Search

Radiometer experiments to detect 21-cm Hydrogen line emission from the Cosmic Dawn and Epoch of Reionization rely upon precise absolute calibration. During calibration, noise generated by amplifiers within the radiometer receiver must be accounted for; however, it is difficult to measure as the noise power varies with source impedance. In this letter, we introduce a convenient method to measure the noise parameters of a receiver system, which is practical for low-frequency receivers used in global 21-cm experiments., Comment: 4 pages, accepted paper for URSI GASS 2023 J08

Noise parameters are a set of four measurable quantities which determine the noise performance of a radio-frequency device under test. The noise parameters of a 2-port device can be extracted by connecting a set of 4 or more source impedances at the device's input, measuring the noise power of the device with each source connected, and then solving a matrix equation. However, sources with high reflection coefficients cannot be used due to a singularity that arises in entries of the matrix. Here, we detail a new method of noise parameter extraction using a singularity-free matrix that is compatible with high-reflection sources. We show that open, short, load and an open cable ("OSLC") can be used to extract noise parameters, and we detail a practical measurement approach. The OSLC approach is particularly well-suited for low-noise amplifier measurement at frequencies below 1 GHz, where alternative methods require physically large apparatus., Comment: IEEE Transactions on Microwave Theory and Techniques

We present first results of a solar radio event observed with the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) at metric wavelengths. We examine a complex event consisting of multiple radio sources/bursts associated with a fast coronal mass ejection (CME) and an M2.1 GOES soft X-ray flare from 2015 September 20. Images of 9--s cadence are used to analyze the event over a 120-minute period, and solar emission is observed out to a distance of $\approx3.5\,R_\odot$, with an instantaneous bandwidth covering 22~MHz within the frequency range of 40--70~MHz. We present our results from the investigation of the radio event, focusing particularly on one burst source that exhibits outward motion, which we classify as a moving type IV burst. We image the event at multiple frequencies and use the source centroids to obtain the velocity for the outward motion. Spatial and temporal comparison with observations of the CME in white light from the LASCO(C2) coronagraph, indicates an association of the outward motion with the core of the CME. By performing graduated-cylindrical-shell (GCS) reconstruction of the CME, we constrain the density in the volume. The electron plasma frequency obtained from the density estimates do not allow us to completely dismiss plasma emission as the underlying mechanism. However, based on source height and smoothness of the emission in frequency and time, we argue that gyrosynchrotron is the more plausible mechanism. We use gyrosynchrotron spectral fitting techniques to estimate the evolving physical conditions during the outward motion of this burst source., Comment: 22 Pages, 10 figures, 1 table

The 21\,cm transition of neutral hydrogen is opening an observational window into the cosmic dawn of the universe---the epoch of first star formation. We use 28\,hr of data from the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) to place upper limits on the spatial power spectrum of 21\,cm emission at $z \approx 18.4$ ($\Delta_{21} \lesssim 10^4\,\text{mK}$), and within the absorption feature reported by the EDGES experiment (Bowman et al. 2018). In the process we demonstrate the first application of the double Karhunen-Lo\`{e}ve transform for foreground filtering, and diagnose the systematic errors that are currently limiting the measurement. We also provide an updated model for the angular power spectrum of low-frequency foreground emission measured from the northern hemisphere, which can be used to refine sensitivity forecasts for next-generation experiments., Comment: The authors really hoping you're having a fantastic day. You keep being awesome!

We have conducted the most sensitive low frequency (below 100 MHz) search to date for prompt, low-frequency radio emission associated with short-duration gamma-ray bursts (GRBs), using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). The OVRO-LWA's nearly full-hemisphere field-of-view ($\sim20$,$000$ square degrees) allows us to search for low-frequency (sub-$100$ MHz) counterparts for a large sample of the subset of GRB events for which prompt radio emission has been predicted. Following the detection of short GRB 170112A by Swift, we used all-sky OVRO-LWA images spanning one hour prior to and two hours following the GRB event to search for a transient source coincident with the position of GRB 170112A. We detect no transient source, with our most constraining $1\sigma$ flux density limit of $650~\text{mJy}$ for frequencies spanning $27~\text{MHz}-84~\text{MHz}$. We place constraints on a number of models predicting prompt, low-frequency radio emission accompanying short GRBs and their potential binary neutron star merger progenitors, and place an upper limit of $L_\text{radio}/L_\gamma \lesssim 7\times10^{-16}$ on the fraction of energy released in the prompt radio emission. These observations serve as a pilot effort for a program targeting a wider sample of both short and long GRBs with the OVRO-LWA, including bursts with confirmed redshift measurements which are critical to placing the most constraining limits on prompt radio emission models, as well as a program for the follow-up of gravitational wave compact binary coalescence events detected by advanced LIGO and Virgo., Comment: 14 pages, 5 figures, ApJ submitted

A host of new low-frequency radio telescopes seek to measure the 21-cm transition of neutral hydrogen from the early universe. These telescopes have the potential to directly probe star and galaxy formation at redshifts $20 \gtrsim z \gtrsim 7$, but are limited by the dynamic range they can achieve against foreground sources of low-frequency radio emission. Consequently, there is a growing demand for modern, high-fidelity maps of the sky at frequencies below 200 MHz for use in foreground modeling and removal. We describe a new widefield imaging technique for drift-scanning interferometers, Tikhonov-regularized $m$-mode analysis imaging. This technique constructs images of the entire sky in a single synthesis imaging step with exact treatment of widefield effects. We describe how the CLEAN algorithm can be adapted to deconvolve maps generated by $m$-mode analysis imaging. We demonstrate Tikhonov-regularized $m$-mode analysis imaging using the Owens Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky north of $\delta=-30^\circ$ with 15 arcmin angular resolution, at frequencies evenly spaced between 36.528 MHz and 73.152 MHz, and $\sim$800 mJy/beam thermal noise. These maps are a 10-fold improvement in angular resolution over existing full-sky maps at comparable frequencies, which have angular resolutions $\ge 2^\circ$. Each map is constructed exclusively from interferometric observations and does not represent the globally averaged sky brightness. Future improvements will incorporate total power radiometry, improved thermal noise, and improved angular resolution -- due to the planned expansion of the OVRO-LWA to 2.6 km baselines. These maps serve as a first step on the path to the use of more sophisticated foreground filters in 21-cm cosmology incorporating the measured angular and frequency structure of all foreground contaminants., Comment: 27 pages, 18 figures

Radio astronomy observatories with high throughput back end instruments require real-time data processing. While computing hardware continues to advance rapidly, development of real-time processing pipelines remains difficult and time-consuming, which can limit scientific productivity. Motivated by this, we have developed Bifrost: an open-source software framework for rapid pipeline development. Bifrost combines a high-level Python interface with highly efficient reconfigurable data transport and a library of computing blocks for CPU and GPU processing. The framework is generalizable, but initially it emphasizes the needs of high-throughput radio astronomy pipelines, such as the ability to process data buffers as if they were continuous streams, the capacity to partition processing into distinct data sequences (e.g., separate observations), and the ability to extract specific intervals from buffered data. Computing blocks in the library are designed for applications such as interferometry, pulsar dedispersion and timing, and transient search pipelines. We describe the design and implementation of the Bifrost framework and demonstrate its use as the backbone in the correlation and beamforming back end of the Long Wavelength Array station in the Sevilleta National Wildlife Refuge, NM., Comment: 25 pages, 13 figures, submitted to JAI. For the code, see https://github.com/ledatelescope/bifrost

Advances in astronomy are intimately linked to advances in digital signal processing (DSP). This special issue is focused upon advances in DSP within radio astronomy. The trend within that community is to use off-the-shelf digital hardware where possible and leverage advances in high performance computing. In particular, graphics processing units (GPUs) and field programmable gate arrays (FPGAs) are being used in place of application-specific circuits (ASICs); high-speed Ethernet and Infiniband are being used for interconnect in place of custom backplanes. Further, to lower hurdles in digital engineering, communities have designed and released general-purpose FPGA-based DSP systems, such as the CASPER ROACH board, ASTRON Uniboard and CSIRO Redback board. In this introductory article, we give a brief historical overview, a summary of recent trends, and provide an outlook on future directions., Comment: Introduction to the Journal of Astronomical Instrumentation (JAI) special issue on Digital Signal Processing in Radio Astronomy, http://www.worldscientific.com/toc/jai/05/04

Low radio frequency solar observations using the Murchison Widefield Array have recently revealed the presence of numerous weak, short-lived and narrow-band emission features, even during moderately quiet solar conditions. These non-thermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hence, necessarily require an automated approach for their detection and characterization. Here, we employ continuous wavelet transform using a mother Ricker wavelet for feature detection from the dynamic spectrum. We establish the efficacy of this approach and present the first statistically robust characterization of the properties of these features. In particular, we examine distributions of their peak flux densities, spectral spans, temporal spans and peak frequencies. We can reliably detect features weaker than 1 SFU, making them, to the best of our knowledge, the weakest bursts reported in literature. The distribution of their peak flux densities follows a power law with an index of -2.23 in the 12-155 SFU range, implying that they can provide an energetically significant contribution to coronal and chromospheric heating. These features typically last for 1-2 seconds and possess bandwidths of about 4-5 MHz. Their occurrence rate remains fairly flat in the 140-210 MHz frequency range. At the time resolution of the data, they appear as stationary bursts, exhibiting no perceptible frequency drift. These features also appear to ride on a broadband background continuum, hinting at the likelihood of them being weak type-I bursts., Comment: 15 pages, 14 figures, Accepted for publication in ApJ

An FPGA-based digital-receiver has been developed for a low-frequency imaging radio interferometer, the Murchison Widefield Array (MWA). The MWA, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia, consists of 128 dual-polarized aperture-array elements (tiles) operating between 80 and 300\,MHz, with a total processed bandwidth of 30.72 MHz for each polarization. Radio-frequency signals from the tiles are amplified and band limited using analog signal conditioning units; sampled and channelized by digital-receivers. The signals from eight tiles are processed by a single digital-receiver, thus requiring 16 digital-receivers for the MWA. The main function of the digital-receivers is to digitize the broad-band signals from each tile, channelize them to form the sky-band, and transport it through optical fibers to a centrally located correlator for further processing. The digital-receiver firmware also implements functions to measure the signal power, perform power equalization across the band, detect interference-like events, and invoke diagnostic modes. The digital-receiver is controlled by high-level programs running on a single-board-computer. This paper presents the digital-receiver design, implementation, current status, and plans for future enhancements., Comment: 14 pages, 7 figures

The FITS (Flexible Image Transport System) data format was developed in the late 1970s for storage and exchange of astronomy-related image data. Since then, it has become a standard file format not only for images, but also for radio interferometer data (e.g. UVFITS, FITS-IDI). But is FITS the right format for next-generation telescopes to adopt? The newer Hierarchical Data Format (HDF5) file format offers considerable advantages over FITS, but has yet to gain widespread adoption within radio astronomy. One of the major holdbacks is that HDF5 is not well supported by data reduction software packages. Here, we present a comparison of FITS, HDF5, and the MeasurementSet (MS) format for storage of interferometric data. In addition, we present a tool for converting between formats. We show that the underlying data model of FITS can be ported to HDF5, a first step toward achieving wider HDF5 support., Comment: Submitted to Proceedings of ADASS XXIV

The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compared to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1-2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal., Comment: 23 pages, 14 figures, accepted for publication in ApJ

In this paper we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise and sample variance in detecting the HI power spectrum from the Epoch of Reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the HI power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k-space. Sensitivity for EoR HI power spectrum detection is estimated for different observing modes with MWA. With 1000 hours of observing on a single field using the 128-tile MWA, EoR detection is feasible (S/N > 1 for $k\lesssim 0.8$ Mpc$^{-1}$). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum., Comment: 19 pages, 14 figures (19 figures in total including subfigures), 2 appendices, in press for publication in ApJ

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives., Comment: Updated with revisions. 32 pages including figures and references. Submitted to PASA

The process of wide-field synthesis imaging is explored, with the aim of understanding the implications of variable, polarised primary beams for forthcoming Epoch of Reionisation experiments. These experiments seek to detect weak signatures from redshifted 21cm emission in deep residual datasets, after suppression and subtraction of foreground emission. Many subtraction algorithms benefit from low side-lobes and polarisation leakage at the outset, and both of these are intimately linked to how the polarised primary beams are handled. Building on previous contributions from a number of authors, in which direction-dependent corrections are incorporated into visibility gridding kernels, we consider the special characteristics of arrays of fixed dipole antennas operating around 100-200 MHz, looking towards instruments such as the Square Kilometre Array (SKA) and the Hydrogen Epoch of Reionization Arrays (HERA). We show that integrating snapshots in the image domain can help to produce compact gridding kernels, and also reduce the need to make complicated polarised leakage corrections during gridding. We also investigate an alternative form for the gridding kernel that can suppress variations in the direction-dependent weighting of gridded visibilities by 10s of dB, while maintaining compact support., Comment: 15 pages, 4 figures. Accepted for publication in JAI

The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. We have used a 32-element MWA prototype interferometer (MWA-32T) to observe two 50-degree diameter fields in the southern sky in the 110 MHz to 200 MHz band in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ~15' angular resolution maps of the two fields. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing low-frequency radio surveys in order to assess the MWA-32T system performance, wide field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110 MHz to 200 MHz band., Comment: 20 pages, 6 tables, 12 figures. 1 online-only machine readable table. Submitted to ApJ

We study the optical properties of the host galaxies of nuclear 22 GHz (lambda=1.35 cm) water masers. To do so, we cross-match the galaxy sample surveyed for water maser emission (123 detections and 3806 non-detections) with the SDSS low-redshift galaxy sample (z<0.05). Out of 1636 galaxies with SDSS photometry, we identify 48 detections; out of the 1063 galaxies that also have SDSS spectroscopy, we identify 33 detections. We find that maser detection rate is higher at higher optical luminosity (M_B), larger velocity dispersion ($\sigma$), and higher [OIII]5007 luminosity, with [OIII]5007 being the dominant factor. These detection rates are essentially the result of the correlations of isotropic maser luminosity with all three of these variables. These correlations are natural if maser strength increases with central black hole mass and the level of AGN activity. We also find that the detection rate is higher in galaxies with higher extinction. Based on these results, we propose that maser surveys seeking to efficiently find masers should rank AGN targets by extinction-corrected [OIII]5007 flux when available. This prioritization would improve maser detection efficiency, from an overall ~ 3% without pre-selection to ~ 16% for the strongest intrinsic [OIII]5007 emitters, by a factor of ~ 5., Comment: Accepted to ApJ., in emulateapj style, 13 pages, 12 figures, 1 long table

We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array (MWA), observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9-201.6 MHz. Though our observing period is characterized as a period of `low' to `medium' activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low frequency radio interferometers come on-line over the next few years., Comment: 15 pages, 5 figures, accepted for publication in Astrophysical Journal Letters. Movies for figures 4 and 5 available at http://www.mwatelescope.org/info/mwa_proto.html

It is shown that the excellent Murchison Radio-astronomy Observatory site allows the Murchison Widefield Array to employ a simple RFI blanking scheme and still calibrate visibilities and form images in the FM radio band. The techniques described are running autonomously in our calibration and imaging software, which is currently being used to process an FM-band survey of the entire southern sky., Comment: Accepted for publication in Proceedings of Science [PoS(RFI2010)016]. 6 pages and 3 figures. Presented at RFI2010, the Third Workshop on RFI Mitigation in Radio Astronomy, 29-31 March 2010, Groningen, The Netherlands

Based on spectroscopic signatures, about one-third of known H$_2$O maser sources in active galactic nuclei (AGN) are believed to arise in highly inclined accretion disks around central engines. These "disk maser candidates" are of interest primarily because angular structure and rotation curves can be resolved with interferometers, enabling dynamical study. We identify five new disk maser candidates in studies with the Green Bank Telescope, bringing the total number published to 30. We discovered two (NGC1320, NGC17) in a survey of 40 inclined active galaxies (v_{sys}< 20000 kms^{-1}). The remaining three disk maser candidates were identified in monitoring of known sources: NGC449, NGC2979, NGC3735. We also confirm a previously marginal case in UGC4203. For the disk maser candidates reported here, inferred rotation speeds are 130-500 kms^{-1}. Monitoring of three more rapidly rotating candidate disks (CG211, NGC6264, VV340A) has enabled measurement of likely orbital centripetal acceleration, and estimation of central masses (2-7x10^7 M_\odot) and mean disk radii (0.2-0.4pc). Accelerations may ultimately permit estimation of distances when combined with interferometer data. This is notable because the three AGN are relatively distant (10000

Modern graphics processing units (GPUs) are inexpensive commodity hardware that offer Tflop/s theoretical computing capacity. GPUs are well suited to many compute-intensive tasks including digital signal processing. We describe the implementation and performance of a GPU-based digital correlator for radio astronomy. The correlator is implemented using the NVIDIA CUDA development environment. We evaluate three design options on two generations of NVIDIA hardware. The different designs utilize the internal registers, shared memory and multiprocessors in different ways. We find that optimal performance is achieved with the design that minimizes global memory reads on recent generations of hardware. The GPU-based correlator outperforms a single-threaded CPU equivalent by a factor of 60 for a 32 antenna array, and runs on commodity PC hardware. The extra compute capability provided by the GPU maximises the correlation capability of a PC while retaining the fast development time associated with using standard hardware, networking and programming languages. In this way, a GPU-based correlation system represents a middle ground in design space between high performance, custom built hardware and pure CPU-based software correlation. The correlator was deployed at the Murchison Widefield Array 32 antenna prototype system where it ran in real-time for extended periods. We briefly describe the data capture, streaming and correlation system for the prototype array., Comment: 11 pages, to appear in PASP

The Murchison Widefield Array (MWA) is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations, but is initially focused on three key science projects. These are detection and characterization of 3-dimensional brightness temperature fluctuations in the 21cm line of neutral hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10, solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources,and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broad-band active dipoles, arranged into 512 tiles comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5km in diameter, with a small number of outliers extending to 3km. All tile-tile baselines are correlated in custom FPGA-based hardware, yielding a Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point spread function (PSF) quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment,allowing full exploitation of the instrumental capabilities., Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings of the IEEE

A soft ultraviolet radiation field, 10.2 eV < E <13.6 eV, that permeates neutral intergalactic gas during the Epoch of Reionization (EoR) excites the 2p (directly) and 2s (indirectly) states of atomic hydrogen. Because the 2s state is metastable, the lifetime of atoms in this level is relatively long, which may cause the 2s state to be overpopulated relative to the 2p state. It has recently been proposed that for this reason, neutral intergalactic atomic hydrogen gas may be detected in absorption in its 3-cm fine-structure line (2s_1/2 -> 2p_3/2) against the Cosmic Microwave Background out to very high redshifts. In particular, the optical depth in the fine-structure line through neutral intergalactic gas surrounding bright quasars during the EoR may reach tau~1e-5. The resulting surface brightness temperature of tens of micro K (in absorption) may be detectable with existing radio telescopes. Motivated by this exciting proposal, we perform a detailed analysis of the transfer of Lyman beta,gamma,delta,... radiation, and re-analyze the detectability of the fine-structure line in neutral intergalactic gas surrounding high-redshift quasars. We find that proper radiative transfer modeling causes the fine-structure absorption signature to be reduced tremendously to tau< 1e-10. We therefore conclude that neutral intergalactic gas during the EoR cannot reveal its presence in the 3-cm fine-structure line to existing radio telescopes., Comment: 7 pages, 4 figures, MNRAS in press; v2. some typos fixed

Of 104 AGN known to exhibit H$_2$O maser emission, X-ray data that enable estimation of column densities, or lower limits, are available for 42. Contributing to this, we report analysis of new and archival X-ray data for 8 galaxies and collation of values for three more. Maser emission is indicative of large columns of cold gas, and in five of the eight new cases, maser spectra point toward origins in accretion disks viewed close-to edge-on (a.k.a. "disk-maser" systems). In these, we detect hard continuum and Fe K$\alpha$ emission with equivalent widths on the order of 1 keV, which is consistent with Compton reflection, fluorescence by cold material, and obscuring columns $\ga 10^{24}$ cm$^{-2}$. Reviewing the full sample of 42, 95% exhibit N$_{\rm H} >10^{23}$ cm$^{-2}$ and 60% exhibit N$_{\rm H} >10^{24}$ cm$^{-2}$. Half of these are now recognized to be disk masers (up from 13); in this sub-sample, which is likely to be more homogeneous vis-\'a-vis the origin of maser emission, 76% exhibit N$_{\rm H} >10^{24}$ cm$^{-2}$. The probability of a common parent distribution of columns for disk-masers and other AGN masers is <3%. Because ground-based surveys of AGN to detect new disk masers are relatively unbiased with respect to X-ray brightness and comparatively inexpensive, they may also be efficient guides for the sensitive pointed X-ray observations required to identify Compton-thick objects outside of shallow surveys., Comment: 14 pages, 3 tables and 2 figures. Accepted for publication by ApJL

We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC3393. The maser emission has a linear distribution oriented at a position angle of $\sim -34\degr$, perpendicular to both the kpc-scale radio jet and the axis of the narrow line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical center, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of $(3.1\pm 0.2) \times 10^7 M_{\sun}$ within $0.36\pm 0.02$ pc ($1.48\pm 0.06$ mas), which corresponds to a mean mass density of $\sim10^{8.2} M_{\sun}$ pc$^{-3}$. We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of $5\pm 1$ km s $^{-1}$ yr$^{-1}$ in the $\sim3880$ km s$^{-1}$ maser feature, which is most likely located along the line of sight to the dynamical center of the system. From the acceleration of this feature, we estimate a disk radius of $0.17\pm 0.02$ pc, which is smaller than the inner disk radius ($0.36\pm 0.02$ pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical center evidently occurs much closer to the center than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC4258 and NGC1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc., Comment: 22 pages and 4 figures (2 of these figures contain 2 eps files) accepted by the Astrophysical Journal

We report the discovery of water maser emission in five active galactic nuclei (AGN) with the 100-m Green Bank Telescope (GBT). The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1 sigma (0.3 arcsec radio and 1.3 arcsec optical) and most likely mark the locations of the embedded central engines. The spectra of three sources, 2MASX J08362280+3327383, NGC 6264, and UGC 09618 NED02, display the characteristic spectral signature of emission from an edge-on accretion disk with maximum orbital velocity of ~700, ~800, and ~1300 km s^-1, respectively. We also present a GBT spectrum of a previously known source MRK 0034 and interpret the narrow Doppler components reported here as indirect evidence that the emission originates in an edge-on accretion disk with orbital velocity of ~500 km s^-1. We obtained a detection rate of 12 percent (5 out of 41) among Seyfert 2 and LINER systems with 10000 km s^-1 < v_sys < 15000 km s^-1. For the 30 nuclear water masers with available hard X-ray data, we report a possible relationship between unabsorbed X-ray luminosity (2-10 keV) and total isotropic water maser luminosity, L_{2-10} proportional to L_{H2O}^{0.5+-0.1}, consistent with the model proposed by Neufeld and Maloney in which X-ray irradiation and heating of molecular accretion disk gas by the central engine excites the maser emission., Comment: 16 pages, 5 tables, 3 figures, to appear in the November 10, 2006, v651n2 issue of the Astrophysical Journal

We present an analysis of polarimetric observations at 22 GHz of the water vapor masers in NGC 4258 obtained with the VLA and the GBT. We do not detect any circular polarization in the spectrum indicative of Zeeman-induced splitting of the maser lines of water, a non-paramagnetic molecule. We have improved the 1-sigma upper limit estimate on the toroidal component of the magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from 300 mG to 90 mG. We have developed a new method for the analysis of spectra with blended features and derive a 1-sigma upper limit of 30 mG on the radial component of the magnetic field at a radius of 0.14 pc. Assuming thermal and magnetic pressure balance, we estimate an upper limit on the mass accretion rate of ~10^(-3.7) M_sun/yr for a total magnetic field of less than 130 mG. We discuss the ramifications of our results on current maser models proposed to explain the observed maser emission structure and the consequences for current accretion theories. We find from our magnetic field limits that the thin-disk model and the jet-disk model are better candidates for accounting for the extremely low-luminosity nature of NGC 4258, than models that include advection-dominated accretion flows., Comment: 20 pages, including 10 figures and 2 tables. Accepted for publication in the Astrophysical Journal

We report monitoring of the 0.3-10 keV spectrum of NGC4258 with XMM over 1.5 years.We als o report reprocessing of an overlapping series of archival Chandra observations. By including earlier ASCA and SAX observations, we present a new, nine-year time series of models fit to the X-ray spectrum of NGC4258. Over the nine years, the photoelectric absorbing column (~10^23 cm^-2) did not vary detectably, except for a ~40% drop between two ASCA epochs separated by 3 years and a ~60% rise between two XMM epochs separated by just 5 months. In contrast, factor of 2-3 changes are seen in absorbed flux on the timescale of years. These are uncorrelated with changes in absorbing column and indicative of central engine variability. The most rapid change in luminosity (5-10 keV) that we detect is ~30% over 19 days. The warped disk, a known source of H2O maser emission in NGC4258, is believed to cross the line of sight to the central engine. We propose that the variations in absorbing column arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius where the disk crosses the line of sight. We estimate that the inhomogeneities are ~10^15 cm in size at the crossing radius of 0.29 pc, slightly smaller than the expected scale height of the disk. This result thus provides strong evidence that the warped accretion disk is the absorber. This is the first direct confirmation that obscuration in type-2 AGN may, in some cases, arise in thin, warped accretion disks, rather than in geometrically thick tori. We do not detect Fe Kalpha line emission in any of our XMM spectra. We do not observe evidence of absorption lines in the XMM or reprocessed Chandra data., Comment: 36 pages,14 figures. Accepted for publication in ApJ

We have mapped, for the first time, the full velocity extent of the water maser emission in NGC 3079. The largely north-south distribution of emission, aligned with a kpc-scale molecular disk, and the segregation of blue- and red-shifted emission on the sky are suggestive of a nearly edge-on molecular disk on pc-scales. Positions and line-of-sight velocities of blue- and red-shifted maser emission are consistent with a central mass of ~2 x 10^6 Msun enclosed within a radius of ~0.4 pc. The corresponding mean mass density of 10^6.8 Msun pc^-3 is suggestive of a central black hole, which is consistent with the detection of hard X-ray excess (20-100 keV) and an Fe Kalpha line from the nucleus. Because the rotation curve traced by the maser emission is flat, the mass of the pc-scale disk is significant with respect to the central mass. Since the velocity dispersion of the maser features does not decrease with radius and constitutes a large fraction of the orbital velocity, the disk is probably thick and flared. The rotation curve and the physical conditions necessary to support maser emission imply a Toomre Q-parameter that is << 1. Thus, the disk is most likely clumpy, and we argue that it is probably forming stars. Overall, the accretion disk in NGC 3079 stands in contrast to the compact, thin, warped, differentially rotating disk in the archetypal maser galaxy NGC 4258 (abridged)., Comment: 41 pages, 13 figures, to appear in the 2005 January 10 issue of the Astrophysical Journal. High resolution versions of the figures and of the paper are available at http://cfa-www.harvard.edu/~pkondratko/publications/NGC3079/

We report the discovery of two binary millisecond pulsars in the core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20 cm. PSR J2140-2310A (M30A) is an eclipsing 11-ms pulsar in a 4-hr circular orbit and PSR J2140-23B (M30B) is a 13-ms pulsar in an as yet undetermined but most likely highly eccentric (e>0.5) and relativistic orbit. Timing observations of M30A with a 20-month baseline have provided precise determinations of the pulsar's position (within 4" of the optical centroid of the cluster), and spin and orbital parameters, which constrain the mass of the companion star to be m_2 >~ 0.1Msun. The position of M30A is coincident with a possible thermal X-ray point source found in archival Chandra data which is most likely due to emission from hot polar caps on the neutron star. In addition, there is a faint (V_555 ~ 23.8) star visible in archival HST F555W data that may be the companion to the pulsar. Eclipses of the pulsed radio emission from M30A by the ionized wind from the compact companion star show a frequency dependent duration (\propto\nu^{-\alpha} with \alpha ~ 0.4-0.5) and delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms. Future observations of M30 may allow both the measurement of post-Keplerian orbital parameters from M30B and the detection of new pulsars due to the effects of strong diffractive scintillation., Comment: 10 pages, 6 figures, Submitted to ApJ. This version includes many recommended modifications, an improved structure, a new author, and a completely redone optical analysis

We analyze recent simultaneous Chandra/RXTE observations of the Seyfert 2 galaxy NGC 4945. The unprecedented spatial resolution of Chandra means we are able to separate the spectra of the nucleus, starburst and superwind regions, while the RXTE data extend the spectrum to higher energies. The extreme absorbing column of $N_H\sim 4\times 10^{24}$ cm$^{-2}$ means that the nucleus is only seen directly above 8--10 keV, while the lower energy spectrum from the nuclear region in Chandra is dominated by reflection. By contrast, the superwind is dominated by emission from hot plasma, but the starburst region contains both hot plasma and reflection signatures. To form a reflected spectrum requires that the starburst region contains clumps of cool, optically thick material, perhaps star forming cores, which are irradiated by 7--10 keV photons from the nucleus. Since photons of this energy are obscured along the line sight then this confirms the result of Madejski et al. (2000) that the extreme absorbtion material is disk-like rather than a torus. However, the IR/optical limits on the lack of high excitation emission lines show that by contrast the lower energy photons from the nucleus are obscured in all directions. We discuss the complex absorption structure revealed by these observations, and propose an an overall source geometry in which the nucleus is completely embedded in material with $N_H\sim 10^{23}$ cm$^{-2}$, Comment: Accepted in ApJ

We report the detection of a new 3.06 ms binary pulsar in the globular cluster NGC6544 using a Fourier-domain ``acceleration'' search. With an implied companion mass of ~0.01 solar masses and an orbital period of only P_b~1.7 hours, it displays very similar orbital properties to many pulsars which are eclipsed by their companion winds. The orbital period is the second shortest of known binary pulsars after 47 Tuc R. The measured flux density of 1.3 +/- 0.4 mJy at 1332 MHz indicates that the pulsar is almost certainly the known steep-spectrum point source near the core of NGC6544., Comment: Accepted by ApJ Letters on 11 October 2000, 5 pages

We have conducted VLBI observations of water maser and radio continuum emission in the nucleus of NGC 3079. The 22 GHz maser emission arises in compact clumps, distributed along an axis that is aligned with the major axis of the galactic disk. The velocities of the masers are consistent with their lying in the inner parsec of a molecular disk rotating in the same sense as the rest of the galaxy. However, the velocity field has a significant non-rotational component, which may indicate supersonic turbulence. The bright maser emission is not coincident with any detected compact 22 GHz continuum source, suggesting the high apparent luminosity of the maser may not due to beamed amplification of continuum emission. We observed two compact continuum sources that have inverted spectra between 5 and 8 GHz, and steep spectra between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We detected a third continuum component that lies along the same axis as the other two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser emission was detected near this axis, which may indicate a second population of masers associated with the jet., Comment: 31 pages, includes 8 figures. To appear in ApJ, Vol 495, 10 March 1998. Full-resolution figures and color plate available at http://cfa-www.harvard.edu/~trotter/ngc3079.html

The XMM target for this program is ostensibly a "normal" galaxy, but the presence of water maser emission indicated that it may be an obscured AGN. Our primary goal is to test this hypothesis; detection hard X-ray emission and a reflection-dominated spectrum would indicate an AGN is present. Demonstration that the local universe contains obscured AGN is important to constraining models of the hard cosmic X-ray background, as is identification of efficient methods to locate them (e.g., ground-based detection of maser emission at microwave frequencies).

The goal of this project was detection of variability in the X-ray absorption column of the AGN in NGC4258 through monitoring with the XMM satellite. We have accomplished this goal and submitted the results to ApJ for publication in a paper entitled, "X-ray Luminosity and Absorption Column Fluctuations in the H2O Maser Galaxy NGC4258 from Weeks to Years," by Fruscione, A., Greenhill, L.J., Filippedco, A.V., Moran, J.M., Hermstein, J.R., and Galle, E. We have received a favorable referee report and expect the article will appear in 2005. To complete the project, we reduced our four epochs of XMM data for NGC4258, one archival XMM observation, and all existing Chandra datasets for NGC4258 (with the latest calibration tables and a grid of corrections for pileup). Self-consistent reduction of all these data permitted detailed comparison that could not have been accomplished simply by taking published model fits that appear in the literature. To accumulate a broader monitoring record, we combined the Chandra and XMM results with those published for SAX and ASCA. We modeled the Chandra and XMM data self-consistently with partially absorbed, hard power-law, soft thermal plasmas, and soft power-law components. Over nine years, the photo-electric absorbing column exhibited a 40% drop between two ASCA epochs separated by 3 years and a 60% rise between two XMM epochs separated by just 5 months. In contract, uncorrelated factor of of 2-3 changes were seen in absorbed flux on te timescale of years, which suggests intrinsic variability of the central engine. The warped disk that is a known source of H2O maser emission in 4258 is believed to cross the line of sight to the central engine. We have proposed that the variations in absorbing column arise from inhomogeneities in the rotating disk, as they sweep across the line of sight. We estimate from the XMM data that the inhomogeneities are about 1E+15 cm in size at radii greater than 0.27 pc. This is consistent with the estimated radius of the disk crossing estimated entirely independently, and it solidifies evidence that the warped accretion disk is the absorber in this (and possible other) AGN.

The target galactic nucleus is ostensibly "normal," but the presence of water maser emission indicates that it may be an obscured AGN. Our primary goal has been to test this hypothesis through the detection hard X-ray emission and an X-ray spectrum characteristic of heavy absorption. The data for this program became available in May 2003. Light curves show that most of the data are good and that the background was well behaved. We have constructed images and found the X-ray emission in this target is a hard and may be extended. The presence of an AGN has been confirmed through observation with XMM. This is one of many edge-on normal galaxies that may contain AGN. The orientation of the extension will be compared to the structure of molecular gas observed at high angular resolution with VLBI. These radio observations were completed in August 2003. Correlation was completed in late 2003. The data have yet to be delivered by the ATNF. Since the start of the program, we have also obtained six radio spectra, on top of the two obtained at or close to the epoch of discovery for the maser. Early indications were that the maser emission exhibited unusually broad velocity extent and that it "jumped in velocity on time scales of weeks. This behavior has been speculated to be the signature of emission excited by jet-ISM interactions. After more extensive and more sensitive radio study, we find that (1) the emission does not in fact change velocity centroid substantially and (2) it comprises narrower, more "normal" emission features. The joint interpretation of radio and XMM data will be possible once the VLBI images and position-resolved X-ray spectra have been made, probably in summer 2004. If the radio emission arises in an accretion disk, rather than a jet, then we may predict that the VLBI images will exhibit velocity-position gradients orthogonal to the extension of X-ray emission, which presumably traces an optical narrow line or outflow region.

The goal of our program is to characterize time variation of X-ray emission from the AGN in NGC4258 and to look for correlations with radio emission (maser line and continuum). Characterization of the X-ray properties includes estimation of absorbing column, flux, and Fe K emission line strength. In the past year we have completed reduction of five epochs of XMM data and 42 epochs of radio spectroscopic and imaging data. We obtained four of the X-ray epochs as part of our own program. The fifth epoch is public and was obtained by a GTO program. In addition to the XMM data, we have retrieved and reduced four epochs of data from the CXO archive. The radio data had been obtained with the VLA, VLBA, and Effelsberg lOOm antenna. Taken together, the X-ray and radio data are a unique time monitoring database. We have found that the Fe line faded over the period mid-1999 to mid-2000 and is now almost undetectable by XMM. In this same period, the radio emission showed no unusual activity. We infer that the line emission arises either in a time varying disk corona much less than 1 pc in size or from a portion of the disk well within the inner edge of the molecular disk responsible for the radio line emission. We have found that variations in X-ray absorbing column, source flux, and radio emission are not correlated, from which we argue that the molecular disk is either not clumpy or that it has a very low areal filling factor since it is believed to cross the line of sight to the central engine at a radius of about 0.3 pc. In the past month, the public CXO data we have analyzed has been published by another group, which claims the provocative discovery of time variable Fe K absorption lines at one epoch. We do not see these features in our reduction of the same data or in our own XMM data. This will be emphasized in our publication.

We present new images of the Orion BN/KL infrared complex at 4.8, 8.0, 8.9,9.9, 10.4, 11.7, 12.5, 17.6, 18.1,20.0 and 22.0 um obtained with the 10-meter Keck I telescope, with typically 0.3 arcsec resolution at 12.5 um. The multi-wavelength observational image data is registered in a stack and a dust emission/extinction model is fitted to the resulting spectrum of each pixel to create a diffraction-limited "image" of the temperature, opacity and luminosity of the emitting dust, as well as the circumstellar and line-of-sight dust extinction. New source structure, temperature, opacity and luminosity detail is seen in the vicinity of IRc2-IRc7. The model results are used to develop a more complete picture of the structure and energetics of the BN/KL, infrared complex.

The goal of this project is to better understand whether the well known, warped accretion disk around the supermassive black hole in NGC4258 is responsible for the observed X-ray absorbing column of gas. The active galactic nuclei (AGN) paradigm suggests that doughnut-like tori obscure black holes, not warped disks. Using radio interferometry, we have mapped the disk and its orientation. Through time monitoring radio observations we are following the rotation and variability in the disk structure. The disk crosses the line of sight to the black hole, if it is clumpy, then we will observe variations in the X-ray spectrum of NGC4258 on month to year time scales. We have reduced 14 epochs of very long base interferometry (VLBI) observations that we use to characterize the structure of the maser-laden accretion disk and its evolution. Five epochs of data remain to be reduced in the next year. We have received distributions of data for two of our four intended epochs of X-ray Multimirror Mission (XMM) observation. Because the first two epochs are relatively close in time, and because community understanding of how to calibrate XMM data has until recently been relatively immature, we are waiting until we receive the third epoch of data before we reduce the several datasets. Delivery of the third epoch should take place late this year. The last epoch of XMM observation will take place in mid-2002. In the next year, we will begin to combine constraints on disk clumpiness with estimates of X-ray variability observed by XMM.