \times 10^9 \, M_\odot$ emitting GWs in the Virgo Cluster. Finally, we estimated the number of potentially detectable sources given our current strain upper limits based on galaxies in Two Micron All-Sky Survey (2MASS) and merger rates from the Illustris cosmological simulation project. Only 34 out of 75,000 realizations of the local Universe contained a detectable source, from which we concluded it was unsurprising that we did not detect any individual sources given our current sensitivity to GWs., Comment: 10 pages, 11 figures. Accepted by Astrophysical Journal. Please send any comments/questions to S. J. Vigeland (vigeland@uwm.edu)

The rare intermittent pulsars pose some of the most challenging questions surrounding the pulsar emission mechanism, but typically have relatively minimal low-frequency ($\lesssim$ 300 MHz) coverage. We present the first low-frequency detection of the intermittent pulsar J1107-5907 with the Murchison Widefield Array (MWA) at 154 MHz and the simultaneous detection from the recently upgraded Molonglo Observatory Synthesis Telescope (UTMOST) at 835 MHz, as part of an on-going observing campaign. During a 30-minute simultaneous observation, we detected the pulsar in its bright emission state for approximately 15 minutes, where 86 and 283 pulses were detected above a signal-to-noise threshold of 6 with the MWA and UTMOST, respectively. Of the detected pulses, 51 had counterparts at both frequencies and exhibited steep spectral indices for both the bright main pulse component and the precursor component. We find that the bright state pulse energy distribution is best parameterised by a log-normal distribution at both frequencies, contrary to previous results which suggested a power law distribution. Further low-frequency observations are required in order to explore in detail aspects such as pulse-to-pulse variability, intensity modulations and to better constrain the signal propagation effects due to the interstellar medium and intermittency characteristics at these frequencies. The spectral index, extended profile emission covering a large fraction of pulse longitude, and the broadband intermittency of PSR J1107-5907 suggests that future low-frequency pulsar searches, for instance those planned with SKA-Low, will be in an excellent position to find and investigate new pulsars of this type., Comment: 19 pages, 7 figures, 4 tables, accepted for publication in ApJ; minor updates to keep in-line with published version

We present new 0.6-10 GHz observations of the binary neutron star merger GW170817 covering the period up to 300 days post-merger, taken with the Karl G. Jansky Very Large Array, the Australia Telescope Compact Array, the Giant Metrewave Radio Telescope and the MeerKAT telescope. We use these data to precisely characterize the decay phase of the late-time radio light curve. We find that the temporal decay is consistent with a power-law slope of t^-2.2, and that the transition between the power-law rise and decay is relatively sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated) outflow, but is instead the classic signature of a relativistic jet. This provides strong observational evidence that GW170817 produced a successful jet, and directly demonstrates the link between binary neutron star mergers and short-hard GRBs. Using simple analytical arguments, we derive constraints on the geometry and the jet opening angle of GW170817. These results are consistent with those from our companion Very Long Baseline Interferometry (VLBI) paper, reporting superluminal motion in GW170817., Comment: 11 pages, 3 figures, 3 tables. Accepted for publication in ApJ Letters

We report on a search for low-frequency radio variability in 944 bright (> 4Jy at 154 MHz) unresolved, extragalactic radio sources monitored monthly for several years with the Murchison Widefield Array. In the majority of sources we find very low levels of variability with typical modulation indices < 5%. We detect 15 candidate low frequency variables that show significant long term variability (>2.8 years) with time-averaged modulation indices M = 3.1 - 7.1%. With 7/15 of these variable sources having peaked spectral energy distributions, and only 5.7% of the overall sample having peaked spectra, we find an increase in the prevalence of variability in this spectral class. We conclude that the variability seen in this survey is most probably a consequence of refractive interstellar scintillation and that these objects must have the majority of their flux density contained within angular diameters less than 50 milli-arcsec (which we support with multi-wavelength data). At 154 MHz we demonstrate that interstellar scintillation time-scales become long (~decades) and have low modulation indices, whilst synchrotron driven variability can only produce dynamic changes on time-scales of hundreds of years, with flux density changes less than one milli-jansky (without relativistic boosting). From this work we infer that the low frequency extra-galactic southern sky, as seen by SKA-Low, will be non-variable on time-scales shorter than one year., Comment: 19 pages, 11 figures

We present the results of a coordinated campaign conducted with the Murchison Widefield Array (MWA) to shadow Fast Radio Bursts (FRBs) detected by the Australian Square Kilometre Array Pathfinder (ASKAP) at 1.4 GHz, which resulted in simultaneous MWA observations of seven ASKAP FRBs. We de-dispersed the $24$ $\times$ $1.28$ MHz MWA images across the $170-200$ MHz band taken at 0.5 second time resolution at the known dispersion measures (DMs) and arrival times of the bursts and searched both within the ASKAP error regions (typically $\sim$ $10$ arcmin $\times$ $10$ arcmin), and beyond ($4$ deg $\times$ $4$ deg). We identified no candidates exceeding a $5\sigma$ threshold at these DMs in the dynamic spectra. These limits are inconsistent with the mean fluence scaling of $\alpha=-1.8 \pm 0.3$ (${\cal F}_\nu \propto \nu^\alpha$, where $\nu$ is the observing frequency) that is reported for ASKAP events, most notably for the three high fluence (${\cal F}_{1.4\,{\rm GHz}} \gtrsim 100$ Jy ms) FRBs 171020, 180110 and 180324. Our limits show that pulse broadening alone cannot explain our non-detections, and that there must be a spectral turnover at frequencies above 200 MHz. We discuss and constrain parameters of three remaining plausible spectral break mechanisms: free-free absorption, intrinsic spectral turn-over of the radiative processes, and magnification of signals at ASKAP frequencies by caustics or scintillation. If free-free absorption were the cause of the spectral turnover, we constrain the thickness of the absorbing medium in terms of the electron temperature, $T$, to $< 0.03$ $(T/10^4 K)^{-1.35}$ pc for FRB 171020., Comment: 9 pages, 3 figures , 2 tables (accepted for publication in ApJL)

We describe the motivation and design details of the "Phase II" upgrade of the Murchison Widefield Array (MWA) radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the MWA in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing MWA core. These new tiles enhance the surface brightness sensitivity of the MWA and will improve the ability of the MWA to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ~3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the array u,v coverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of MWA continuum images. The upgrade retains all of the features that have underpinned the MWA's success (large field-of-view, snapshot image quality, pointing agility) and boosts the scientific potential with enhanced imaging capabilities and by enabling new calibration strategies., Comment: 11 pages, 8 figs. Accepted for publication in PASA

We provide the first in situ measurements of antenna element (tile) beam shapes of the Murchison Widefield Array (MWA), a low radio-frequency interferometer and an SKA precursor. Most current MWA processing pipelines use an assumed beam shape, errors in which can cause absolute and relative flux density errors, as well as polarisation 'leakage'. This makes understanding the primary beam of paramount importance, especially for sensitive experiments such as a measurement of the 21 cm line from the epoch of reionisation (EoR). The calibration requirements for measuring the EoR 21 cm line are so extreme that tile to tile beam variations may affect our ability to make a detection. Measuring the primary beam shape from visibilities alone is challenging, as multiple instrumental, atmospheric, and astrophysical factors contribute to uncertainties in the data. Building on the methods of Neben et al. (2015), we tap directly into the receiving elements of the MWA before any digitisation or correlation of the signal. Using ORBCOMM satellite passes we are able to produce all-sky maps for 4 separate tiles in the XX polarisation. We find good agreement with the cutting-edge 'fully' embedded element (FEE) model of Sokolowski et al. (2017), and observe that the MWA beamformers consistently recreate beam shapes to within ~1dB in the reliable areas of our beam maps. We also clearly observe the effects of a missing dipole from a tile in one of our beam maps, and show that the FEE model is able to reproduce this modified beam shape. We end by motivating and outlining additional onsite experiments to further constrain the primary beam behaviour., Comment: 13 pages, 7 figures Accepted to PASA

Studying the gravitational-wave sky with pulsar timing arrays (PTAs) is a key science goal for the Square Kilometre Array (SKA) and its pathfinder telescopes. With current PTAs reaching sub-microsecond timing precision, making accurate measurements of interstellar propagation effects and mitigating them effectively has become increasingly important to realise PTA goals. As these effects are much stronger at longer wavelengths, low-frequency observations are most appealing for characterizing the interstellar medium (ISM) along the sight lines toward PTA pulsars. The Murchison Widefield Array (MWA) and the Engineering Development Array (EDA), which utilizes MWA technologies, present promising opportunities for undertaking such studies, particularly for PTA pulsars located in the southern sky. Such pulsars are also the prime targets for PTA efforts planned with the South African MeerKAT, and eventually with the SKA. In this paper we report on observations of two bright southern millisecond pulsars PSRs J0437-4715 and J2145-0750 made with these facilities; MWA observations sampling multiple frequencies across the 80-250 MHz frequency range, while the EDA providing direct-sampled baseband data to yield a large instantaneous usable bandwidth of $\sim$200 MHz. Using these observations, we investigate various aspects relating to pulsar emission and ISM properties, such as spectral evolution of the mean pulse shape, scintillation as a function of frequency, chromaticity in interstellar dispersion, and flux density spectra at low frequencies. Systematic and regular monitoring observations will help ascertain the role of low-frequency measurements in PTA experiments, while simultaneously providing a detailed characterization of the ISM toward the pulsars, which will be useful in devising optimal observing strategies for future PTA experiments., Comment: 23 pages, 10 Figures, 4 Tables, Accepted to ApJS

Interferometric arrays seeking to measure the 21 cm signal from the Epoch of Reionization must contend with overwhelmingly bright emission from foreground sources. Accurate recovery of the 21 cm signal will require precise calibration of the array, and several new avenues for calibration have been pursued in recent years, including methods using redundancy in the antenna configuration. The newly upgraded Phase II of Murchison Widefield Array (MWA) is the first interferometer that has large numbers of redundant baselines while retaining good instantaneous UV-coverage. This array therefore provides a unique opportunity to compare redundant calibration with sky-model based algorithms. In this paper, we present the first results from comparing both calibration approaches with MWA Phase II observations. For redundant calibration, we use the package OMNICAL, and produce sky-based calibration solutions with the analysis package Fast Holographic Deconvolution (FHD). There are three principal results. (1) We report the success of OMNICAL on observations of ORBComm satellites, showing substantial agreement between redundant visibility measurements after calibration. (2) We directly compare OMNICAL calibration solutions with those from FHD, and demonstrate these two different calibration schemes give extremely similar results. (3) We explore improved calibration by combining OMNICAL and FHD. We evaluate these combined methods using power spectrum techniques developed for EoR analysis and find evidence for marginal improvements mitigating artifacts in the power spectrum. These results are likely limited by signal-to-noise in the six hours of data used, but suggest future directions for combining these two calibration schemes., Comment: 20 pages, 11 figures. Accepted to ApJ

PSR J0636+5128 is a millisecond pulsar in one of the most compact pulsar binaries known, with a 96\,min orbital period. The pulsar mass function suggests a very low-mass companion, similar to that seen in so-called "black widow" binaries. Unlike in most of those, however, no radio eclipses by material driven off from the companion were seen leading to the possibility that the companion was a degenerate remnant of a carbon-oxygen white dwarf. We report the discovery of the optical counterpart of its companion in images taken with the Gemini North and Keck~I telescopes. The companion varies between $r=25$ and $r=23$ on the 96\,min orbital period of the binary, caused by irradiation from the pulsar's energetic wind. We modeled the multi-color lightcurve using parallax constraints from pulsar timing and determine a companion mass of $(1.71\pm0.23)\times 10^{-2}\,M_\odot$, a radius of $(7.6\pm1.4)\times 10^{-2}\,R_\odot$, and a mean density of $54\pm26\,{\rm g\,cm}^{-3}$, all for an assumed neutron star mass of $1.4\,M_\odot$. This makes the companion to PSR J0636+5128 one of the densest of the "black widow" systems. Modeling suggests that the composition is not predominantly hydrogen, perhaps due to an origin in an ultra-compact X-ray binary., Comment: 4 figures, 1 table. Submitted to ApJ on June 29, 2018. Accepted on July 20, 2018