Search

We report the discovery of the millisecond pulsar (MSP) PSR J1950+2414 ($P=4.3$ ms) in a binary system with an eccentric ($e=0.08$) 22-day orbit in Pulsar ALFA survey observations with the Arecibo telescope. Its companion star has a median mass of 0.3 $M_\odot$ and is most likely a white dwarf. Fully recycled MSPs like this one are thought to be old neutron stars spun-up by mass transfer from a companion star. This process should circularize the orbit, as is observed for the vast majority of binary MSPs, which predominantly have orbital eccentricities $e < 0.001$. However, four recently discovered binary MSPs have orbits with $0.027 < e < 0.44$; PSR J1950+2414 is the fifth such system to be discovered. The upper limits for its intrinsic spin period derivative and inferred surface magnetic field strength are comparable to those of the general MSP population. The large eccentricities are incompatible with the predictions of the standard recycling scenario: something unusual happened during their evolution. Proposed scenarios are a) initial evolution of the pulsar in a triple system which became dynamically unstable, b) origin in an exchange encounter in an environment with high stellar density, c) rotationally delayed accretion-induced collapse of a super-Chandrasekhar white dwarf, and d) dynamical interaction of the binary with a circumbinary disk. We compare the properties of all five known eccentric MSPs with the predictions of these formation channels. Future measurements of the masses and proper motion might allow us to firmly exclude some of the proposed formation scenarios., Comment: 9 pages, 2 figures, 2 tables; accepted for publication in The Astrophysical Journal

The on-going PALFA survey at the Arecibo Observatory began in 2004 and is searching for radio pulsars in the Galactic plane at 1.4 GHz. Observations since 2009 have been made with new wider-bandwidth spectrometers than were previously employed in this survey. A new data reduction pipeline has been in place since mid-2011 which consists of standard methods using dedispersion, searches for accelerated periodic sources, and search for single pulses, as well as new interference-excision strategies and candidate selection heuristics. This pipeline has been used to discover 41 pulsars, including 8 millisecond pulsars (MSPs; P < 10 ms), bringing the PALFA survey's discovery totals to 145 pulsars, including 17 MSPs, and one Fast Radio Burst (FRB). The pipeline presented here has also re-detected 188 previously known pulsars including 60 found in PALFA data by re-analyzing observations previously searched by other pipelines. A comprehensive description of the survey sensitivity, including the effect of interference and red noise, has been determined using synthetic pulsar signals with various parameters and amplitudes injected into real survey observations and subsequently recovered with the data reduction pipeline. We have confirmed that the PALFA survey achieves the sensitivity to MSPs predicted by theoretical models. However, we also find that compared to theoretical survey sensitivity models commonly used there is a degradation in sensitivity to pulsars with periods P >= 100 ms that gradually becomes up to a factor of ~10 worse for P > 4 s at DM < 150 pc/cc. This degradation of sensitivity at long periods is largely due to red noise. We find that 35 +- 3% of pulsars are missed despite being bright enough to be detected in the absence of red noise. This reduced sensitivity could have implications on estimates of the number of long-period pulsars in the Galaxy., Comment: 24 pages, 13 figures, submitted to ApJ

We report 21-yr timing of one of the most precise pulsars: PSR J1713+0747. Its pulse times of arrival are well modeled by a comprehensive pulsar binary model including its three-dimensional orbit and a noise model that incorporates correlated noise such as jitter and red noise. Its timing residuals have weighted root mean square $\sim 92$ ns. The new dataset allows us to update and improve previous measurements of the system properties, including the masses of the neutron star ($1.31\pm0.11$ $M_{\odot}$) and the companion white dwarf ($0.286\pm0.012$ $M_{\odot}$) and the parallax distance $1.15\pm0.03$ kpc. We measured the intrinsic change in orbital period, $\dot{P}^{\rm Int}_{\rm b}$, is $-0.20\pm0.17$ ps s$^{-1}$, which is not distinguishable from zero. This result, combined with the measured $\dot{P}^{\rm Int}_{\rm b}$ of other pulsars, can place a generic limit on potential changes in the gravitational constant $G$. We found that $\dot{G}/G$ is consistent with zero [$(-0.6\pm1.1)\times10^{-12}$ yr$^{-1}$, 95\% confidence] and changes at least a factor of $31$ (99.7\% confidence) more slowly than the average expansion rate of the Universe. This is the best $\dot{G}/G$ limit from pulsar binary systems. The $\dot{P}^{\rm Int}_{\rm b}$ of pulsar binaries can also place limits on the putative coupling constant for dipole gravitational radiation $\kappa_D=(-0.9\pm3.3)\times10^{-4}$ (95\% confidence). Finally, the nearly circular orbit of this pulsar binary allows us to constrain statistically the strong-field post-Newtonian parameters $\Delta$, which describes the violation of strong equivalence principle, and $\hat{\alpha}_3$, which describes a breaking of both Lorentz invariance in gravitation and conservation of momentum. We found, at 95\% confidence, $\Delta<0.01$ and $\hat{\alpha}_3<2\times10^{-20}$ based on PSR J1713+0747., Comment: 14 pages, 6 figures. Published on ApJ, 809, 41 (2015)

We have discovered 21 Rotating Radio Transients (RRATs) in data from the Green Bank Telescope (GBT) 350-MHz Drift-scan and the Green Bank North Celestial Cap pulsar surveys using a new candidate sifting algorithm. RRATs are pulsars with sporadic emission that are detected through their bright single pulses rather than Fourier domain searches. We have developed {\tt RRATtrap}, a single-pulse sifting algorithm that can be integrated into pulsar survey data analysis pipelines in order to find RRATs and Fast Radio Bursts. We have conducted follow-up observations of our newly discovered sources at several radio frequencies using the GBT and Low Frequency Array (LOFAR), yielding improved positions and measurements of their periods, dispersion measures, and burst rates, as well as phase-coherent timing solutions for four of them. The new RRATs have dispersion measures (DMs) ranging from 15 to 97 pc cm$^{-3}$, periods of 240 ms to 3.4 s, and estimated burst rates of 20 to 400 pulses hr$^{-1}$ at 350 MHz. We use this new sample of RRATs to perform statistical comparisons between RRATs and canonical pulsars in order to shed light on the relationship between the two populations. We find that the DM and spatial distributions of the RRATs agree with those of the pulsars found in the same survey. We find evidence that slower pulsars (i.e. $P>200$ ms) are preferentially more likely to emit bright single pulses than are faster pulsars ($P<200$ ms), although this conclusion is tentative. Our results are consistent with the proposed link between RRATs, transient pulsars, and canonical pulsars as sources in various parts of the pulse activity spectrum., Comment: 18 pages, 13 figures, 5 tables, published in ApJ

We present timing observations of four millisecond pulsars discovered in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. PSRs J1552-4937 and J1843-1448 are isolated objects with spin periods of 6.28 and 5.47 ms respectively. PSR J1727-2946 is in a 40-day binary orbit and has a spin period of 27 ms. The 4.43-ms pulsar J1813-2621 is in a circular 8.16-day binary orbit around a low-mass companion star with a minimum companion mass of 0.2 solar masses. Combining these results with detections from five other Parkes multibeam surveys, gives a well-defined sample of 56 pulsars with spin periods below 20 ms. We develop a likelihood analysis to constrain the functional form which best describes the underlying distribution of spin periods for millisecond pulsars. The best results were obtained with a log-normal distribution. A gamma distribution is less favoured, but still compatible with the observations. Uniform, power-law and Gaussian distributions are found to be inconsistent with the data. Galactic millisecond pulsars being found by current surveys appear to be in agreement with a log-normal distribution which allows for the existence of pulsars with periods below 1.5 ms., Comment: 10 pages, 4 tables, 7 figures, accepted for publication in MNRAS

Among efforts to detect gravitational radiation, pulsar timing arrays are uniquely poised to detect "memory" signatures, permanent perturbations in spacetime from highly energetic astrophysical events such as mergers of supermassive black hole binaries. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) observes dozens of the most stable millisecond pulsars using the Arecibo and Green Bank radio telescopes in an effort to study, among other things, gravitational wave memory. We herein present the results of a search for gravitational wave bursts with memory (BWMs) using the first five years of NANOGrav observations. We develop original methods for dramatically speeding up searches for BWM signals. In the directions of the sky where our sensitivity to BWMs is best, we would detect mergers of binaries with reduced masses of $10^9$ $M_\odot$ out to distances of 30 Mpc; such massive mergers in the Virgo cluster would be marginally detectable. We find no evidence for BWMs. However, with our non-detection, we set upper limits on the rate at which BWMs of various amplitudes could have occurred during the time spanned by our data--e.g., BWMs with amplitudes greater than $10^{-13}$ must occur at a rate less than 1.5 yr$^{-1}$., Comment: 12 pages, 7 figures. We will submit this article to ApJ two weeks from now. Questions or comments should be directed to Dustin Madison (drm252@cornell.edu)

We present the discovery of five millisecond pulsars (MSPs) from the PALFA Galactic plane survey using Arecibo. Four of these (PSRs J0557+1551, J1850+0244, J1902+0300, and J1943+2210) are binary pulsars whose companions are likely white dwarfs, and one (PSR J1905+0453) is isolated. Phase-coherent timing solutions, ranging from $\sim$1 to $\sim$3 years in length, and based on observations from the Jodrell Bank and Arecibo telescopes, provide precise determinations of spin, orbital, and astrometric parameters. All five pulsars have large dispersion measures ($>100$ pc cm$^{-3}$, within the top 20% of all known Galactic field MSPs) and are faint (1.4 GHz flux density < 0.1 mJy, within the faintest 5% of all known Galactic field MSPs), illustrating PALFA's ability to find increasingly faint, distant MSPs in the Galactic plane. In particular, PSR J1850+0244 has a dispersion measure of 540 pc cm$^{-3}$, the highest of all known MSPs. Such distant, faint MSPs are important input for accurately modeling the total Galactic MSP population., Comment: 12 pages, 7 figures, ApJ in press

On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (nanoHz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition. In this chapter, we describe how timing observations using SKA1 will contribute to detecting GWs, or can confirm a detection if a first signal already has been identified when SKA1 commences observations. We describe how SKA observations will identify the source(s) of a GW signal, search for anisotropies in the background, improve models of galaxy evolution, test theories of gravity, and characterise the early inspiral phase of a SMBHB system. We describe the impact of the large number of millisecond pulsars to be discovered by the SKA; and the observing cadence, observation durations, and instrumentation required to reach the necessary sensitivity. We describe the noise processes that will influence the achievable precision with the SKA. We assume a long-term timing programme using the SKA1-MID array and consider the implications of modifications to the current design. We describe the possible benefits from observations using SKA1-LOW. Finally, we describe GW detection prospects with SKA1 and SKA2, and end with a description of the expectations of GW astronomy., Comment: 19 pages, 3 figures, to be published in: "Advancing Astrophysics with the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)037

Globular clusters are highly efficient radio pulsar factories. These pulsars can be used as precision probes of the clusters' structure, gas content, magnetic field, and formation history; some of them are also highly interesting in their own right because they probe exotic stellar evolution scenarios as well as the physics of dense matter, accretion, and gravity. Deep searches with SKA1-MID and SKA1-LOW will plausibly double to triple the known population. Such searches will only require one to a few tied-array beams, and can be done during early commissioning of the telescope - before an all-sky pulsar survey using hundreds to thousands of tied-array beams is feasible. With SKA2 it will be possible to observe most of the active radio pulsars within a large fraction of the Galactic globular clusters, an estimated population of 600 - 3700 observable pulsars (those beamed towards us). This rivals the total population of millisecond pulsars that can be found in the Galactic field; fully characterizing it will provide the best-possible physical laboratories as well as a rich dynamical history of the Galactic globular cluster system., Comment: 15 pages, 5 figures, to be published in: "Advancing Astrophysics with the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)047

We report on the determination of astrometric, spin and orbital parameters for PSR J1518+0204C, a "black widow" binary millisecond pulsar in the globular cluster M5. The accurate position and orbital parameters obtained from radio timing allowed us to search for the optical companion. By using WFC3/HST images we identified a very faint variable star (m_F390W > 24.8, m_F606W > 24.3, m_F814W > 23.1) located at only 0.25" from the pulsar's timing position. Due to its strong variability, this star is visible only in a sub-sample of images. However, the light curve obtained folding the available data with the orbital parameters of the pulsar shows a maximum at the pulsar inferior conjunction and a possible minimum at the pulsar superior conjunction. Furthermore, the shape of the optical modulation indicates a heating process possibly due to the pulsar wind. This is the first identification of an optical companion to a black widow pulsar in the dense stellar environment of a globular cluster., Comment: Accepted for publication by ApJ; 24 Pages, 5 Figures, 1 Table

The radio millisecond pulsar J1713+0747 is regarded as one of the highest-precision clocks in the sky, and is regularly timed for the purpose of detecting gravitational waves. The International Pulsar Timing Array collaboration undertook a 24-hour global observation of PSR J1713+0747 in an effort to better quantify sources of timing noise in this pulsar, particularly on intermediate (1 - 24 hr) timescales. We observed the pulsar continuously over 24 hr with the Arecibo, Effelsberg, GMRT, Green Bank, LOFAR, Lovell, Nancay, Parkes, and WSRT radio telescopes. The combined pulse times-of-arrival presented here provide an estimate of what sources of timing noise, excluding DM variations, would be present as compared to an idealized root-N improvement in timing precision, where N is the number of pulses analyzed. In the case of this particular pulsar, we find that intrinsic pulse phase jitter dominates arrival time precision when the S/N of single pulses exceeds unity, as measured using the eight telescopes that observed at L-band/1.4 GHz. We present first results of specific phenomena probed on the unusually long timescale (for a single continuous observing session) of tens of hours, in particular interstellar scintillation, and discuss the degree to which scintillation and profile evolution affect precision timing. This paper presents the data set as a basis for future, deeper studies., Comment: Published in the Astrophysical Journal

We describe an ongoing search for pulsars and dispersed pulses of radio emission, such as those from rotating radio transients (RRATs) and fast radio bursts (FRBs), at 350 MHz using the Green Bank Telescope. With the Green Bank Ultimate Pulsar Processing Instrument, we record 100 MHz of bandwidth divided into 4,096 channels every 81.92 $\mu s$. This survey will cover the entire sky visible to the Green Bank Telescope ($\delta > -40^\circ$, or 82% of the sky) and outside of the Galactic Plane will be sensitive enough to detect slow pulsars and low dispersion measure ($<$30 $\mathrm{pc\,cm^{-3}}$) millisecond pulsars (MSPs) with a 0.08 duty cycle down to 1.1 mJy. For pulsars with a spectral index of $-$1.6, we will be 2.5 times more sensitive than previous and ongoing surveys over much of our survey region. Here we describe the survey, the data analysis pipeline, initial discovery parameters for 62 pulsars, and timing solutions for 5 new pulsars. PSR J0214$+$5222 is an MSP in a long-period (512 days) orbit and has an optical counterpart identified in archival data. PSR J0636$+$5129 is an MSP in a very short-period (96 minutes) orbit with a very low mass companion (8 $M_\mathrm{J}$). PSR J0645$+$5158 is an isolated MSP with a timing residual RMS of 500 ns and has been added to pulsar timing array experiments. PSR J1434$+$7257 is an isolated, intermediate-period pulsar that has been partially recycled. PSR J1816$+$4510 is an eclipsing MSP in a short-period orbit (8.7 hours) and may have recently completed its spin-up phase., Comment: 18 pages, 10 figures, 5 tables, accepted by ApJ

The Pulsar Arecibo L-band Feed Array (PALFA) Survey uses the ALFA 7-beam receiver to search both inner and outer Galactic sectors visible from Arecibo ($32^{\circ}\lesssim \ell \lesssim 77^{\circ}$ and $168^{\circ}\lesssim \ell \lesssim 214^{\circ}$) close to the Galactic plane ($|b|\lesssim5^{\circ}$) for pulsars. In this paper we detail a precursor survey of this region with PALFA, which observed a subset of the full region (slightly more restrictive in $\ell$ and $|b|\lesssim1^{\circ}$) and detected 45 pulsars. For both Galactic millisecond and normal pulsar populations, we compare the survey's detections with simulations to model these populations and, in particular, to estimate the number of observable pulsars in the Galaxy. We place 95\% confidence intervals of 82,000 to 143,000 on the number of detectable normal pulsars and 9,000 to 100,000 on the number of detectable millisecond pulsars in the Galactic disk. These are consistent with previous estimates. Given the most likely population size in each case (107,000 and 15,000 for normal and millisecond pulsars, respectively) we extend survey detection simulations to predict that, when complete, the full PALFA survey should have detected $1,000\substack{+330 \\ -230}$ normal pulsars and $30\substack{+200 \\ -20}$ millisecond pulsars. Identical estimation techniques predict that $490\substack{+160 \\ -115}$ normal pulsars and $12\substack{+70 \\ -5}$ millisecond pulsars would be detected by the beginning of 2014; at the time, the PALFA survey had detected 283 normal pulsars and 31 millisecond pulsars, respectively., Comment: 10 pages, 5 figures

Recent work has exploited pulsar survey data to identify temporally isolated, millisecond-duration radio bursts with large dispersion measures (DMs). These bursts have been interpreted as arising from a population of extragalactic sources, in which case they would provide unprecedented opportunities for probing the intergalactic medium; they may also be linked to new source classes. Until now, however, all so-called fast radio bursts (FRBs) have been detected with the Parkes radio telescope and its 13-beam receiver, casting some concern about the astrophysical nature of these signals. Here we present FRB 121102, the first FRB discovery from a geographic location other than Parkes. FRB 121102 was found in the Galactic anti-center region in the 1.4-GHz Pulsar ALFA survey with the Arecibo Observatory with a DM = 557.4 $\pm$ 3 pc cm$^{-3}$, pulse width of $3\; \pm 0.5$ ms, and no evidence of interstellar scattering. The observed delay of the signal arrival time with frequency agrees precisely with the expectation of dispersion through an ionized medium. Despite its low Galactic latitude ($b = -0.2^{\circ}$), the burst has three times the maximum Galactic DM expected along this particular line-of-sight, suggesting an extragalactic origin. A peculiar aspect of the signal is an inverted spectrum; we interpret this as a consequence of being detected in a sidelobe of the ALFA receiver. FRB 121102's brightness, duration, and the inferred event rate are all consistent with the properties of the previously detected Parkes bursts., Comment: 9 pages, 3 figures, submitted to ApJ

The Double Pulsar, PSR J$0737$$-$$3039$A/B, is a unique system in which both neutron stars have been detected as radio pulsars. As shown in Ferdman et al., there is no evidence for pulse profile evolution of the A pulsar, and the geometry of the pulsar was fit well with a double-pole circular radio beam model. Assuming a more realistic polar cap model with a vacuum retarded dipole magnetosphere configuration including special relativistic effects, we create synthesized pulse profiles for A given the best-fit geometry from the simple circular beam model. By fitting synthesized pulse profiles to those observed from pulsar A, we constrain the geometry of the radio beam, namely the half-opening angle and the emission altitude, to be $30^\circ$ and $10$ neutron star radii, respectively. Combining the observational constraints of PSR J$0737$$-$$3039$A/B, we are able to construct the full three-dimensional orbital geometry of the Double Pulsar. The relative angle between the spin axes of the two pulsars ($\Delta_S$) is estimated to be ($138^\circ \pm 5^\circ$) at the current epoch and will likely remain constant until tidal interactions become important in $\sim$$85$ Myr, at merger., Comment: 20 pages, 5 figures, Accepted by ApJ on 1 April 2014

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project currently observes 43 pulsars using the Green Bank and Arecibo radio telescopes. In this work we use a subset of 17 pulsars timed for a span of roughly five years (2005--2010). We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Within the timing data, we perform a search for continuous gravitational waves from individual supermassive black hole binaries in circular orbits using robust frequentist and Bayesian techniques. We find that there is no evidence for the presence of a detectable continuous gravitational wave; however, we can use these data to place the most constraining upper limits to date on the strength of such gravitational waves. Using the full 17 pulsar dataset we place a 95% upper limit on the sky-averaged strain amplitude of $h_0\lesssim 3.8\times 10^{-14}$ at a frequency of 10 nHz. Furthermore, we place 95% \emph{all sky} lower limits on the luminosity distance to such gravitational wave sources finding that the $d_L \gtrsim 425$ Mpc for sources at a frequency of 10 nHz and chirp mass $10^{10}{\rm M}_{\odot}$. We find that for gravitational wave sources near our best timed pulsars in the sky, the sensitivity of the pulsar timing array is increased by a factor of $\sim$4 over the sky-averaged sensitivity. Finally we place limits on the coalescence rate of the most massive supermassive black hole binaries., Comment: 21 pages, 9 figures. Matches version submitted to ApJ

We report here a series of observations of the interstellar scintillation (ISS) of the double pulsar J0737$-$3039 over the course of 18 months. As in earlier work (Coles et al., 2005) the basic phenomenon is the variation in the ISS caused by the changing transverse velocities of each pulsar, the ionized interstellar medium (IISM), and the Earth. The transverse velocity of the binary system can be determined both by VLBI and timing observations. The orbital velocity and inclination is almost completely determined from timing observations, but the direction of the orbital angular momentum is not known. Since the Earth's velocity is known, and can be compared with the orbital velocity by its effect on the timescale of the ISS, we can determine the orientation $\Omega$ of the pulsar orbit with respect to equatorial coordinates ($\Omega = 65\pm2$ deg). We also resolve the ambiguity ($i= 88.7$ or $91.3$ deg) in the inclination of the orbit deduced from the measured Shapiro delay by our estimate $i=88.1\pm0.5$ deg. This relies on analysis of the ISS over both frequency and time and provides a model for the location, anisotropy, turbulence level and transverse phase gradient of the IISM. We find that the IISM can be well-modeled during each observation, typically of a few orbital periods, but its turbulence level and mean velocity vary significantly over the 18 months., Comment: Accepted for publication in Astrophysical Journal

We present updated analyses of pulse profiles and their arrival-times from PSR B1534+12, a 37.9-ms radio pulsar in orbit with another neutron star. A high-precision timing model is derived from twenty-two years of timing data, and accounts for all astrophysical processes that systematically affect pulse arrival-times. Five "post-Keplerian" parameters are measured that represent relativistic corrections to the standard Keplerian quantities of the pulsar's binary orbit. These relativistic parameters are then used to test general relativity by comparing the measurements with their predicted values. We conclude that relativity theory is confirmed to within 0.17% of its predictions. Furthermore, we derive the following astrophysical results from our timing analysis: a distance of $d_{\rm GR} = 1.051 \pm 0.005$ kpc to the pulsar-binary system, by relating the "excess" orbital decay to Galactic parameters; evidence for pulse "jitter" in PSR B1534+12 due to short-term magnetospheric activity; and evolution in pulse-dispersion properties. As a secondary study, we also present several analyses on pulse-structure evolution and its connection to relativistic precession of the pulsar's spin axis. The precession-rate measurement yields a value of $\Omega_1^{\rm spin}$ = $0.59^{+0.12}_{-0.08}$ $^{\circ}$/year (68% confidence) that is consistent with expectations, and represents an additional test of relativistic gravity., Comment: Accepted for publication in ApJ on 6 April 2014. v3: added references, a structure-function analysis/discussion, edited MCMC figure and timing-error analysis, corrected minor errors in calculations for distance and MCMC results, and used most up-to-date planetary ephemeris for timing

Gravitationally bound three-body systems have been studied for hundreds of years and are common in our Galaxy. They show complex orbital interactions, which can constrain the compositions, masses, and interior structures of the bodies and test theories of gravity, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, B1620-26 (with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multi-wavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13) Msun, where Msun is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15) Msun and 0.4101(3) Msun), as well as the inclinations of the orbits (both approximately 39.2 degrees). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity., Comment: 17 pages, 3 figures, 1 table. Published online by Nature on 5 Jan 2014. Extremely minor differences with published version may exist

We report on timing observations of the recently discovered binary pulsar PSR J1952+2630 using the Arecibo Observatory. The mildly recycled 20.7-ms pulsar is in a 9.4-hr orbit with a massive, M_WD > 0.93 M_sun, white dwarf (WD) companion. We present, for the first time, a phase-coherent timing solution, with precise spin, astrometric, and Keplerian orbital parameters. This shows that the characteristic age of PSR J1952+2630 is 77 Myr, younger by one order of magnitude than any other recycled pulsar-massive WD system. We derive an upper limit on the true age of the system of 50 Myr. We investigate the formation of PSR J1952+2630 using detailed modelling of the mass-transfer process from a naked helium star on to the neutron star following a common-envelope phase (Case BB Roche-lobe overflow). From our modelling of the progenitor system, we constrain the accretion efficiency of the neutron star, which suggests a value between 100 and 300% of the Eddington accretion limit. We present numerical models of the chemical structure of a possible oxygen-neon-magnesium WD companion. Furthermore, we calculate the past and the future spin evolution of PSR J1952+2630, until the system merges in about 3.4 Gyr due to gravitational wave emission. Although we detect no relativistic effects in our timing analysis we show that several such effects will become measurable with continued observations over the next 10 years; thus PSR J1952+2630 has potential as a testbed for gravitational theories., Comment: 12 pages, 10 figures, to be published in MNRAS

We report a 5.4\sigma\ detection of pulsed gamma rays from PSR B1821-24 in the globular cluster M28 using ~44 months of Fermi Large Area Telescope (LAT) data that have been reprocessed with improved instrument calibration constants. We constructed a phase-coherent ephemeris, with post-fit residual RMS of 3 \mu s, using radio data spanning ~23.2 years, enabling measurements of the multi-wavelength light curve properties of PSR B1821-24 at the milliperiod level. We fold RXTE observations of PSR B1821-24 from 1996 to 2007 and discuss implications on the emission zones. The gamma-ray light curve consists of two peaks, separated by 0.41$\pm$0.02 in phase, with the first gamma-ray peak lagging the first radio peak by 0.05$\pm$0.02 in phase, consistent with the phase of giant radio pulses. We observe significant emission in the off-peak interval of PSR B1821-24 with a best-fit LAT position inconsistent with the core of M28. We do not detect significant gamma-ray pulsations at the spin or orbital periods from any other known pulsar in M28, and we place limits on the number of energetic pulsars in the cluster. The derived gamma-ray efficiency, ~2%, is typical of other gamma-ray pulsars with comparable spin-down power, suggesting that the measured spin-down rate ($2.2\times10^{36}$ erg s$^{-1}$) is not appreciably distorted by acceleration in the cluster potential. This confirms PSR B1821-24 as the second very energetic millisecond pulsar in a globular cluster and raises the question of whether these represent a separate class of objects that only form in regions of very high stellar density, Comment: 5 figures, 2 tables, accepted for publication in ApJ

In this paper, we present a novel artificial intelligence (AI) program that identifies pulsars from recent surveys using image pattern recognition with deep neural nets---the PICS (Pulsar Image-based Classification System) AI. The AI mimics human experts and distinguishes pulsars from noise and interferences by looking for patterns from candidate. The information from each pulsar candidate is synthesized in four diagnostic plots, which consist of up to thousands pixel of image data. The AI takes these data from each candidate as its input and uses thousands of such candidates to train its ~9000 neurons. Different from other pulsar selection programs which use pre-designed patterns, the PICS AI teaches itself the salient features of different pulsars from a set of human-labeled candidates through machine learning. The deep neural networks in this AI system grant it superior ability in recognizing various types of pulsars as well as their harmonic signals. The trained AI's performance has been validated with a large set of candidates different from the training set. In this completely independent test, PICS ranked 264 out of 277 pulsar-related candidates, including all 56 previously known pulsars, to the top 961 (1%) of 90008 test candidates, missing only 13 harmonics. The first non-pulsar candidate appears at rank 187, following 45 pulsars and 141 harmonics. In other words, 100% of the pulsars were ranked in the top 1% of all candidates, while 80% were ranked higher than any noise or interference. The performance of this system can be improved over time as more training data are accumulated. This AI system has been integrated into the PALFA survey pipeline and has discovered six new pulsars to date., Comment: 29 pages, 9 figures, two tables, accepted by ApJ

We present a study of PSR J1723-2837, an eclipsing, 1.86 ms millisecond binary radio pulsar discovered in the Parkes Multibeam survey. Radio timing indicates that the pulsar has a circular orbit with a 15 hr orbital period, a low-mass companion, and a measurable orbital period derivative. The eclipse fraction of ~15% during the pulsar's orbit is twice the Roche lobe size inferred for the companion. The timing behavior is significantly affected by unmodeled systematics of astrophysical origin, and higher-order orbital period derivatives are needed in the timing solution to account for these variations. We have identified the pulsar's (non-degenerate) companion using archival ultraviolet, optical, and infrared survey data and new optical photometry. Doppler shifts from optical spectroscopy confirm the star's association with the pulsar and indicate a pulsar-to-companion mass ratio of 3.3 +/- 0.5, corresponding to a companion mass range of 0.4 to 0.7 Msun and an orbital inclination angle range of between 30 and 41 degrees, assuming a pulsar mass range of 1.4-2.0 Msun. Spectroscopy indicates a spectral type of G for the companion and an inferred Roche-lobe-filling distance that is consistent with the distance estimated from radio dispersion. The features of PSR J1723-2837 indicate that it is likely a "redback" system. Unlike the five other Galactic redbacks discovered to date, PSR J1723-2837 has not been detected as a gamma-ray source with Fermi. This may be due to an intrinsic spin-down luminosity that is much smaller than the measured value if the unmeasured contribution from proper motion is large., Comment: 11 pages, including 8 figures and 5 tables. Accepted by the Astrophysical Journal

We describe the first X-ray observations of five short orbital period ($P_B < 1$ day), $\gamma$-ray emitting, binary millisecond pulsars. Four of these, PSRs J0023+0923, J1124$-$3653, J1810+1744, and J2256$-$1024 are `black-widow' pulsars, with degenerate companions of mass $\ll0.1 M_{\odot}$, three of which exhibit radio eclipses. The fifth source, PSR J2215+5135, is an eclipsing `redback' with a near Roche-lobe filling $\sim$0.2 solar mass non-degenerate companion. Data were taken using the \textit{Chandra X-Ray Observatory} and covered a full binary orbit for each pulsar. Two pulsars, PSRs J2215+5135 and J2256$-$1024, show significant orbital variability while PSR J1124$-$3653 shows marginal orbital variability. The lightcurves for these three pulsars have X-ray flux minima coinciding with the phases of the radio eclipses. This phenomenon is consistent with an intrabinary shock emission interpretation for the X-rays. The other two pulsars, PSRs J0023+0923 and J1810+1744, are fainter and do not demonstrate variability at a level we can detect in these data. All five spectra are fit with three separate models: a power-law model, a blackbody model, and a combined model with both power-law and blackbody components. The preferred spectral fits yield power-law indices that range from 1.3 to 3.2 and blackbody temperatures in the hundreds of eV. The spectrum for PSR J2215+5135 shows a significant hard X-ray component, with a large number of counts above 2 keV, which is additional evidence for the presence of intrabinary shock emission and is similar to what has been detected in the low-mass X-ray binary to millisecond pulsar transition object PSR J1023+0038., Comment: 8 pages, 6 figures, 2 tables, submitted to ApJ

The binary pulsar J2222-0137 is an enigmatic system containing a partially recycled millisecond pulsar and a companion of unknown nature. Whilst the low eccentricity of the system favors a white dwarf companion, an unusual double neutron star system is also a possibility, and optical observations will be able to distinguish between these possibilities. In order to allow the absolute luminosity (or upper limit) of the companion object to be properly calibrated, we undertook astrometric observations with the Very Long Baseline Array to constrain the system distance via a measurement of annual geometric parallax. With these observations, we measure the parallax of the J2222-0137 system to be 3.742 +0.013 -0.016 milliarcseconds, yielding a distance of 267.3 +1.2 -0.9 pc, and measure the transverse velocity to be 57.1 +0.3 -0.2 km/s. Fixing these parameters in the pulsar timing model made it possible to obtain a measurement of Shapiro delay and hence the system inclination, which shows that the system is nearly edge-on (sin i = 0.9985 +/- 0.0005). Furthermore, we were able to detect the orbital motion of J2222-0137 in our VLBI observations and measure the longitude of ascending node. The VLBI astrometry yields the most accurate distance obtained for a radio pulsar to date, and is furthermore the most accurate parallax for any radio source obtained at "low" radio frequencies (below ~5 GHz, where the ionosphere dominates the error budget). Using the astrometric results, we show the companion to J2222-0137 will be easily detectable in deep optical observations if it is a white dwarf. Finally, we discuss the implications of this measurement for future ultra-high-precision astrometry, in particular in support of pulsar timing arrays., Comment: 22 pages, 7 figures, accepted for publication in ApJ

It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales., Comment: 43 pages, 9 figures, 4 table. Published by Nature on 26 Sep 2013. Includes Supplementary information. Minor differences with published version may exist

Modern radio pulsar surveys produce a large volume of prospective candidates, the majority of which are polluted by human-created radio frequency interference or other forms of noise. Typically, large numbers of candidates need to be visually inspected in order to determine if they are real pulsars. This process can be labor intensive. In this paper, we introduce an algorithm called PEACE (Pulsar Evaluation Algorithm for Candidate Extraction) which improves the efficiency of identifying pulsar signals. The algorithm ranks the candidates based on a score function. Unlike popular machine-learning based algorithms, no prior training data sets are required. This algorithm has been applied to data from several large-scale radio pulsar surveys. Using the human-based ranking results generated by students in the Arecibo Remote Command enter programme, the statistical performance of PEACE was evaluated. It was found that PEACE ranked 68% of the student-identified pulsars within the top 0.17% of sorted candidates, 95% within the top 0.34%, and 100% within the top 3.7%. This clearly demonstrates that PEACE significantly increases the pulsar identification rate by a factor of about 50 to 1000. To date, PEACE has been directly responsible for the discovery of 47 new pulsars, 5 of which are millisecond pulsars that may be useful for pulsar timing based gravitational-wave detection projects., Comment: 7 pages, 4 figures, accepted by MNRAS

We have made extensive observations of 35 distant slow (non-recycled) pulsars discovered in the ongoing Arecibo PALFA pulsar survey. Timing observations of these pulsars over several years at Arecibo Observatory and Jodrell Bank Observatory have yielded high-precision positions and measurements of rotation properties. Despite being a relatively distant population, these pulsars have properties that mirror those of the previously known pulsar population. Many of the sources exhibit timing noise, and one underwent a small glitch. We have used multifrequency data to measure the interstellar scattering properties of these pulsars. We find scattering to be higher than predicted along some lines of sight, particularly in the Cygnus region. Lastly, we present XMM-Newton and Chandra observations of the youngest and most energetic of the pulsars, J1856+0245, which has previously been associated with the GeV-TeV pulsar wind nebula HESS J1857+026., Comment: Published in the Astrophysical Journal

We report the Fermi Large Area Telescope discovery of gamma-ray pulsations from the 22.7 ms pulsar A in the double pulsar system J0737-3039A/B. This is the first mildly recycled millisecond pulsar (MSP) detected in the GeV domain. The 2.7 s companion object PSR J0737-3039B is not detected in gamma rays. PSR J0737-3039A is a faint gamma-ray emitter, so that its spectral properties are only weakly constrained; however, its measured efficiency is typical of other MSPs. The two peaks of the gamma-ray light curve are separated by roughly half a rotation and are well offset from the radio and X-ray emission, suggesting that the GeV radiation originates in a distinct part of the magnetosphere from the other types of emission. From the modeling of the radio and the gamma-ray emission profiles and the analysis of radio polarization data, we constrain the magnetic inclination $\alpha$ and the viewing angle $\zeta$ to be close to 90$^\circ$, which is consistent with independent studies of the radio emission from PSR J0737-3039A. A small misalignment angle between the pulsar's spin axis and the system's orbital axis is therefore favored, supporting the hypothesis that pulsar B was formed in a nearly symmetric supernova explosion as has been discussed in the literature already., Comment: Published in the Astrophysical Journal (ApJ 768, 169, 2013). Updated to reflect changes made at the proof stage

Einstein@Home aggregates the computer power of hundreds of thousands of volunteers from 193 countries, to search for new neutron stars using data from electromagnetic and gravitational-wave detectors. This paper presents a detailed description of the search for new radio pulsars using Pulsar ALFA survey data from the Arecibo Observatory. The enormous computing power allows this search to cover a new region of parameter space; it can detect pulsars in binary systems with orbital periods as short as 11 minutes. We also describe the first Einstein@Home discovery, the 40.8 Hz isolated pulsar PSR J2007+2722, and provide a full timing model. PSR J2007+2722's pulse profile is remarkably wide with emission over almost the entire spin period. This neutron star is most likely a disrupted recycled pulsar, about as old as its characteristic spin-down age of 404 Myr. However there is a small chance that it was born recently, with a low magnetic field. If so, upper limits on the X-ray flux suggest but can not prove that PSR J2007+2722 is at least ~ 100 kyr old. In the future, we expect that the massive computing power provided by volunteers should enable many additional radio pulsar discoveries., Comment: ArXiv version updated to correspond to the published ApJ version. This includes pulse profiles for PSR J2007+2722 at 327 and 430 MHz, correction of a sign error in the polarimetry conventions, improvements to the estimate of the expected shortest orbital period of a Galactic DNS system, and other changes made in refereeing and proof. 32 pages, 19 figures, 7 tables

The double pulsar system PSR J0737-3039A/B is a double neutron star binary, with a 2.4-hour orbital period, which has allowed measurement of relativistic orbital perturbations to high precision. The low mass of the second-formed neutron star, as well as the low system eccentricity and proper motion, point to a different evolutionary scenario compared to most other known double neutron star systems. We describe analysis of the pulse profile shape over 6 years of observations, and present the resulting constraints on the system geometry. We find the recycled pulsar in this system, PSR J0737-3039A, to be a near-orthogonal rotator, with an average separation between its spin and magnetic axes of 90 +/- 11 +/- 5 deg. Furthermore, we find a mean 95% upper limit on the misalignment between its spin and orbital angular momentum axes of 3.2 deg, assuming that the observed emission comes from both magnetic poles. This tight constraint lends credence to the idea that the supernova that formed the second pulsar was relatively symmetric, possibly involving electron-capture onto an O-Ne-Mg core., Comment: 12 pages, 2 tables, 5 figures. Accepted for publication by the Astrophysical Journal

In the last few years, over 43 millisecond radio pulsars have been discovered by targeted searches of unidentified gamma-ray sources found by the Fermi Gamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in compact binaries with low-mass companions. These systems often show eclipses of the pulsar signal and are commonly known as black widows and redbacks because the pulsar is gradually destroying its companion. In this paper, we report on the optical discovery of four strongly irradiated millisecond pulsar companions. All four sources show modulations of their color and luminosity at the known orbital periods from radio timing. Light curve modelling of our exploratory data shows that the equilibrium temperature reached on the companion's dayside with respect to their nightside is consistent with about 10-30% of the available spin-down energy from the pulsar being reprocessed to increase the companion's dayside temperature. This value compares well with the range observed in other irradiated pulsar binaries and offers insights about the energetics of the pulsar wind and the production of gamma-ray emission. In addition, this provides a simple way of estimating the brightness of irradiated pulsar companions given the pulsar spin-down luminosity. Our analysis also suggests that two of the four new irradiated pulsar companions are only partially filling their Roche lobe. Some of these sources are relatively bright and represent good targets for spectroscopic follow-up. These measurements could enable, among other things, mass determination of the neutron stars in these systems., Comment: 11 pages, 5 tables, 1 figure, 4 online tables. ApJ submitted and refereed

The on-going PALFA survey is searching the Galactic plane (|b| < 5 deg., 32 < l < 77 deg. and 168 < l < 214 deg.) for radio pulsars at 1.4 GHz using ALFA, the 7-beam receiver installed at the Arecibo Observatory. By the end of August 2012, the PALFA survey has discovered 100 pulsars, including 17 millisecond pulsars (P < 30 ms). Many of these discoveries are among the pulsars with the largest DM/P ratios, proving that the PALFA survey is capable of probing the Galactic plane for millisecond pulsars to a much greater depth than any previous survey. This is due to the survey's high sensitivity, relatively high observing frequency, and its high time and frequency resolution. Recently the rate of discoveries has increased, due to a new more sensitive spectrometer, two updated complementary search pipelines, the development of online collaborative tools, and access to new computing resources. Looking forward, focus has shifted to the application of artificial intelligence systems to identify pulsar-like candidates, and the development of an improved full-resolution pipeline incorporating more sophisticated radio interference rejection. The new pipeline will be used in a complete second analysis of data already taken, and will be applied to future survey observations. An overview of recent developments, and highlights of exciting discoveries will be presented., Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and Opportunities after 80 years", J. van Leeuwen (ed.); 6 pages, 4 figures

We present the discovery and phase-coherent timing of four highly dispersed millisecond pulsars (MSPs) from the Arecibo PALFA Galactic plane survey: PSRs J1844+0115, J1850+0124, J1900+0308, and J1944+2236. Three of the four pulsars are in binary systems with low-mass companions, which are most likely white dwarfs, and which have orbital periods on the order of days. The fourth pulsar is isolated. All four pulsars have large dispersion measures (DM > 100 pc cm-3), are distant (> 3.4 kpc), faint at 1.4 GHz (< 0.2 mJy), and are fully recycled (with spin periods P between 3.5 and 4.9 ms). The three binaries also have very small orbital eccentricities, as expected for tidally circularized, fully recycled systems with low-mass companions. These four pulsars have DM/P ratios that are among the highest values for field MSPs in the Galaxy. These discoveries bring the total number of confirmed MSPs from the PALFA survey to fifteen. The discovery of these MSPs illustrates the power of PALFA for finding weak, distant MSPs at low-Galactic latitudes. This is important for accurate estimates of the Galactic MSP population and for the number of MSPs that the Square Kilometer Array can be expected to detect., Comment: 23 pages, including 7 figures and 2 tables. Accepted to the Astrophysical Journal

We present two millisecond pulsar discoveries from the PALFA survey of the Galactic plane with the Arecibo telescope. PSR J1955+2527 is an isolated pulsar with a period of 4.87 ms, and PSR J1949+3106 has a period of 13.14 ms and is in a 1.9-day binary system with a massive companion. Their timing solutions, based on 4 years of timing measurements with the Arecibo, Green Bank, Nan\c{c}ay and Jodrell Bank telescopes, allow precise determination of spin and astrometric parameters, including precise determinations of their proper motions. For PSR J1949+3106, we can clearly detect the Shapiro delay. From this we measure the pulsar mass to be 1.47(+0.43/-0.31) solar masses, the companion mass to be 0.85(+0.14/-0.11) solar masses and the orbital inclination to be i = 79.9(+1.6/-1.9) degrees, where uncertainties correspond to +/- 1-\sigma\ confidence levels. With continued timing, we expect to also be able to detect the advance of periastron for the J1949+3106 system. This effect, combined with the Shapiro delay, will eventually provide very precise mass measurements for this system and a test of general relativity., Comment: 29 pages, 4 tables, 7 figures; accepted for publication to The Astrophysical Journal

The recently discovered transitional millisecond pulsar system J1023+0038 exposes a crucial evolutionary phase of recycled neutron stars for multiwavelength study. The system, comprising the neutron star itself, its stellar companion, and the surrounding medium, is visible across the electromagnetic spectrum from the radio to X-ray/gamma-ray regimes and offers insight into the recycling phase of millisecond pulsar evolution. Here, we report on multiple-epoch astrometric observations with the Very Long Baseline Array (VLBA) which give a system parallax of 0.731 +/- 0.022 milliarcseconds (mas) and a proper motion of 17.98 +/- 0.05 mas/yr. By combining our results with previous optical observations, we are able to use the parallax distance of 1368+42-39 pc to estimate the mass of the pulsar as 1.71 +/- 0.16 solar masses, and we are also able to measure the 3D space velocity of the system as 126 +/- 5 km/s. Despite the precise nature of the VLBA measurements, the remaining ~3% distance uncertainty dominates the 0.16 solar mass error on our mass estimate., Comment: 5 pages, 2 tables, 1 figure; accepted for publication in ApJL

PSR J1740-3052 is a young pulsar in orbit around a companion that is most likely a B-type main-sequence star. Since its discovery more than a decade ago, data have been taken at several frequencies with instruments at the Green Bank, Parkes, Lovell, and Westerbork telescopes. We measure scattering timescales in the pulse profiles and dispersion measure changes as a function of binary orbital phase and present evidence that both of these vary as would be expected due to a wind from the companion star. Using pulse arrival times that have been corrected for the observed periodic dispersion measure changes, we find a timing solution spanning 1997 November to 2011 March. This includes measurements of the advance of periastron and the change in the projected semimajor axis of the orbit and sets constraints on the orbital geometry. From these constraints, we estimate that the pulsar received a kick of at least ~50 km/s at birth. A quasi-periodic signal is present in the timing residuals with a period of 2.2 times the binary orbital period. The origin of this signal is unclear., Comment: 10 pages, 5 figures; accepted for publication in MNRAS

We investigate the changes in polarization position angle in radiation from pulsar A around the eclipse in the Double Pulsar system PSR J0737-3039A/B at the 20 cm and 50 cm wavelengths using the Parkes 64-m telescope. The changes are ~2\sigma\ during and shortly after the eclipse at 20 cm but less significant at 50 cm. We show that the changes in position angle during the eclipse can be modelled by differential synchrotron absorption in the eclipse regions. Position angle changes after the eclipse are interpreted as Faraday rotation in the magnetotail of pulsar B. Implied charge densities are consistent with the Goldreich-Julian density, suggesting that the particle energies in the magnetotail are mildly relativistic., Comment: Accepted for publication in The Astrophysical Journal Letters

The energetic, eclipsing millisecond pulsar J1816+4510 was recently discovered in a low-frequency radio survey with the Green Bank Telescope. With an orbital period of 8.7 hr and minimum companion mass of 0.16 Msun it appears to belong to an increasingly important class of pulsars that are ablating their low-mass companions. We report the discovery of the gamma-ray counterpart to this pulsar, and present a likely optical/ultraviolet counterpart as well. Using the radio ephemeris we detect pulsations in the unclassified gamma-ray source 2FGL J1816.5+4511, implying an efficiency of ~25% in converting the pulsar's spin-down luminosity into gamma-rays and adding PSR J1816+4510 to the large number of millisecond pulsars detected by Fermi. The likely optical/UV counterpart was identified through position coincidence (<0.1") and unusual colors. Assuming that it is the companion, with R=18.27+/-0.03 mag and effective temperature >15,000 K it would be among the brightest and hottest of low-mass pulsar companions, and appears qualitatively different from other eclipsing pulsar systems. In particular, current data suggest that it is a factor of two larger than most white dwarfs of its mass, but a factor of four smaller than its Roche lobe. We discuss possible reasons for its high temperature and odd size, and suggest that it recently underwent a violent episode of mass loss. Regardless of origin, its brightness and the relative unimportance of irradiation make it an ideal target for a mass, and hence a neutron star mass, determination., Comment: 10 pages, 6 figures. Accepted for publication in the Astrophysical Journal

We present an analysis of high-precision pulsar timing data taken as part of the North American Nanohertz Observatory for Gravitational waves (NANOGrav) project. We have observed 17 pulsars for a span of roughly five years using the Green Bank and Arecibo radio telescopes. We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Sub-microsecond timing residuals are obtained in nearly all cases, and the best root-mean-square timing residuals in this set are ~30-50 ns. We present methods for analyzing post-fit timing residuals for the presence of a gravitational wave signal with a specified spectral shape. These optimally take into account the timing fluctuation power removed by the model fit, and can be applied to either data from a single pulsar, or to a set of pulsars to detect a correlated signal. We apply these methods to our dataset to set an upper limit on the strength of the nHz-frequency stochastic supermassive black hole gravitational wave background of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of the two best pulsars in the set, PSRs J1713+0747 and J1909-3744., Comment: To be submitted to ApJ

The double pulsar PSR J0737-3039A/B displays short, 30 s eclipses that arise around conjunction when the radio waves emitted by pulsar A are absorbed as they propagate through the magnetosphere of its companion pulsar B. These eclipses offer a unique opportunity to probe directly the magnetospheric structure and the plasma properties of pulsar B. We have performed a comprehensive analysis of the eclipse phenomenology using multi-frequency radio observations obtained with the Green Bank Telescope. We have characterized the periodic flux modulations previously discovered at 820 MHz by McLaughlin et al., and investigated the radio frequency dependence of the duration and depth of the eclipses. Based on their weak radio frequency evolution, we conclude that the plasma in pulsar B's magnetosphere requires a large multiplicity factor (~ 10^5). We also found that, as expected, flux modulations are present at all radio frequencies in which eclipses can be detected. Their complex behavior is consistent with the confinement of the absorbing plasma in the dipolar magnetic field of pulsar B as suggested by Lyutikov & Thompson and such a geometric connection explains that the observed periodicity is harmonically related to pulsar B's spin frequency. We observe that the eclipses require a sharp transition region beyond which the plasma density drops off abruptly. Such a region defines a plasmasphere which would be well inside the magnetospheric boundary of an undisturbed pulsar. It is also two times smaller than the expected standoff radius calculated using the balance of the wind pressure from pulsar A and the nominally estimated magnetic pressure of pulsar B., Comment: 9 pages, 7 figures, 3 tables, ApJ in press

We present high-precision timing of five millisecond pulsars (MSPs) carried out for more than seven years; four pulsars are in binary systems and one is isolated. We are able to measure the pulsars' proper motions and derive an estimate for their space velocities. The measured two-dimensional velocities are in the range 70-210 km/s, consistent with those measured for other MSPs. We also use all the available proper motion information for isolated and binary MSPs to update the known velocity distribution for these populations. As found by earlier works, we find that the velocity distribution of binary and isolated MSPs are indistinguishable with the current data. Four of the pulsars in our observing program are highly recycled with low-mass white dwarf companions and we are able to derive accurate binary parameters for these systems. For three of these binary systems we are able to place initial constraints on the pulsar masses with best-fit values in the range 1.0-1.6 M_sun. The implications of the results presented here to our understanding of binary pulsar evolution are discussed. The updated parameters for the binary systems studied here, together with recently discovered similar systems, allowed us to update previous limits on the the violation of the strong equivalence principle through the parameter |Delta| to 4.6x10^-3 (95% confidence) and the violation of Lorentz-invariance/momentum-conservation through the parameter |hat{alpha}_3| to 5.5x10^-20 (95% confidence)., Comment: 24 pages, 8 figures; accepted for publication in The Astrophysical Journal

We report the discovery of the 20.7 ms binary pulsar J1952+2630, made using the distributed computing project Einstein@Home in Pulsar ALFA survey observations with the Arecibo telescope. Follow-up observations with the Arecibo telescope confirm the binary nature of the system. We obtain a circular orbital solution with an orbital period of 9.4 hr, a projected orbital radius of 2.8 lt-s, and a mass function of f = 0.15 solar masses by analysis of spin period measurements. No evidence of orbital eccentricity is apparent; we set a 2-sigma upper limit e < 1.7e-3. The orbital parameters suggest a massive white dwarf companion with a minimum mass of 0.95 solar masses, assuming a pulsar mass of 1.4 solar masses. Most likely, this pulsar belongs to the rare class of intermediate mass binary pulsars. Future timing observations will aim to determine the parameters of this system further, measure relativistic effects, and elucidate the nature of the companion star., Comment: 5 pages, 3 figures, 1 table; updated to published version

We report on the identification of a near-infrared counterpart to the massive (>11 Msun) binary companion of pulsar J1740-3052. An accurate celestial position of PSR J1740-3052 is determined from interferometric radio observations. Adaptive optics corrected near-infrared imaging observations show a counterpart at the interferometric position of the pulsar. The counterpart has Ks=15.87+-0.10 and J-Ks>0.83. Based on distance and absorption estimates from models of the Galactic electron and dust distributions these observed magnitudes are consistent with those of a main-sequence star as the binary companion. We argue that this counterpart is the binary companion to PSR J1740-3052 and thus rule out a stellar mass black hole as the pulsar companion., Comment: 6 pages, 3 figures, 1 table. Accepted for publication in MNRAS

The Australian Square Kilometre Array Pathfinder (ASKAP) is a 36-element array with a 30-square-degree field of view being built at the proposed SKA site in Western Australia. We are conducting a Design Study for pulsar observations with ASKAP, planning both timing and search observations. We provide an overview of the ASKAP telescope and an update on pulsar-related progress., Comment: To appear in proceedings of "Radio Pulsars: An astrophysical key to unlock the secrets of the Universe"

(abridged) PSR J1903+0327, a millisecond pulsar in an eccentric (e = 0.44) 95-day orbit with a (~ 1Msun) companion poses a challenge to our understanding of stellar evolution in binary and multiple-star systems. Here we describe optical and radio observations which rule out most of the scenarios proposed to explain formation of this system. Radio timing measurements of three post-Keplerian effects yield the most precise measurement of the mass of a millisecond pulsar to date: 1.667 +/- 0.021 solar masses (99.7% confidence limit) (...). Optical spectroscopy of a proposed main sequence counterpart show that its orbital motion mirrors the pulsar's 95-day orbit; being therefore its binary companion (...) The optical detection also provides a measurement of the systemic radial velocity of the binary; this and the proper motion measured from pulsar timing allow the determination of the systemic 3-D velocity in the Galaxy. We find that the system is always within 270 pc of the plane of the Galaxy, but always more than 3 kpc away from the Galactic centre. Thus an exchange interaction in a dense stellar environment (like a globular cluster or the Galactic centre) is not likely to be the origin of this system. We suggest that after the supernova that formed it, the neutron star was in a tight orbit with a main-sequence star, the present companion was a tertiary farther out. The neutron star then accreted matter from its evolving inner MS companion, forming a millisecond pulsar. The former donor star then disappears, either due to a chaotic 3-body interaction with the outer star (caused by the expansion of the inner orbit that necessarily results from mass transfer), or in the case of a very compact inner system, due to ablation/accretion by the newly formed millisecond pulsar., Comment: 16 pages in emulate MNRAS format, 11 figures, 2 tables, accepted for publication in MNRAS

We report on the optical identification of the companion star to the eclipsing millisecond pulsar PSR J1824-2452H in the galactic globular cluster M28 (NGC 6626). This star is at only 0.2" from the nominal position of the pulsar and it shows optical variability (~ 0.25 mag) that nicely correlates with the pulsar orbital period. It is located on the blue side of the cluster main sequence, ~1.5 mag fainter than the turn-off point. The observed light curve shows two distinct and asymmetric minima, suggesting that the companion star is suffering tidal distortion from the pulsar. This discovery increases the number of non-degenerate MSP companions optically identified so far in globular clusters (4 out of 7), suggesting that these systems could be a common outcome of the pulsar recycling process, at least in dense environments where they can be originated by exchange interactions., Comment: accepted for publication on ApJ, 17 pages, 5 figures

High-precision pulsar timing relies on a solar-system ephemeris in order to convert times of arrival (TOAs) of pulses measured at an observatory to the solar system barycenter. Any error in the conversion to the barycentric TOAs leads to a systematic variation in the observed timing residuals; specifically, an incorrect planetary mass leads to a predominantly sinusoidal variation having a period and phase associated with the planet's orbital motion about the Sun. By using an array of pulsars (PSRs J0437-4715, J1744-1134, J1857+0943, J1909-3744), the masses of the planetary systems from Mercury to Saturn have been determined. These masses are consistent with the best-known masses determined by spacecraft observations, with the mass of the Jovian system, 9.547921(2)E-4 Msun, being significantly more accurate than the mass determined from the Pioneer and Voyager spacecraft, and consistent with but less accurate than the value from the Galileo spacecraft. While spacecraft are likely to produce the most accurate measurements for individual solar system bodies, the pulsar technique is sensitive to planetary system masses and has the potential to provide the most accurate values of these masses for some planets., Comment: Accepted for publication in ApJ

Einstein@Home aggregates the computer power of hundreds of thousands of volunteers from 192 countries to "mine" large data sets. It has now found a 40.8 Hz isolated pulsar in radio survey data from the Arecibo Observatory taken in February 2007. Additional timing observations indicate that this pulsar is likely a disrupted recycled pulsar. PSR J2007+2722's pulse profile is remarkably wide with emission over almost the entire spin period; the pulsar likely has closely aligned magnetic and spin axes. The massive computing power provided by volunteers should enable many more such discoveries., Comment: 8 pages, 1 figure