Search

We report the results of careful astrometric measurements of the Cannonball pulsar J0002+6216 carried out over three years using the High Sensitivity Array (HSA). We significantly refine the proper motion to $\mu=35.3\pm0.6$ mas yr$^{-1}$ and place new constraints on the distance, with the overall effect of lowering the velocity and increasing the inferred age to $47.60\pm0.80$ kyr. Although the pulsar is brought more in line with the standard natal kick distribution, this new velocity has implications for the morphology of the pulsar wind nebula that surrounds it, the density of the interstellar medium through which it travels, and the age of the supernova remnant (CTB 1) from which it originates., Comment: 12 pages, 5 figures, 4 tables

From an on-going survey of the Galactic bulge, we have discovered a number of compact, steep spectrum radio sources. In this present study we have carried out more detailed observations for two of these sources, located 43 arcmin and 12.7 deg from the Galactic Center. Both sources have a very steep spectrum (alpha ~ -3) and are compact, with upper limits on the angular size of 1-2 arcsec. Their flux densities appear to be relatively steady on timescales of years, months, and hours, with no indications of rapid variability or transient behavior. We detect significant circularly polarized emission from both sources, but only weak or upper limits on linear polarization. Neither source has a counterpart at other wavelengths and deep, high-frequency searches fail to find pulsations. We compare their source properties with other known compact, non-thermal source populations in the bulge (e.g. X-ray binaries, magnetars, the Burper, cataclysmic variables). Our existing data support the hypothesis that they are scatter broadened millisecond or recycled pulsars, either at the bulge or along the line of sight. We also consider the possibility that they may be a new population of Galactic radio sources which share similar properties as pulsars but lack pulsations; a hypothesis that can be tested by future large-scale synoptic surveys., Comment: 12 pages, 4 figures. Submitted to MNRAS

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

We present the discovery of a nuclear transient with the Caltech-NRAO Stripe 82 Survey (CNSS), a dedicated radio transient survey carried out with the Karl G. Jansky Very Large Array (VLA). This transient, CNSS J001947.3+003527, exhibited a turn-on over a timescale of $\lesssim$1 yr, increasing in flux density at 3 GHz from $<0.14$ mJy in 2014 February to $4.4\pm0.1$ mJy in 2015 March, reaching a peak luminosity of $5\times10^{28} \text{erg s}^{-1} \text{Hz}^{-1}$ around 2015 October. The association of CNSS J0019+00 with the nucleus (Gaia and our VLBI positions are consistent to within 1 pc) of a nearby S0 Seyfert galaxy at 77 Mpc, together with the radio spectral evolution, implies that this transient is most likely a tidal disruption event (TDE). Our equipartition analysis indicates the presence of a $\sim$15,000 km s$^{-1}$ outflow, having energy $\sim$10$^{49}$ erg. We derive the radial density profile for the circumnuclear material in the host galaxy to be proportional to $R^{-2.5}$. All of these properties suggest resemblance with radio-detected thermal TDEs like ASASSN-14li and XMMSL1 J0740-85. No significant X-ray or optical emission is detected from CNSS J0019+00, although this may simply be due to the thermal emission being weak during our late-time follow-up observations. From the CNSS survey we have obtained the first unbiased measurement of the rate of radio TDEs, $R(>500 \mu{\rm Jy})$ of about $2\times10^{-3}$ deg$^{-2}$, or equivalently a volumetric rate of about 10 Gpc$^{-3}$ yr$^{-1}$. This rate implies that all-sky radio surveys such as the VLA Sky Survey and those planned with ASKAP, will find many tens of radio TDEs over the next few years., Comment: 18 pages, 7 figures, ApJ accepted

We have carried out a dedicated transient survey of 300 deg$^2$ of the SDSS Stripe 82 region using the Giant Meterwavelength Radio Telescope (GMRT) at 150 MHz. Our multi-epoch observations, together with the TGSS survey, allow us to probe variability and transient activity on four different timescales, beginning with 4 hours, and up to 4 years. Data calibration, RFI flagging, source finding and transient search were carried out in a semi-automated pipeline incorporating the SPAM recipe. This has enabled us to produce superior-quality images and carry out reliable transient search over the entire survey region in under 48 hours post-observation. Among the few thousand unique point sources found in our 5$\sigma$ single-epoch catalogs (flux density thresholds of about 24 mJy, 20 mJy, 16 mJy and 18 mJy on the respective timescales), we find $<$0.08%, 0.01%, $<$0.06% and 0.05% to be variable (beyond a significance of 4$\sigma$ and fractional variability of 30%) on timescales of 4 hours, 1 day, 1 month and 4 years respectively. This is substantially lower than that in the GHz sky, where $\sim$1% of the persistent point sources are found to be variable. Although our survey was designed to probe a superior part of the transient phase space, our transient sources did not yield any significant candidates. The transient (preferentially extragalactic) rate at 150 MHz is therefore $<$0.005 on timescales of 1 month and 4 years, and $<$0.002 on timescales of 1 day and 4 hours, beyond 7$\sigma$ detection threshold. We put these results in the perspective with the previous studies and give recommendations for future low-frequency transient surveys., Comment: 9 pages, 7 figures. Published in MNRAS; doi: 10.1093/mnras/stz2918

We have carried out VLA imaging and a Fermi timing analysis of the 115 ms gamma-ray and radio pulsar PSR J0002+6216. We found that the pulsar lies at the apex of a narrowly collimated cometary-like 7 arcmin tail of non-thermal radio emission which we identify as a bow-shock pulsar wind nebula. The tail of the nebula points back toward the geometric center of the supernova remnant CTB 1 (G116.9+0.2) 28 arcmin away, at a position angle $\theta_\mu=113^\circ$. We measure a proper motion with 2.9$\sigma$ significance from a Fermi timing analysis giving $\mu$=115$\pm$33 mas yr$^{-1}$ and $\theta_\mu=121^\circ\pm{13}^\circ$, corresponding to a large transverse pulsar velocity of 1100 km s$^{-1}$ at a distance of 2 kpc. This proper motion is of the right magnitude and direction to support the claim that PSR J0002+6216 was born from the same supernova that produced CTB 1. We explore the implications for pulsar birth periods, asymmetric supernova explosions, and mechanisms for pulsar natal kick velocities., Comment: 14 pages, 4 figures, accepted for publication in ApJL

As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations., Comment: submitted to Astro2020

Telescope slew and settle time markedly reduces the efficiency of wide-field multi-epoch surveys for sensitive interferometers with small fields of view. The overheads can be mitigated through the use of On-the-Fly Mosaicing (OTFM), where the the antennas are driven at a non-sidereal rate and visibilities are recorded continuously. Here we introduce the OTFM technique for the VLA, and describe its implementation for the Caltech-NRAO Stripe 82 Survey (CNSS), a dedicated 5-epoch survey for slow transients at S band (2-4 GHz). We also describe the OTFSim tool for planning dynamically-scheduled OTFM observations on the VLA, the latest imaging capabilities for OTFM in CASA, and present a comparison of OTFM observations with pointed observations. Using the subset of our observations from the CNSS pilot and final surveys, we demonstrate that the wide-band and wide-field OTFM observations with the VLA can be imaged accurately, and that this technique offers a more efficient alternative to standard mosaicing for multi-epoch shallow surveys such as the CNSS and the VLA Sky Survey (VLASS). We envisage that the new OTFM mode will facilitate new synoptic surveys and high-frequency mapping experiments on the VLA., Comment: 15 pages, 14 figures, Accepted for publication in ApJ

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

The binary neutron star merger GW170817 was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 41+/-3 Mpc. The radio and X-ray afterglows of GW170817 exhibited delayed onset, a gradual rise in the emission with time as t^0.8, a peak at about 150 days post-merger, followed by a relatively rapid decline. To date, various models have been proposed to explain the afterglow emission, including a choked-jet cocoon and a successful-jet cocoon (a.k.a. structured jet). However, the observational data have remained inconclusive as to whether GW170817 launched a successful relativistic jet. Here we show, through Very Long Baseline Interferometry, that the compact radio source associated with GW170817 exhibits superluminal motion between two epochs at 75 and 230 days post-merger. This measurement breaks the degeneracy between the models and indicates that, while the early-time radio emission was powered by a wider-angle outflow (cocoon), the late-time emission was most likely dominated by an energetic and narrowly-collimated jet, with an opening angle of <5 degrees, and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the growing evidence linking binary neutron star mergers and short gamma-ray bursts., Comment: 42 pages, 4 figures (main text), 2 figures (supplementary text), 2 tables. Referee and editor comments incorporated

We apply a newly-developed On-the-Fly mosaicing technique on the NSF's Karl G. Jansky Very Large Array (VLA) at 3 GHz in order to carry out a sensitive search for an afterglow from the Advanced LIGO binary black hole merger event GW 151226. In three epochs between 1.5 and 6 months post-merger we observed a 100 sq. deg region, with more than 80% of the survey region having a RMS sensitivity of better than 150 uJy/beam, in the northern hemisphere having a merger containment probability of 10%. The data were processed in near-real-time, and analyzed to search for transients and variables. No transients were found but we have demonstrated the ability to conduct blind searches in a time-frequency phase space where the predicted afterglow signals are strongest. If the gravitational wave event is contained within our survey region, the upper limit on any late-time radio afterglow from the merger event at an assumed mean distance of 440 Mpc is about 1e29 erg/s/Hz. Approximately 1.5% of the radio sources in the field showed variability at a level of 30%, and can be attributed to normal activity from active galactic nuclei. The low rate of false positives in the radio sky suggests that wide-field imaging searches at a few Gigahertz can be an efficient and competitive search strategy. We discuss our search method in the context of the recent afterglow detection from GW 170817 and radio follow-up in future gravitational wave observing runs., Comment: 11 pages. 6 figures. 1 table. Accepted for publication in ApJ Letters

We present continued radio follow-up observations of PTF11qcj, a highly energetic broad-lined Type Ic supernova (SN), with a radio peak luminosity comparable to that of the $\gamma$-ray burst (GRB) associated SN 1998bw. The latest observations, carried out with the Karl G. Jansky Very Large Array (VLA), extend up to $\sim$5 years after the PTF11qcj optical discovery. The radio light curve shows a double-peak profile, possibly associated with density variations in the circumstellar medium (CSM), or with the presence of an off-axis GRB jet. Optical spectra of PTF11qcj taken during both peaks of the radio light curve do not show the broad H$\alpha$ features typically expected from H-rich circumstellar interaction. Modeling of the second radio peak within the CSM interaction scenario requires a flatter density profile and an enhanced progenitor mass-loss rate compared to those required to model the first peak. Although our radio data alone cannot rule out the alternative scenario of an off-axis GRB powering the second radio peak, the implied off-axis GRB parameters are unusual compared to typical values found for cosmological long GRBs. Deep X-ray observations carried out around the time of the second radio peak could have helped distinguish between the density variation and off-axis GRB scenarios. Future VLBA measurements of the PTF11qcj radio ejecta may unambiguously rule out the off-axis GRB jet scenario., Comment: 13 pages, 6 figures

We describe an image-based method that uses two radio criteria, compactness and spectral index, to identify promising pulsar candidates among Fermi Large Area Telescope (LAT) unassociated sources. These criteria are applied to those radio sources from the Giant Metrewave Radio Telescope all-sky survey at 150 MHz (TGSS ADR1) found within the error ellipses of unassociated sources from the 3FGL catalog and a preliminary source list based on 7 years of LAT data. After follow-up interferometric observations to identify extended or variable sources, a list of 16 compact, steep-spectrum candidates is generated. An ongoing search for pulsations in these candidates, in gamma rays and radio, has found six millisecond pulsars and one normal pulsar. A comparison of this method with existing selection criteria based on gamma-ray spectral and variability properties suggests that the pulsar discovery space using Fermi may be larger than previously thought. Radio imaging is a hitherto underutilized source selection method that can be used, as with other multi-wavelength techniques, in the search for Fermi pulsars., Comment: MNRAS, in press (13 pages, 12 figures, 4 tables)

The radio spectral index is a powerful probe for classifying cosmic radio sources and understanding the origin of the radio emission. Combining data at 147 MHz and 1.4 GHz from the TIFR GMRT Sky Survey (TGSS) and the NRAO VLA Sky Survey (NVSS), we produced a large-area radio spectral index map of ~80 per cent of the sky (Dec > -40 deg), as well as a radio spectral index catalogue containing 1,396,515 sources, of which 503,647 are not upper or lower limits. Almost every TGSS source has a detected counterpart, while this is true only for 36 per cent of NVSS sources. We released both the map and the catalogue to the astronomical community. The catalogue is analysed to discover systematic behaviours in the cosmic radio population. We find a differential spectral behaviour between faint and bright sources as well as between compact and extended sources. These trends are explained in terms of radio galaxy evolution. We also confirm earlier reports of an excess of steep-spectrum sources along the galactic plane. This corresponds to 86 compact and steep-spectrum source in excess compared to expectations. The properties of this excess are consistent with normal non-recycled pulsars, which may have been missed by pulsation searches due to larger than average scattering along the line of sight., Comment: 15 pages, 15 figures, accepted MNRAS

GW170817 is the first gravitational wave detection of a binary neutron star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 Mpc. It has been proposed that the observed gamma-ray, X-ray and radio emission is due to an ultra-relativistic jet launched during the merger, directed away from our line of sight. The presence of such a jet is predicted from models positing neutron star mergers as the central engines driving short-hard gamma-ray bursts (SGRBs). Here we show that the radio light curve of GW170817 has no direct signature of an off-axis jet afterglow. While we cannot rule out the existence of a jet pointing elsewhere, the observed gamma-rays could not have originated from such a jet. Instead, the radio data requires a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high velocity tail of the neutron-rich material dynamically ejected during the merger or a cocoon of material that breaks out when a jet transfers its energy to the dynamical ejecta. The cocoon scenario can explain the radio light curve of GW170817 as well as the gamma-rays and X-rays (possibly also ultraviolet and optical emission), and hence is the model most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron star mergers, giving rise to a heretofore unidentified population of radio, ultraviolet, X-ray and gamma-ray transients in the local universe., Comment: 41 pages, 4 figures (main text), 4 figures (supplementary text), 1 table. This revised version incorporates comments from the referees

We present the results of a community study aimed at exploring some of the scientific opportunities that the next generation Very Large Array (ngVLA) could open in the field of multi-messenger time-domain astronomy. We focus on compact binary mergers, golden astrophysical targets of ground-based gravitational wave (GW) detectors such as advanced LIGO. A decade from now, a large number of these mergers is likely to be discovered by a world-wide network of GW detectors. This will enable the identification of host galaxies, either directly through detection of electromagnetic (EM) counterparts, or indirectly by probing potential anisotropies in the spatial distribution of mergers. Identifying the host galaxy population of GW mergers would provide a way to constrain the efficiency of various binary neutron star (NS) or black hole (BH) formation scenarios, and the merger timescale distributions as linked to merger rates in early- and late-type galaxies. We discuss how a radio array with ~10x the sensitivity of the current Karl G. Jansky VLA and ~10x the resolution, would enable resolved radio continuum studies of binary merger hosts, probing regions of the galaxy undergoing star formation (which can be heavily obscured by dust and gas), AGN components, and mapping the offset distribution of the mergers with respect to the host galaxy light. For compact binary mergers containing at least one NS and with associated EM counterparts, we show how the ngVLA would enable direct size measurements of the relativistic merger ejecta and probe, for the first time directly, their dynamics., Comment: 8 pages, 5 figures, 1 Table, published as an ngVLA memo at https://library.nrao.edu/ngvla.shtml

Merging neutron stars offer an exquisite laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart EM170817 to gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic dataset, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma-rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultra-relativistic jets. Instead, we suggest that breakout of a wide-angle, mildly-relativistic cocoon engulfing the jet elegantly explains the low-luminosity gamma-rays, the high-luminosity ultraviolet-optical-infrared and the delayed radio/X-ray emission. We posit that all merging neutron stars may lead to a wide-angle cocoon breakout; sometimes accompanied by a successful jet and sometimes a choked jet., Comment: Science, in press DOI 10.1126/science.aap9455, 83 pages, 3 tables, 16 figures

Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultra-relativistic jet viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will distinguish between these models and very long baseline interferometry will have the capability to directly measure the angular velocity and geometry of the debris., Comment: Science, in press, 26 pages, DOI:10.1126/science.aap9855

We present the discovery, classification, and radio-to-X-ray follow-up observations of iPTF17cw, a broad-lined (BL) type Ic supernova (SN) discovered by the intermediate Palomar Transient Factory (iPTF). Although unrelated to the gravitational wave trigger, this SN was discovered as a happy by-product of the extensive observational campaign dedicated to the follow-up of Advanced LIGO event GW170104. The spectroscopic properties and inferred peak bolometric luminosity of iPTF17cw are most similar to the gamma-ray burst (GRB) associated SN 1998bw, while the shape of the r-band light curve is most similar to that of the relativistic SN 2009bb. Karl G. Jansky Very Large Array (VLA) observations of the iPTF17cw field reveal a radio counterpart about 10 times less luminous than SN 1998bw, and with peak radio luminosity comparable to that of SN 2006aj/GRB 060218 and SN 2010bh/GRB 100316D. Our radio observations of iPTF17cw imply a relativistically expanding outflow. However, further late-time observations with the VLA in its most extended configuration are needed to confirm fading of iPTF radio counterpart at all frequencies. X-ray observations carried out with Chandra reveal the presence of an X-ray counterpart with luminosity similar to that of SN 2010bh/GRB 100316D. Searching the Fermi catalog for possible gamma-rays reveals that GRB 161228B is spatially and temporally compatible with iPTF17cw. The similarity to SN 1998bw and SN 2009bb, the radio and X-ray detections, and the potential association with GRB 161228B, all point to iPTF17cw being a new candidate member of the rare sample of optically-discovered engine-driven BL-Ic SNe associated with relativistic ejecta., Comment: 13 pages, 9 figures, submitted to ApJ

We present multi-wavelength follow-up campaigns by the AstroSat-CZTI and GROWTH collaborations to search for an electromagnetic counterpart to the gravitational wave event GW170104. At the time of the GW170104 trigger, the AstroSat CZTI field-of-view covered 50.3\% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of $\approx 4.5 \times 10^{-7}~{\rm erg~cm}^{-2}{\rm~s}^{-1}$ for a 1\,s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB~170105A, with only a fortuitous spatial coincidence with GW170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models., Comment: ApJ accepted - updated to match version, 14 pages, 8 figures, 3 tables

We present the VLA-COSMOS 3 GHz Large Project based on 384 hours of observations with the Karl G. Jansky Very Large Array (VLA) at 3 GHz (10 cm) toward the two square degree Cosmic Evolution Survey (COSMOS) field. The final mosaic reaches a median rms of 2.3 uJy/beam over the two square degrees at an angular resolution of 0.75". To fully account for the spectral shape and resolution variations across the broad (2 GHz) band, we image all data with a multiscale, multifrequency synthesis algorithm. We present a catalog of 10,830 radio sources down to 5 sigma, out of which 67 are combined from multiple components. Comparing the positions of our 3 GHz sources with those from the Very Long Baseline Array (VLBA)-COSMOS survey, we estimate that the astrometry is accurate to 0.01" at the bright end (signal-to-noise ratio, S/N_3GHz > 20). Survival analysis on our data combined with the VLA-COSMOS 1.4~GHz Joint Project catalog yields an expected median radio spectral index of alpha=-0.7. We compute completeness corrections via Monte Carlo simulations to derive the corrected 3 GHz source counts. Our counts are in agreement with previously derived 3 GHz counts based on single-pointing (0.087 square degrees) VLA data. In summary, the VLA-COSMOS 3 GHz Large Project simultaneously provides the largest and deepest radio continuum survey at high (0.75") angular resolution to date, bridging the gap between last-generation and next-generation surveys., Comment: 19 pages, 18 figures, 5 tables, accepted for publication in A&A

The Fermi Large Area Telescope has detected an extended region of GeV emission toward the Galactic Center that is currently thought to be powered by dark matter annihilation or a population of young and/or millisecond pulsars. In a test of the pulsar hypothesis, we have carried out an initial search of a 20 deg**2 area centered on the peak of the galactic center GeV excess. Candidate pulsars were identified as a compact, steep spectrum continuum radio source on interferometric images and followed with targeted single-dish pulsation searches. We report the discovery of the recycled pulsar PSR 1751-2737 with a spin period of 2.23 ms. PSR 1751-2737 appears to be an isolated recycled pulsar located within the disk of our Galaxy, and it is not part of the putative bulge population of pulsars that are thought to be responsible for the excess GeV emission. However, our initial success in this small pilot survey suggests that this hybrid method (i.e. wide-field interferometric imaging followed up with single dish pulsation searches) may be an efficient alternative strategy for testing whether a putative bulge population of pulsars is responsible for the GeV excess., Comment: MNRAS, in press

We present radio follow-up observations carried out with the Karl G. Jansky Very Large Array during the first observing run (O1) of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). A total of three gravitational wave triggers were followed up during the ~4 months of O1, from September 2015 to January 2016. Two of these triggers, GW150914 and GW151226, are binary black hole merger events of high significance. A third trigger, G194575, was subsequently declared as an event of no interest (i.e., a false alarm). Our observations targeted selected optical transients identified by the intermediate Palomar Transient Factory (iPTF) in the Advanced LIGO error regions of the three triggers, and a limited region of the gravitational wave localization area of G194575 not accessible to optical telescopes due to Sun constraints, where a possible high-energy transient was identified. No plausible radio counterparts to GW150914 and GW151226 were found, in agreement with expectations for binary black hole mergers. We show that combining optical and radio observations is key to identifying contaminating radio sources that may be found in the follow-up of gravitational wave triggers, such as emission associated to star formation and AGN. We discuss our results in the context of the theoretical predictions for radio counterparts to gravitational wave transients, and describe our future plans for the radio follow-up of Advanced LIGO (and Virgo) triggers., Comment: 13 pages, 6 figures, accepted for publication in ApJ Letters. This version addresses Referee's comments

We report on a search for steep spectrum radio sources within the 95% confidence error ellipses of the Fermi unassociated sources from the Large Array Telescope (LAT). Using existing catalogs and the newly released GMRT all-sky survey at 150 MHz we identify compact radio sources that are bright at MHz frequencies but faint or absent at GHz frequencies. Such steep spectrum radio sources are rare and constitute a sample of pulsar candidates, selected independently of period, dispersion measure, interstellar scattering and orbital parameters. We find point-like, steep spectrum candidates toward 11 Fermi sources. Based on the gamma-ray/radio positional coincidence, the rarity of such radio sources, and the properties of the 3FGL sources themselves, we argue that many of these sources could be pulsars. They may have been missed by previous radio periodicity searches due to interstellar propagation effects or because they lie in an unusually tight binary. If this hypothesis is correct, then renewed gamma-ray and radio periodicity searches at the positions of the steep spectrum radio sources may reveal pulsations., Comment: MNRAS, in press

We have used the 150 MHz radio continuum survey (TGSS ADR) from the Giant Metrewave Radio Telescope (GMRT) to search for phase-averaged emission toward all well-localized radio pulsars north of -53deg Declination. We detect emission toward 200 pulsars with high confidence (>=5-sigma) and another 88 pulsars at fainter levels. We show that most of our identifications are likely from pulsars, except for a small number where the measured flux density is confused by an associated supernova or pulsar-wind nebula, or a globular cluster. We investigate the radio properties of the 150 MHz sample and we find an unusually high number of gamma-ray binary millisecond pulsars with very steep spectral indices. We also note a discrepancy in the measured flux densities between GMRT and LOFAR pulsar samples, suggesting that the flux density scale for the LOFAR pulsar sample may be in error by approximately a factor two. We carry out a separate search of 30 well-localized gamma-ray, radio-quiet pulsars in an effort to detect a widening of the radio beam into the line-of-sight at lower frequencies. No steep spectrum emission was detected either toward individual pulsars or in a weighted stack of all 30 images., Comment: ApJ, in press

We report on a search for radio transients at 340 MHz with the Jansky Very Large Array (VLA) Low band Ionospheric and Transient Experiment (VLITE). Between 2015 July 29 and 2015 September 27, operating in commensal mode, VLITE imaged approximately 2800 pointings covering 12,000 deg$^2$ on the sky, sampling timescales ranging from tens of seconds to several hours on a daily basis. In addition, between 2015 February 25 and 2015 May 9, VLITE observed 55 epochs of roughly 2-4 hours each toward the COSMOS field. Using existing radio source catalogs we have searched all the daily VLITE images for transients, while for the COSMOS field we compared individual images and the summed image to search for new sources in repeated observations of the same field. The wide range of timescales makes VLITE sensitive to both coherent and incoherent transient source classes. No new transients are found, allowing us to set stringent upper limits on transients at milliJansky levels and at low frequencies where comparatively few such surveys have been carried out to date. An all-sky isotropic rate of bursting radio transients with similar rates, duration, and intensity as the unusual transient GCRT J1745$-$3009, discovered in wide-field monitoring toward the Galactic center, is ruled out with high confidence. The resulting non-detections allows us to argue that this is a coherent source, whose properties most resemble the growing class of nulling pulsars. We end with a discussion of the future prospects for the detection of transients by VLITE and other experiments., Comment: submitted to ApJ

A radio source that faded over six days, with a redshift of $z\approx0.5$ host, has been identified by Keane et al. (2016) as the transient afterglow to a fast radio burst (FRB 150418). We report follow-up radio and optical observations of the afterglow candidate and find a source that is consistent with an active galactic nucleus. If the afterglow candidate is nonetheless a prototypical FRB afterglow, existing slow-transient surveys limit the fraction of FRBs that produce afterglows to 0.25 for afterglows with fractional variation, $m=2|S_1-S_2|/(S_1+S_2)\geq0.7$, and 0.07 for $m\geq1$, at 95% confidence. In anticipation of a barrage of bursts expected from future FRB surveys, we provide a simple framework for statistical association of FRBs with afterglows. Our framework properly accounts for statistical uncertainties, and ensures consistency with limits set by slow-transient surveys., Comment: Accepted version (ApJL)

We present the first full release of a survey of the 150 MHz radio sky, observed with the Giant Metrewave Radio Telescope between April 2010 and March 2012 as part of the TGSS project. Aimed at producing a reliable compact source survey, our automated data reduction pipeline efficiently processed more than 2000 hours of observations with minimal human interaction. Through application of innovative techniques such as image-based flagging, direction-dependent calibration of ionospheric phase errors, correcting for systematic offsets in antenna pointing, and improving the primary beam model, we created good quality images for over 95 percent of the 5336 pointings. Our data release covers 36,900 square degrees (or 3.6 pi steradians) of the sky between -53 deg and +90 deg DEC, which is 90 percent of the total sky. The majority of pointing images have a background RMS noise below 5 mJy/beam with an approximate resolution of 25" x 25" (or 25" x 25" / cos (DEC - 19 deg) for pointings south of 19 deg DEC). We have produced a catalog of 0.62 Million radio sources derived from an initial, high reliability source extraction at the 7 sigma level. For the bulk of the survey, the measured overall astrometric accuracy is better than 2" in RA and DEC, while the flux density accuracy is estimated at ~10 percent. Within the scope of the TGSS ADR project, the source catalog as well as 5336 mosaic images (5 deg x 5 deg) and an image cutout service, are made publicly available online as a service to the astronomical community. Next to enabling a wide range of different scientific investigations, we anticipate that these survey products provide a solid reference for various new low-frequency radio aperture array telescopes (LOFAR, LWA, MWA, SKA-low), and can play an important role in characterizing the EoR foreground. The TGSS ADR project aims at continuously improving the quality of the survey data products., Comment: 29 pages, 27 figures. Accepted for publication in A&A. Data products available via http://tgssadr.strw.leidenuniv.nl/

The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. Only a small fraction of the total localized region was immediately visible in the Northern night sky, due both to sun-angle and elevation constraints. Here, we report on the transient candidates identified and rapid follow-up undertaken to determine the nature of each candidate. Even in the small area imaged of 126 sq deg, after extensive filtering, 8 candidates were deemed worthy of additional follow-up. Within two hours, all 8 were spectroscopically classified by the Keck II telescope. Curiously, even though such events are rare, one of our candidates was a superluminous supernova. We obtained radio data with the Jansky Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes. This end-to-end discovery and follow-up campaign bodes well for future searches in this post-detection era of gravitational waves., Comment: Revised

We have commenced a multi-year program, the Caltech-NRAO Stripe 82 Survey (CNSS), to search for radio transients with the Jansky VLA in the SDSS Stripe 82 region. The CNSS will deliver five epochs over the entire $\sim$270 deg$^2$ of Stripe 82, an eventual deep combined map with a rms noise of $\sim$40 $\mu$Jy and catalogs at a frequency of 3 GHz, and having a spatial resolution of 3". This first paper presents the results from an initial pilot survey of a 50 deg$^2$ region of Stripe 82, involving four epochs spanning logarithmic timescales between one week and 1.5 years, with the combined map having a median rms noise of 35 $\mu$Jy. This pilot survey enabled the development of the hardware and software for rapid data processing, as well as transient detection and follow-up, necessary for the full 270 deg$^2$ survey. Classification of variable and transient sources relied heavily on the wealth of multi-wavelength data in the Stripe 82 region, supplemented by repeated mapping of the region by the Palomar Transient Factory. $3.9^{+0.5}_{-0.9}$% of the detected point sources were found to vary by greater than 30%, consistent with similar studies at 1.4 GHz and 5 GHz. Multi-wavelength photometric data and light curves suggest that the variability is mostly due to shock-induced flaring in the jets of AGN. Although this was only a pilot survey, we detected two bona fide transients, associated with an RS CVn binary and a dKe star. Comparison with existing radio survey data revealed additional highly variable and transient sources on timescales between 5-20 years, largely associated with renewed AGN activity. The rates of such AGN possibly imply episodes of enhanced accretion and jet activity occurring once every $\sim$40,000 years in these galaxies. We compile the revised radio transient rates and make recommendations for future transient surveys and joint radio-optical experiments. (Abridged), Comment: 26 pages, 22 figures, 4 tables. Accepted for publication in the Astrophysical Journal. Data products (images, catalogs, tables, and light curves) available at http://tauceti.caltech.edu/stripe82 . A regularly-updated compilation of radio transient surveys is available at http://www.tauceti.caltech.edu/kunal/radio-transient-surveys/index.html

Long duration gamma-ray bursts are a rare subclass of stripped-envelope core-collapse supernovae that launch collimated relativistic outflows (jets). All gamma-ray-burst-associated supernovae are spectroscopically of Type Ic with broad lines, but the fraction of broad-lined Type Ic supernovae harboring low-luminosity gamma-ray-burst remains largely unconstrained. Some supernovae should be accompanied by off-axis $\gamma$-ray burst jets that remain invisible initially, but then emerge as strong radio sources (as the jets decelerate). However, this critical prediction of the jet model for gamma-ray bursts has yet to be verified observationally. Here, we present K. G. Jansky Very Large Array observations of 15 broad-lined supernovae of Type Ic discovered by the Palomar Transient Factory in an untargeted manner. Most of the supernovae in our sample exclude radio emission observationally similar to that of the radio-loud, relativistic SN\,1998bw. We constrain the fraction of 1998bw-like broad-lined Type Ic supernovae to be <~ 41% (99.865% confidence). Most of the events in our sample also exclude off-axis jets similar to GRB 031203 and GRB 030329, but we cannot rule out off-axis gamma-ray-bursts expanding in a low-density wind environment. Three supernovae in our sample are detected in the radio. PTF11qcj and PTF14dby show late-time radio emission with average ejecta speeds of ~(0.3-0.4)c, on the dividing line between relativistic and "ordinary" supernovae. The speed of PTF11cmh radio ejecta is poorly constrained. We estimate that <~ 85% (99.865% confidence) of the broad-lined Type Ic supernovae in our sample may harbor off-axis $\gamma$-ray-bursts expanding in media with densities in the range probed by this study., Comment: 20 pages, 10 figures; accepted for publication in ApJ. This version addresses Referee's comments

We present our extensive observational campaign on the Swift-discovered GRB141121A, al- most ten years after its launch. Our observations covers radio through X-rays, and extends for more than 30 days after discovery. The prompt phase of GRB 141121A lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is E{\gamma},iso = 8.0x10^52 erg. Due to the long prompt duration, GRB141121A falls into the recently discovered class of UL-GRBs. Peculiar features of this burst are a flat early-time optical light curve and a radio-to-X-ray rebrightening around 3 days after the burst. The latter is followed by a steep optical-to-X-ray decay and a much shallower radio fading. We analyze GRB 141121A in the context of the standard forward-reverse shock (FS,RS) scenario and we disentangle the FS and RS contributions. Finally, we comment on the puzzling early-time (t ~3 d) behavior of GRB 141121A, and suggest that its interpretation may require a two-component jet model. Overall, our analysis confirms that the class of UL-GRBs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful GRB explosions, and future missions (like SVOM, XTiDE, or ISS-Lobster) will provide many more of such objects., Comment: 25 pages, 5 pages, accepted for publication on The Astrophysical Journal (June 2015)

At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with peak luminosities ($L_{\rm iso} < 10^{48.5}~\rm{erg\,s}^{-1}$) substantially lower than the average of the more distant ones ($L_{\rm iso} > 10^{49.5}~\rm{erg\,s}^{-1}$). The properties of several low-luminosity (low-$L$) GRBs indicate that they can be due to shock break-out, as opposed to the emission from ultrarelativistic jets. Owing to this, it is highly debated how both populations are connected, and whether there is a continuum between them. The burst at redshift $z=0.283$ from 2012 April 22 is one of the very few examples of intermediate-$L$ GRBs with a $\gamma$-ray luminosity of $L\sim10^{48.9}~\rm{erg\,s}^{-1}$ that have been detected up to now. Together with the robust detection of its accompanying supernova SN 2012bz, it has the potential to answer important questions on the origin of low- and high-$L$ GRBs and the GRB-SN connection. We carried out a spectroscopy campaign using medium- and low-resolution spectrographs at 6--10-m class telescopes, covering the time span of 37.3 days, and a multi-wavelength imaging campaign from radio to X-ray energies over a duration of $\sim270$ days. Furthermore, we used a tuneable filter centred at H$\alpha$ to map star formation in the host galaxy and the surrounding galaxies. We used these data to extract and model the properties of different radiation components and incorporate spectral-energy-distribution fitting techniques to extract the properties of the host galaxy. Modelling the light curve and spectral energy distribution from the radio to the X-rays revealed the blast-wave to expand with an initial Lorentz factor of $\Gamma_0\sim60$, low for a high-$L$ GRB, and that the afterglow had an exceptional low peak luminosity-density of $\lesssim2\times10^{30}~\rm{erg\,s}^{-1}\,\rm{Hz}^{-1}$ in the sub-mm. [Abridged], Comment: 30 pages, 17 figures, 9 tables; abstract is abridged; images are shown at reduced resolution; comments are welcome

We have used the Karl G. Jansky Very Large Array (VLA) to search for 36 GHz and 44 GHz methanol (CH3OH) lines in a sample of 21 Galactic supernova remnants (SNRs). Mainly the regions of the SNRs with 1720 MHz OH masers were observed. Despite the limited spatial extent covered in our search, methanol masers were detected in both G1.4-0.1 and W28. Additional masers were found in SgrAEast. More than 40 masers were found in G1.4-0.1 which we deduce are due to interactions between the SNR and at least two separate molecular clouds. The six masers in W28 are associated with the molecular cloud that is also associated with the OH maser excitation. We discuss the possibility that the methanol maser may be more numerous in SNRs than the OH maser, but harder to detect due to observational constraints., Comment: 9 pages

We present a wide dataset of gamma-ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814A the best., Comment: Version resubmitted to MNRAS. 29 pages, 8 figures, 6 tables. Small changes to the abstract. Sections 3 and 4 slightly expanded and modified. Results unchanged. Section 4 has been broken into smaller subsections. Figure 6 improved. Added Table 6 with a summary of the models. Typos and references fixed

We present the results of our radio observational campaign on GRB 100418a, for which we used the Australia Telescope Compact Array (ATCA), Very Large Array (VLA) and the Very Long Baseline Array (VLBA). GRB 100418a was a peculiar GRB with unusual X-ray and optical afterglow profiles featuring a plateau phase with a very shallow rise. This observed plateau phase was believed to be due to a continued energy injection mechanism which powered the forward shock, giving rise to an unusual and long-lasting afterglow. The radio afterglow of GRB 100418a was detectable several weeks after the prompt emission. We conducted long-term monitoring observations of the afterglow and attempted to test the energy injection model advocating that the continuous energy injection is due to shells of material moving at a wide range of Lorentz factors. We obtained an upper limit of $\gamma$ $<$ 7 for the expansion rate of the GRB 100418a radio afterglow, indicating that the range-of-Lorentz factor model could only be applicable for relatively slow moving ejecta. A preferred explanation could be that continued activity of the central engine may have powered the long-lasting afterglow., Comment: 8 pages, 3 figures, accepted for publication in ApJ

We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst of the past 29 years. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z=0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 d after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies with no significant late-time flaring or rebrightening activity. The entire dataset from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past due to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments., Comment: Accepted to ApJ. Light curves and SEDs are available at http://www.astro.caltech.edu/~dperley/grb/130427a/data/

We report on the discovery of compact, narrow OH line emission from the hydroxl molecule at 1720 MHz toward the extended TeV source J1834-087. The origin of this high energy emission is unknown; it could be powered by one or more candidate neutron stars (leptonic) or by cosmic rays interacting with dense gas (hadronic). The OH emission is detected near the center of J1834-087, coincident with the radio continuum of the supernova remnant W41, and the radial velocity of the line is the same velocity as a giant molecular cloud along the line of sight. We argue that the OH is maser emission stimulated by the interaction of the W41 shock with the molecular cloud. The known correlation between gamma-ray bright supernova remnants and OH masers favors a hadronic interpretation for this high energy emission., Comment: 4 pages, 1 figure. ApJ Letters, submitted

We present the discovery, classification, and extensive panchromatic (from radio to X-ray) follow-up observations of PTF11qcj, a supernova discovered by the Palomar Transient Factory. PTF11qcj is located at a distance of dL ~ 124 Mpc. Our observations with the Karl G. Jansky Very Large Array show that this event is radio-loud: PTF11qcj reached a radio peak luminosity comparable to that of the famous gamma-ray-burst-associated supernova 1998bw (L_{5GHz} ~ 10^{29} erg/s/Hz). PTF11qcj is also detected in X-rays with the Chandra observatory, and in the infrared band with Spitzer. Our multi-wavelength analysis probes the supernova interaction with circumstellar material. The radio observations suggest a progenitor mass-loss rate of ~10^{-4} Msun/yr x (v_w/1000 km/s), and a velocity of ~(0.3-0.5)c for the fastest moving ejecta (at ~10d after explosion). However, these estimates are derived assuming the simplest model of supernova ejecta interacting with a smooth circumstellar material characterized by radial power-law density profile, and do not account for possible inhomogeneities in the medium and asphericity of the explosion. The radio light curve shows deviations from such a simple model, as well as a re-brightening at late times. The X-ray flux from PTF11qcj is compatible with the high-frequency extrapolation of the radio synchrotron emission (within the large uncertainties). An IR light echo from pre-existing dust is in agreement with our infrared data. Our analysis of pre-explosion data from the Palomar Transient Factory suggests that a precursor eruption of absolute magnitude M_r ~ -13 mag may have occurred ~ 2.5 yr prior to the supernova explosion. Based on our panchromatic follow-up campaign, we conclude that PTF11qcj fits the expectations from the explosion of a Wolf-Rayet star. Precursor eruptions may be a feature characterizing the final pre-explosion evolution of such stars., Comment: 43 pages, 15 figures; this version matches the one published in ApJ (includes minor changes that address the Referee's comments.)

We report on an analysis of the Extended Chandra Deep Field South (E-CDFS) region using archival data from the Very Large Array, with the goal of studying radio variability and transients at the sub-mJy level. The 49 epochs of E-CDFS observations at 1.4 GHz sample timescales from one day to 3 months. We find that only a fraction (1%) of unresolved radio sources above 40 uJy are variable at the 4-sigma level. There is no evidence that the fractional variability changes along with the known transition of radio source populations below one milliJansky. Optical identifications of the sources show that the variable radio emission is associated with the central regions of an active galactic nucleus or a star-forming galaxy. After a detailed comparison of the efficacy of various source-finding algorithms, we use the best to carry out a transient search. No transients were found. This implies that the areal density of transients with peak flux density greater than 0.21 mJy is less than 0.37 deg^-2 (at a confidence level of 95%). This result is approximately an order of magnitude below the transient rate measured at 5 GHz by Bower et al. (2007) but it is consistent with more recent upper limits from Frail et al. (2012). Our findings suggest that the radio sky at 1.4 GHz is relatively quiet. For multi-wavelength transient searches, such as the electromagnetic counterparts to gravitational waves, this frequency may be optimal for reducing the high background of false positives., Comment: 22 pages, 13 figures (25 if individual subfigures are counted), 10 tables. Accepted for publication in The Astrophysical Journal

We present the gamma-ray, X-ray, optical and radio data for GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, a sudden and prominent rebrightening in the optical band occurs followed by a fast decay in both bands. This optical rebrightening is accompanied by possible chromatic variations. We discuss possible interpretations, such as double component scenarios and internal dissipation mechanism, with their virtues and drawbacks. We also compare GRB100814A with other Swift bursts that show optical rebrightenings with similar properties., Comment: 5 pages, 2 figures. Proceedings of Fermi-Swift 2012 GRB conference held in Munich. Accepted for publication by "Proceedings of Science"

(Abridged). The optical light curve of some SNe may be powered by the outward diffusion of the energy deposited by the explosion shock in optically thick circumstellar matter (CSM). Recently, it was shown that the radiation-mediated and -dominated shock in an optically thick wind must transform into a collisionless shock and can produce hard X-rays. The X-rays are expected to peak at late times, relative to maximum visible light. Here we report on a search, using Swift and Chandra, for X-ray emission from 28 SNe that belong to classes whose progenitors are suspected to be embedded in dense CSM (IIn/Ibn/SLSN-I). Two SNe in our sample have X-ray properties that are roughly consistent with the expectation for X-rays from a collisionless shock in optically thick CSM. Therefore, we suggest that their optical light curves are powered by shock breakout in CSM. We show that two other events were too X-ray bright during the SN maximum optical light to be explained by the shock breakout model. We conclude that the light curves of some, but not all, type-IIn/Ibn SNe are powered by shock breakout in CSM. For the rest of the SNe in our sample, including all the SLSN-I events, our X-ray limits are not deep enough and were typically obtained at too early times to conclude about their nature. We argue that the optical light curves of SNe, for which the X-ray emission peaks at late times, are likely powered by the diffusion of shock energy from a dense CSM. We comment about the possibility to detect some of these events in radio., Comment: 12 pages, 7 figures, submitted to ApJ

We report on a re-analysis of archival data from the Very Large Array for a sample of ten long duration radio transients reported by Bower and others. These transients have an implied all-sky rate that would make them the most common radio transient in the sky and yet most have no quiescent counterparts at other wavelengths and therefore no known progenitor (other than Galactic neutron stars). We find that more than half of these transients are due to rare data artifacts. The remaining sources have lower signal-to-noise ratio (SNR) than initially reported by 1 to 1.5-sigma. This lowering of SNR matters greatly since the sources are at the threshold. We are unable to decisively account for the differences. By two orthogonal criteria one source appears to be a good detection. Thus the rate of long duration radio transients without optical counterparts is, at best, comparable to that of the class of recently discovered Swift J1644+57 nuclear radio transients. We revisit the known and expected classes of long duration radio transients and conclude that the dynamic radio sky remains a rich area for further exploration. Informed by the experience of past searches for radio transients, we suggest that future surveys pay closer attention to rare data errors and ensure that a wealth of sensitive multi-wavelength data be available in advance of the radio observations and that the radio searches should have assured follow-up resources., Comment: ApJ, submitted

We present the discovery of PTF 10vgv, a Type Ic supernova detected by the Palomar Transient Factory, using the Palomar 48-inch telescope (P48). R-band observations of the PTF 10vgv field with P48 probe the supernova emission from its very early phases (about two weeks before R-band maximum), and set limits on its flux in the week prior to the discovery. Our sensitive upper limits and early detections constrain the post-shock-breakout luminosity of this event. Via comparison to numerical (analytical) models, we derive an upper-limit of R \lesssim 4.5 Rsun (R \lesssim 1 Rsun) on the radius of the progenitor star, a direct indication in favor of a compact Wolf-Rayet star. Applying a similar analysis to the historical observations of SN 1994I, yields R \lesssim 1/4 Rsun for the progenitor radius of this supernova., Comment: 16 pages; 3 figures; accepted for publication in ApJ Letters; minor changes to match published version (results unchanged)

Active galactic nuclei (AGN), powered by long-term accretion onto central supermassive black holes, produce relativistic jets with lifetimes of greater than one million yr that preclude observations at birth. Transient accretion onto a supermassive black hole, for example through the tidal disruption of a stray star, may therefore offer a unique opportunity to observe and study the birth of a relativistic jet. On 2011 March 25, the Swift {\gamma}-ray satellite discovered an unusual transient source (Swift J164449.3+573451) potentially representing such an event. Here we present the discovery of a luminous radio transient associated with Swift J164449.3+573451, and an extensive set of observations spanning centimeter to millimeter wavelengths and covering the first month of evolution. These observations lead to a positional coincidence with the nucleus of an inactive galaxy, and provide direct evidence for a newly-formed relativistic outflow, launched by transient accretion onto a million solar mass black hole. While a relativistic outflow was not predicted in this scenario, we show that the tidal disruption of a star naturally explains the high-energy properties, radio luminosity, and the inferred rate of such events. The weaker beaming in the radio compared to {\gamma}-rays/X-rays, suggests that radio searches may uncover similar events out to redshifts of z ~ 6., Comment: 24 pages, 5 figures, 2 tables

We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt gamma-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at t_rest = 37 s after the onset of the GRB. This optical flash appears to be distinct from the subsequent emerging afterglow emission, for which we present comprehensive broadband radio to X-ray light curves to 13 days post-burst and rigorously test the standard fireball model. The intrinsic extinction toward GRB 080603A is high (A_V,z = 0.8 mag), and the well-sampled X-ray-to-near-infrared spectral energy distribution is interesting in requiring an LMC2 extinction profile, in contrast to the majority of GRBs. Comparison of the gamma-ray and extinction-corrected optical flux densities of the flash rules out an inverse-Compton origin for the prompt gamma-rays; instead, we suggest that the optical flash could originate from the inhomogeneity of the relativistic flow. In this scenario, a large velocity irregularity in the flow produces the prompt gamma-rays, followed by a milder internal shock at a larger radius that would cause the optical flash. Flat gamma-ray spectra, roughly F propto nu^-0.1, are observed in many GRBs. If the flat spectrum extends down to the optical band in GRB 080603A, the optical flare could be explained as the low-energy tail of the gamma-ray emission. If this is indeed the case, it provides an important clue to understanding the nature of the emission process in the prompt phase of GRBs and highlights the importance of deep (R> 20 mag), rapid follow-up observations capable of detecting faint, prompt optical emission., Comment: 22 pages, 11 figures, accepted to MNRAS

We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations show this transient was coincident with a galaxy at redshift z=0.147, and reached an apparent magnitude of V ~ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be M_V =-22.7 approximately 45 days after maximum light. Based on our unfiltered optical photometry the peak optical emission was L = 1.3 x 10^45 erg s^-1, and over a period of 287 rest-frame days had an integrated bolometric luminosity of 1.2 x 10^52 erg. Analysis of the pre-outburst SDSS spectrum of the source shows features consistent with a Narrow-line Seyfert1 (NLS1) galaxy. High-resolution HST and Keck followup observations show the event occurred within 150pc of nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival CSS and SDSS data to investigate the three most likely sources of such an event; 1) an extremely luminous supernova; 2) the tidal disruption of a star by the massive nuclear black hole; 3) variability of the central AGN. We find that CSS100217 was likely an extremely luminous type IIn supernova that occurred within range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity., Comment: submitted to ApJ

We present the results of a 5 GHz survey with the Very Large Array, designed to search for short-lived (<1 day) transients and to characterize the variability of radio sources at milli-Jansky levels. A total sky area of 2.66 deg^2, spread over 141 fields at low Galactic latitudes was observed 16 times with a cadence sampling timescales of days, months and years. Most of the data were searched for transients in near real time. Candidates were followed up using visible light telescopes (1-2 hr delays) and the X-Ray Telescope on board the Swift satellite. The final processing of the data revealed a single possible transient with a flux density of 2.4 mJy. This implies a transients, >1.8 mJy, sky surface density of 0.039 (-0.032/+0.13) deg^-2. This areal density is consistent with the sky surface density of transients from the Bower et al. survey extrapolated to 1.8 mJy. Our observed transient areal density is consistent with a Neutron Stars (NSs) origin for these events. Furthermore, we use the data to measure the sources variability on days to years time scales, and we present the variability structure function of 5 GHz sources. The mean structure function shows a fast increase on ~1 day time scale, followed by a slower increase on time scales of up to 10 days. On time scales between 10-60 days the structure function is roughly constant. We find that >30% of the unresolved sources brighter than 1.8 mJy are variable at the >4 sigma confidence level, presumably due mainly to refractive scintillation., Comment: Submitted to ApJ, 19 pages, 8 figures

We present numerical solutions of the 2D relativistic hydrodynamics equations describing the deceleration and expansion of highly relativistic conical jets, of opening angles 0.05R/c the emission of radiation from the jet blast wave is similar to that of a spherical blast wave carrying the same energy. Thus, the total (calorimetric) energy of GRB blast waves may be estimated with only a small fractional error based on t>R/c observations., Comment: 5 pages, 5 figures

We present the discovery and follow-up observations of a broad-line type-Ic supernova (SN), PTF 10bzf (SN 2010ah), detected by the Palomar Transient Factory (PTF) on 2010 February 23. The SN distance is \cong 218 Mpc, greater than GRB 980425 / SN 1998bw and GRB 060218 / SN 2006aj, but smaller than the other SNe firmly associated with gamma-ray bursts (GRBs). We conducted a multi-wavelength follow-up campaign with Palomar-48 inch, Palomar 60-inch, Gemini-N, Keck, Wise, Swift, the Allen Telescope Array, CARMA, WSRT, and EVLA. Here we compare the properties of PTF 10bzf with those of SN 1998bw and other broad-line SNe. The optical luminosity and spectral properties of PTF 10bzf suggest that this SN is intermediate, in kinetic energy and amount of 56Ni, between non GRB-associated SNe like 2002ap or 1997ef, and GRB-associated SNe like 1998bw. No X-ray or radio counterpart to PTF 10bzf was detected. X-ray upper-limits allow us to exclude the presence of an underlying X-ray afterglow as luminous as that of other SN-associated GRBs like GRB 030329 or GRB 031203. Early-time radio upper-limits do not show evidence for mildly-relativistic ejecta. Late-time radio upper-limits rule out the presence of an underlying off-axis GRB, with energy and wind density similar to the SN-associated GRB 030329 and GRB 031203. Finally, by performing a search for a GRB in the time window and at the position of PTF 10bzf, we find that no GRB in the IPN catalog could be associated with this SN., Comment: 34 pages, 7 figures, to appear in ApJ; revised version addresses referee's comments; P60 data added; results unchanged