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This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited., We present centimeter-band and millimeter-band afterglow observations of five long-duration γ-ray bursts (GRBs; GRB 130131A, 130420B, 130609A, 131229A, 140713A) with dust-obscured optical afterglow emission, known as “dark” GRBs. We detect the radio afterglow of two of the dark GRBs (GRB 130131A and 140713A), along with a tentative detection of a third (GRB 131229A) with the Karl G. Jansky Very Large Array (VLA). Supplemented by three additional VLA-detected dark GRBs from the literature, we present uniform modeling of their broadband afterglows. We derive high line-of-sight dust extinctions of AV,GRB ≈ 2.2– ≳ 10.6 mag. Additionally, we model the host galaxies of the six bursts in our sample, and derive host galaxy dust extinctions of AV,Host ≈ 0.3–4.7 mag. Across all tested γ-ray (fluence and duration) and afterglow properties (energy scales, geometries, and circumburst densities), we find dark GRBs to be representative of more typical unobscured long GRBs, except in fluence, for which observational biases and inconsistent classification may influence the dark GRB distribution. Additionally, we find that AV,GRB is not related to a uniform distribution of dust throughout the host, nor to the extremely local environment of the burst, indicating that a larger-scale patchy dust distribution is the cause of the high line-of-sight extinction. Since radio observations are invaluable to revealing heavily dust-obscured GRBs, we make predictions for the detection of radio emission from host star formation with the next-generation VLA. © 2022. The Author(s). Published by the American Astronomical Society., G.S. acknowledges support for this work was provided by the NSF through Student Observing Support award SOSP20B-001 from the NRAO. The Fong Group at Northwestern acknowledges support by the National Science Foundation under grant No. AST-1814782 and CAREER grant No. AST-2047919. W.F. gratefully acknowledges support by the David and Lucile Packard Foundation. T.L. acknowledges support from the Radboud Excellence Initiative. A.J.C.T. acknowledges support from the Spanish Ministry Project PID2020-118491GB-I00, Junta de Andalucía Project P20_01068 and the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). K.D.A. acknowledges support provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51403.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.

We present deep X-ray and radio observations of the Fast Blue Optical Transient (FBOT) AT2020xnd/ZTF20acigmel at $z=0.2433$ from $13$d to $269$d after explosion. AT2020xnd belongs to the category of optically luminous FBOTs with similarities to the archetypal event AT2018cow. AT2020xnd shows luminous radio emission reaching $L_{\nu}\approx8\times10^{29}$ergs$^{-1}$Hz$^{-1}$ at 20GHz and $75$d post explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at $L_{X}\approx6\times10^{42}$ergs$^{-1}$. Interpreting the radio emission in the context of synchrotron radiation from the explosion's shock interaction with the environment we find that AT2020xnd launched a high-velocity outflow ($v\sim$0.1-0.2$c$) propagating into a dense circumstellar medium (effective $\dot M\approx10^{-3}M_{\rm{sol}}$yr$^{-1}$ for an assumed wind velocity of $v_w=1000$kms$^{-1}$). Similar to AT2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT2020xnd a high-redshift analog to AT2018cow, and establish AT2020xnd as the fourth member of the class of optically-luminous FBOTs with luminous multi-wavelength counterparts., Comment: 21 pages, 6 figures, 6 tables. Submitted to ApJ

We present a comprehensive optical and near-infrared census of the fields of 90 short gamma-ray bursts (GRBs) discovered in 2005-2021, constituting all short GRBs for which host galaxy associations are feasible ($\approx$ 60% of the total Swift short GRB population). We contribute 245 new multi-band imaging observations across 49 distinct GRBs and 25 spectra of their host galaxies. Supplemented by literature and archival survey data, the catalog contains 335 photometric and 40 spectroscopic data sets. The photometric catalog reaches $3\sigma$ depths of $\gtrsim 24-27$ mag and $\gtrsim 23-26$ mag for the optical and near-infrared bands, respectively. We identify host galaxies for 84 bursts, in which the most robust associations make up 54% (49/90) of events, while only a small fraction, 6.7%, have inconclusive host associations. Based on new spectroscopy, we determine 17 host spectroscopic redshifts with a range of $z\approx 0.15-1.6$ and find that $\approx$ 25-44% of Swift short GRBs originate from $z>1$. We also present the galactocentric offset catalog for 83 short GRBs. Taking into account the large range of individual measurement uncertainties, we find a median of projected offset of $\approx 7.9$ kpc, for which the bursts with the most robust associations have a smaller median of $\approx 4.9$ kpc. Our catalog captures more high-redshift and low-luminosity hosts, and more highly-offset bursts than previously found, thereby diversifying the population of known short GRB hosts and properties. In terms of locations and host luminosities, the populations of short GRBs with and without detectable extended emission are statistically indistinguishable. This suggests that they arise from the same progenitors, or from multiple progenitors which form and evolve in similar environments. All of the data products are available on the BRIGHT website., Comment: 53 pages, 9 figures, 6 tables, submitted

In the past two decades, high amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole. X-ray emission from the Seyfert 2 galaxy GSN 069 (2MASX J01190869-3411305) at redshift z = 0.018 was first detected in 2010 July and implies an X-ray brightening of more than a factor of 240 over ROSAT observations performed 16 years earlier. The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event. The X-ray spectrum is ultra-soft and can be described by accretion disc emission with luminosity proportional to the fourth power of the disc temperature during long-term evolution. Here we report observations of X-ray quasi-periodic eruptions from the nucleus of GSN 069 over the course of 54 days, 2018 December onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with event duration of just over 1 hour and recurrence time of about 9 hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4x10$^5$ solar masses) with peak X-ray luminosity of ~ 5x10$^{42}$ ergs per second. The warm phase has a temperature of about 120 eV, reminiscent of the typical soft X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei. If the observed properties are not unique to GSN 069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with more massive black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years., Authors' version of a Letter published on-line in Nature on September 11, 2019. Includes Methods and Extended Data. 43 pages, 3 tables and 10 figures

We present detailed radio observations of the tidal disruption event (TDE) AT2019dsg, obtained with the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), and spanning $55-560$ days post-disruption. We find that the peak brightness of the radio emission increases until ~200 days and subsequently begins to decrease steadily. Using the standard equipartition analysis, including the effects of synchrotron cooling as determined by the joint VLA-ALMA spectral energy distributions, we find that the outflow powering the radio emission is in roughly free expansion with a velocity of $\approx 0.07c$, while its kinetic energy increases by a factor of about 5 from 55 to 200 days and plateaus at $\approx 5\times 10^{48}$ erg thereafter. The ambient density traced by the outflow declines as $\approx R^{-1.6}$ on a scale of $\approx (1-4)\times 10^{16}$ cm ($\approx 6300-25000$ $R_s$), followed by a steeper decline to $\approx 6\times 10^{16}$ cm ($\approx 37500$ $R_s$). Allowing for a collimated geometry, we find that to reach even mildly relativistic velocities ($\Gamma=2$) the outflow requires an opening angle of $\theta_j\approx 2^\circ$, which is narrow even by the standards of GRB jets; a truly relativistic outflow requires an unphysically narrow jet. The outflow velocity and kinetic energy in AT2019dsg are typical of previous non-relativistic TDEs, and comparable to those from Type Ib/c supernovae, raising doubts about the claimed association with a high-energy neutrino event., Comment: Accepted to the Astrophysical Journal

We present the complete set of {\it Hubble Space Telescope} imaging of the binary neutron star merger GW170817 and its optical counterpart AT 2017gfo. Including deep template imaging in F814W, F110W, F140W, and F160W at 3.4 years post-merger, we re-analyze the full light curve of AT 2017gfo across 12 bands from 5--1273 rest-frame days after merger. We obtain four new detections of the short $\gamma$-ray burst (GRB) 170817A afterglow from 109--170 rest-frame days post-merger. These detections are consistent with the previously observed $\beta=-0.6$ spectral index in the afterglow light curve with no evidence for spectral evolution. We also analyze our limits in the case of novel late-time optical and IR emission signatures, such as a kilonova afterglow or infrared dust echo, but find our limits are not constraining in these contexts. We use the new data to construct deep optical and infrared stacks, reaching limits of $M=-6.3$ to $-4.6$ mag, to analyze the local environment around AT 2017gfo and low surface brightness features in its host galaxy NGC 4993. We rule out the presence of any globular cluster at the position of AT 2017gfo to $2.3 \times 10^{4} L_{\odot}$, including those with the reddest $V-H$ colors. Finally, we analyze the substructure of NGC 4993 in deep residual imaging, and find shell features which extend up to 71.8\arcsec\ (14.2 kpc) from the center of the galaxy. We find that the shells have a cumulative stellar mass of $6.3\times10^{8} M_{\odot}$, roughly 2% the total stellar mass of NGC 4993, and mass-weighted ages of $>$3 Gyr. We conclude that it was unlikely the GW170817 progenitor system formed in the galaxy merger, which based on dynamical signatures and the stellar population in the shells mostly likely occurred 220--685 Myr ago., Comment: 21 pages, 9 figures, submitted to ApJ

Radio observations of tidal disruption events (TDEs) probe material ejected by the disruption of stars by supermassive black holes (SMBHs), uniquely tracing the formation and evolution of jets and outflows, revealing details of the disruption hydrodynamics, and illuminating the environments around previously-dormant SMBHs. To date, observations reveal a surprisingly diverse population. A small fraction of TDEs (at most a few percent) have been observed to produce radio-luminous mildly relativistic jets. The remainder of the population are radio quiet, producing less luminous jets, non-relativistic outflows or, possibly, no radio emission at all. Here, we review the radio observations that have been made of TDEs to date and discuss possible explanations for their properties, focusing on detected sources and, in particular, on the two best-studied events: Sw J1644+57 and ASASSN-14li. We also discuss what we have learned about the host galaxies of TDEs from radio observations and review constraints on the rates of bright and faint radio outflows in TDEs. Upcoming X-ray, optical, near-IR, and radio surveys will greatly expand the sample of TDEs, and technological advances open the exciting possibility of discovering a sample of TDEs in the radio band unbiased by host galaxy extinction., Resubmitted for publication in Springer Space Science Reviews following referee comments. Chapter in ISSI review "The Tidal Disruption of Stars by Massive Black Holes" vol. 79. Table 2 is available in machine-readable format upon request

We present X-ray and radio observations of the Fast Blue Optical Transient (FBOT) CRTS-CSS161010 J045834-081803 (CSS161010 hereafter) at t=69-531 days. CSS161010 shows luminous X-ray ($L_x\sim5\times 10^{39}\,\rm{erg\,s^{-1}}$) and radio ($L_{\nu}\sim10^{29}\,\rm{erg\,s^{-1}Hz^{-1}}$) emission. The radio emission peaked at ~100 days post transient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blastwave. CSS161010 launched a mildly relativistic outflow with velocity $\Gamma\beta c\ge0.55c$ at ~100 days. This is faster than the non-relativistic AT2018cow ($\Gamma\beta c\sim0.1c$) and closer to ZTF18abvkwla ($\Gamma\beta c\ge0.3c$ at 63 days). The inferred initial kinetic energy of CSS161010 ($E_k\gtrsim10^{51}$ erg) is comparable to that of long Gamma Ray Bursts (GRBs), but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ($\sim0.01-0.1\,\rm{M_{\odot}}$). This is consistent with the lack of observed gamma-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at ~150 Mpc in a dwarf galaxy with stellar mass $M_{*}\sim10^{7}\,\rm{M_{\odot}}$ and specific star formation rate $sSFR\sim 0.3\,\rm{Gyr^{-1}}$. This mass is among the lowest inferred for host-galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally-located intermediate mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare (rate $, Comment: Accepted to ApJL

We thank the anonymous referee for their many suggestions that improved this paper. We thank Miguel Perez-Torres for helpful discussions. MN is supported by a Royal Astronomical Society Research Fellowship. TW is funded in part by European Research Council grant 320360 and by European Commission grant 730980. PGJ and GC acknowledge support from European Research Council Consolidator Grant 647208. GL and PC are supported by a research grant (19054) from Villum Fonden. MG is supported by the Polish NCN MAESTRO Grant 2014/14/A/ST9/00121. NI is partially supported by Polish NCN DAINA Grant 2017/27/L/ST9/03221. IA is a CIFAR Azrieli Global Scholar in the Gravity and the Extreme Universe Program and acknowledges support from that program, from the Israel Science Foundation (grant numbers 2108/18 and 2752/19), from the United States-Israel Binational Science Foundation (BSF), and from the Israeli Council forHigher Education Alon Fellowship. JB, DH, and CP were supported by NASA Grant 80NSSC18K0577. TWCacknowledges the EU Funding underMarie Sklodowska-Curie grant agreement No. 842471. LG was funded by the European Union's Horizon 2020 Framework Programme under theMarie Sklodowska-Curie grant agreement No. 839090. Thiswork has been partially supported by the Spanish grant PGC2018-095317-B-C21 within the European Funds for Regional Development (FEDER). SGGacknowledges support by FCT under Projects CRISP PTDC/FIS-AST-31546 and UIDB/00099/2020. IM is a recipient of the Australian Research Council Future Fellowship FT190100574. TMBwas funded by the CONICYT PFCHA/DOCTORADOBECAS CHILE/2017-72180113. KDA acknowledges support provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51403.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. This work is based on data collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under ESO programmes 1103.D-0328 and 0104.B-0709 and as part of ePESSTO+(the advanced Public ESO Spectroscopic Survey for Transient Objects Survey), observations from the Las Cumbres Observatory network, the 6.5 meterMagellan Telescopes located at Las Campanas Observatory, Chile, the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution and the WilliamHerschel Telescope (programme W19B/P7). Swift data were supplied by the UK Swift Science Data Centre at the University of Leicester. The Liverpool Telescope andWilliam Herschel Telescope are operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council., At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and the faint-and-fast event iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass ≈106M , disrupting a star of ≈1M . By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t2, consistent with a photosphere expanding at constant velocity ( 2000 km s−1), and a line-forming region producing initially blueshifted H and He II profiles with v = 3000–10 000 km s−1. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K. D. Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission – the first time this connection has been observed in a TDE. The light-curve rise begins 29 ± 2 d before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N III) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at ≈1041 erg s−1. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models., Royal Astronomical Society Research Fellowship, European Research Council (ERC) 320360 647208, European Commission European Commission Joint Research Centre 730980, Villum Fonden 19054, Polish NCN MAESTRO Grant 2014/14/A/ST9/00121, Polish NCN DAINA Grant 2017/27/L/ST9/03221, Israel Science Foundation 2108/18 2752/19, US-Israel Binational Science Foundation, Israeli Council forHigher Education Alon Fellowship, National Aeronautics & Space Administration (NASA) 80NSSC18K0577 NAS5-26555, European Union (EU) 842471 839090, European Union (EU) PGC2018-095317-B-C21, Portuguese Foundation for Science and Technology CRISP PTDC/FIS-AST-31546 UIDB/00099/2020, Australian Research Council FT190100574, CONICYT PFCHA/DOCTORADOBECAS CHILE 2017-72180113, NASA through the NASA Hubble Fellowship grant - Space Telescope Science Institute HST-HF2-51403.001-A, European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under ESO programmes 1103.D-0328 0104.B-0709, WilliamHerschel Telescope W19B/P7, Science & Technology Facilities Council (STFC)

AT 2018hyz (=ASASSN-18zj) is a tidal disruption event (TDE) located in the nucleus of a quiescent E+A galaxy at a redshift of $z = 0.04573$, first detected by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present optical+UV photometry of the transient, as well as an X-ray spectrum and radio upper limits. The bolometric light curve of AT 2018hyz is comparable to other known TDEs and declines at a rate consistent with a $t^{-5/3}$ at early times, emitting a total radiated energy of $E = 9\times10^{50}$ erg. An excess bump appears in the UV light curve about 50 days after bolometric peak, followed by a flattening beyond 250 days. The light curve shows an excess bump in the UV about 50 days after bolometric peak lasting for at least 100 days, which may be related to an outflow. We detect a constant X-ray source present for at least 86 days. The X-ray spectrum shows a total unabsorbed flux of $\sim 4\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ and is best fit by a blackbody plus power-law model with a photon index of $\Gamma = 0.8$. A thermal X-ray model is unable to account for photons $> 1$ keV, while the radio non-detection favors inverse-Compton scattering rather than a jet for the non-thermal component. We model the optical and UV light curves using the Modular Open-Source Fitter for Transients (MOSFiT) and find a best fit for a black hole of $5.2\times10^6$ M$_\odot$ partially disrupting a $0.1$ M$_\odot$ star (stripping a mass of $\sim 0.01$ M$_\odot$ for the inferred impact parameter, $\beta=0.6$). The low optical depth implied by the small debris mass may explain how we are able to see hydrogen emission with disk-like line profiles in the spectra of AT 2018hyz (see our companion paper, Short et al.~2020)., Comment: 10 pages, 10 figures, published in MNRAS

Time domain science has undergone a revolution over the past decade, with tens of thousands of new supernovae (SNe) discovered each year. However, several observational domains, including SNe within days or hours of explosion and faint, red transients, are just beginning to be explored. Here, we present the Young Supernova Experiment (YSE), a novel optical time-domain survey on the Pan-STARRS telescopes. Our survey is designed to obtain well-sampled $griz$ light curves for thousands of transient events up to $z \approx 0.2$. This large sample of transients with 4-band light curves will lay the foundation for the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, providing a critical training set in similar filters and a well-calibrated low-redshift anchor of cosmologically useful SNe Ia to benefit dark energy science. As the name suggests, YSE complements and extends other ongoing time-domain surveys by discovering fast-rising SNe within a few hours to days of explosion. YSE is the only current four-band time-domain survey and is able to discover transients as faint $\sim$21.5 mag in $gri$ and $\sim$20.5 mag in $z$, depths that allow us to probe the earliest epochs of stellar explosions. YSE is currently observing approximately 750 square degrees of sky every three days and we plan to increase the area to 1500 square degrees in the near future. When operating at full capacity, survey simulations show that YSE will find $\sim$5000 new SNe per year and at least two SNe within three days of explosion per month. To date, YSE has discovered or observed 8.3% of the transient candidates reported to the International Astronomical Union in 2020. We present an overview of YSE, including science goals, survey characteristics and a summary of our transient discoveries to date., ApJ, in press; more information at https://yse.ucsc.edu/

We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short GRB 200522A, located at a small projected offset of $\approx 1$ kpc from the center of a young, star-forming host galaxy at $z=0.5536$. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our HST observations at a rest-frame time of $\approx2.3$ days, has a luminosity of $\approx (1.3-1.7) \times 10^{42}$ erg s$^{-1}$. This is substantially lower than on-axis short GRB afterglow detections, but is a factor of $\approx 8$-$17$ more luminous than the kilonova of GW170817, and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color ($i-y = -0.08\pm 0.21$; rest-frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broad-band behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a $\approx14^{\circ}$, relativistic ($\Gamma_0 \gtrsim 80$) jet. Models which include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting $\approx 0.3$-$6$ years after the burst for a deposited energy of $\approx 10^{53}$ erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to $\approx\!250$ Mpc., Comment: 33 pages, 13 figures, 5 tables. Submitted to AAS Journals

We present the largest and deepest late-time radio and millimeter survey to date of superluminous supernovae (SLSNe) and long-duration gamma-ray bursts (LGRBs) to search for associated nonthermal synchrotron emission. Using the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), we observed 43 sources at 6 and 100 GHz on a timescale of ∼ 1–19 yr post-explosion. We do not detect radio/millimeter emission from any of the sources, with the exception of a 6 GHz detection of PTF10hgi, as well as the detection of 6 GHz emission near the location of the SLSN PTF12dam, which we associate with its host galaxy. We use our data to place constraints on central engine emission due to magnetar wind nebulae and off-axis relativistic jets. We also explore nonrelativistic emission from the SN ejecta, and place constraints on obscured star formation in the host galaxies. In addition, we conduct a search for fast radio bursts (FRBs) from some of the sources using VLA phased-array observations; no FRBs are detected to a limit of 16 mJy (7σ; 10 ms duration) in about 40 minutes on source per event. A comparison to theoretical models suggests that continued radio monitoring may lead to detections of persistent radio emission on timescales of ≳ a decade.

We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z=0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus of its host galaxy; it peaks at a luminosity of $L \approx 10^{44}$ erg s$^{-1}$; and the spectrum shows a persistent blackbody temperature $T \gtrsim 20,000$ K with broad H I and He II emission. The lines are initially centered at zero velocity, but by 100 days the H I lines disappear while the He II develops a blueshift of $\gtrsim 5,000$ km s$^{-1}$. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of $\sim 10^{6.3} M_\odot$. The host is another quiescent, Balmer-strong galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 days are among the deepest for a TDE at this phase., Comment: Updated to match published version

On 2019 April 25.346 and 26.640 UT the LIGO and Virgo gravitational wave (GW) observatories announced the detection of the first candidate events in Observing Run 3 that contain at least one neutron star. S190425z is a likely binary neutron star (BNS) merger at $d_L = 156 \pm 41$ Mpc, while S190426c is possibly the first NS-BH merger ever detected, at $d_L = 377 \pm 100$ Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5-m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1-m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/NIR kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short GRB afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of short GRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field $\gamma$-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the $\gamma$-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg$^2$ are likely essential., Comment: The first six authors contributed equally to this work

On 2019 August 14 the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo gravitational wave interferometer announced the detection of a binary merger, S190814bv, with a low false alarm rate (FAR) of about 1 in $1.6\times 10^{25}$ years, a distance of $267\pm 52$ Mpc, a 90\% (50\%) localization region of about 23 (5) deg$^2$, and a probability of being a neutron star--black hole (NS-BH) merger of $>99\%$. The LIGO/Virgo Collaboration (LVC) defines NS-BH such that the lighter binary member has a mass of $5$ M$_\odot$, and this classification is in principle consistent with a BH-BH merger depending on the actual upper mass cut-off for neutron stars. Additionally, the LVC designated a probability that the merger led to matter outside the final BH remnant of $, 10 pages, 1 table, 4 figures, accepted to ApJL

We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from $\approx150$ s to $\approx163$ days after the burst, comprising the first joint ALMA-VLA-GMRT observations of a gamma-ray burst (GRB) afterglow. The radio and mm-band data reveal a distinct signature at $\approx3.9$ days, which we interpret as reverse shock (RS) emission. Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock (FS) components. We perform detailed modeling of the full multiwavelength data set, using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission. We uncover and account for all degeneracies in the model parameters. The joint RS-FS modeling reveals a weakly magnetized ($\sigma\approx3\times10^{-3}$), mildly relativistic RS, from which we derive an initial bulk Lorentz factor of $\Gamma_0\approx103$ for the GRB jet. Our results support the hypothesis that low-density environments are conducive to the observability of RS emission. We compare our observations to other events with strong RS detections, and find a likely observational bias selecting for longer lasting, non-relativistic reverse shocks. We present and begin to address new challenges in modeling posed by the present generation of comprehensive, multi-frequency data sets., Comment: Accepted version

We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_{\nu}\propto t^{-1.95\pm0.15}$ and a simple power-law spectrum $F_{\nu}\propto \nu^{-0.575\pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $n\le 9.6 \times 10^{-3}\,\rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}\times 10^{49}\,\rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}\times10^{52}\, \rm{erg}$), jet opening angle $\theta_{0}= 5.9^{+1.0}_{-0.7}\,\rm{deg}$ with characteristic Lorentz factor $\Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} \times 10^{-3}\, \rm{cm^{-3}}$ and viewed $\theta_{obs} = 30.4^{+4.0}_{-3.4}\, \rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are constrained to $\epsilon_{\rm{e}} = 0.18_{-0.13}^{+0.30}$ and $\epsilon_{\rm{B}}=2.3_{-2.2}^{+16.0} \times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\ge2.5\sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}\propto (\Gamma\beta)^{-\alpha}$ into the environment, finding that shallow stratification indexes $\alpha\le6$ are disfavored., Comment: version accepted for publication in ApJL, 13 pages, 6 figures

We present results from an analysis of all data taken by the bicep2/Keck CMB polarization experiments up to and including the 2015 observing season. This includes the first Keck Array observations at 220 GHz and additional observations at 95 and 150 GHz. The Q and U maps reach depths of 5.2, 2.9, and 26 μKCMB arcmin at 95, 150, and 220 GHz, respectively, over an effective area of ≈400 square degrees. The 220 GHz maps achieve a signal to noise on polarized dust emission approximately equal to that of Planck at 353 GHz. We take auto and cross spectra between these maps and publicly available WMAP and Planck maps at frequencies from 23 to 353 GHz. We evaluate the joint likelihood of the spectra versus a multicomponent model of lensed-ΛCDM+r+dust+synchrotron+noise. The foreground model has seven parameters, and we impose priors on some of these using external information from Planck and WMAP derived from larger regions of sky. The model is shown to be an adequate description of the data at the current noise levels. The likelihood analysis yields the constraint r0.05

Massive, rapidly spinning magnetar remnants produced as a result of binary neutron-star (BNS) mergers may deposit a fraction of their energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. Here, we present 6.0 GHz Very Large Array (VLA) observations of nine, low-redshift short gamma-ray bursts (GRBs; z σ limits on radio continuum emission of F ν ≲ 6–20 μJy at the burst positions, or L ν ≲ (0.6–8.3) × 1028 erg s−1 Hz−1. Comparing these limits with new light-curve modeling that properly incorporates relativistic effects, we obtain limits on the energy deposited into the ejecta of E ej ≲ (0.6–6.7) × 1052 erg ( erg) for an ejecta mass of 0.03 M ⊙ (0.1 M ⊙). We present a uniform reanalysis of 27 short GRBs with 5.5–6.0 GHz observations, and find that ≳50% of short GRBs did not form stable magnetar remnants in their mergers. Assuming short GRBs are produced by BNS mergers drawn from the Galactic BNS population plus an additional component of high-mass GW194025-like mergers in a fraction f GW190425 of cases, we place constraints on the maximum mass of a nonrotating neutron star (NS; Tolman–Oppenheimer–Volkoff mass; M TOV), finding for f GW190425 = 0.4; this limit increases for larger values of f GW190425. The detection (or lack thereof) of radio remnants in untargeted surveys such as the VLA Sky Survey (VLASS) could provide more stringent constraints on the fraction of mergers that produce stable remnants. If radio remnants are discovered in VLASS, this suggests that short GRBs are a biased population of BNS mergers in terms of the stability of the remnants they produce.

XMM-Newton performs a survey of the sky in the 0.2-12 keV X-ray band while slewing between observation targets. The sensitivity in the soft X-ray band is comparable with that of the ROSAT all-sky survey, allowing bright transients to be identified in near real-time by a comparison of the flux in both surveys. Several of the soft X-ray flares are coincident with galaxy nuclei and five of these have been interpreted as candidate tidal disruption events (TDE). The first three discovered had a soft X-ray spectrum, consistent with the classical model of TDE, where radiation is released during the accretion phase by thermal processes. The remaining two have an additional hard, power-law component, which in only one case was accompanied by radio emission. Overall the flares decay with the classical index of t−5/3 but vary greatly in the early phase.

BICEP Array is a degree-scale Cosmic Microwave Background (CMB) experiment that will search for primordial B-mode polarization while constraining Galactic foregrounds. BICEP Array will be comprised of four receivers to cover a broad frequency range with channels at 30/40, 95, 150 and 220/270 GHz. The first low-frequency receiver will map synchrotron emission at 30 and 40 GHz and will deploy to the South Pole at the end of 2019. In this paper, we give an overview of the BICEP Array science and instrument, with a focus on the detector module. We designed corrugations in the metal frame of the module to suppress unwanted interactions with the antenna-coupled detectors that would otherwise deform the beams of edge pixels. This design reduces the residual beam systematics and temperature-to-polarization leakage due to beam steering and shape mismatch between polarized beam pairs. We report on the simulated performance of single- and wide-band corrugations designed to minimize these effects. Our optimized design alleviates beam differential ellipticity caused by the metal frame to about 7% over 57% bandwidth (25 to 45 GHz), which is close to the level due the bare antenna itself without a metal frame. Initial laboratory measurements are also presented., 12 pages and 14 figures

We present detailed multi-wavelength observations of GRB 161219B at $z=0.1475$, spanning the radio to X-ray regimes, and the first ALMA light curve of a GRB afterglow. The cm- and mm-band observations before $8.5$ d require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, $n_0\approx 3\times10^{-4}$ cm$^{-3}$ and show that this is a characteristic of all strong RS detections to date. The VLA observations exhibit unexpected rapid variability on $\sim$ minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, $t_{\rm jet}\approx32$ day, yielding a wide jet opening angle of $\theta_{\rm jet}\approx13^{\circ}$, implying beaming corrected $\gamma$-ray and kinetic energies of $E_{\gamma}\approx4.9\times10^{48}$ erg and $E_{\rm K}\approx1.3\times10^{50}$ erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, $R_{\rm B}\approx1$, compared to the FS. These direct, multi-frequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets., Comment: 23 pages, 17 figures, 9 tables. Published in ApJ

Targeting faint polarization patterns arising from Primordial Gravitational Waves in the Cosmic Microwave Background requires excellent observational sensitivity. Optical elements in small aperture experiments such as Bicep3 and Keck Array are designed to optimize throughput and minimize losses from transmission, reflection and scattering at millimeter wavelengths. As aperture size increases, cryostat vacuum windows must withstand larger forces from atmospheric pressure and the solution has often led to a thicker window at the expense of larger transmission loss. We have identified a new candidate material for the fabrication of vacuum windows: with a tensile strength two orders of magnitude larger than previously used materials, woven high-modulus polyethylene could allow for dramatically thinner windows, and therefore significantly reduced losses and higher sensitivity. In these proceedings we investigate the suitability of high-modulus polyethylene windows for ground-based CMB experiments, such as current and future receivers in the Bicep/Keck Array program. This includes characterizing their optical transmission as well as their mechanical behavior under atmospheric pressure. We find that such ultra-thin materials are promising candidates to improve the performance of large-aperture instruments at millimeter wavelengths, and outline a plan for further tests ahead of a possible upcoming field deployment of such a science-grade window., Comment: Published in Proc. SPIE. Presented at SPIE Astronomical Telescopes and Instrumentation Conference 10708: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI, June 2018. 14 pages, 7 figures

BICEP Array is the newest multi-frequency instrument in the BICEP/Keck Array program. It is comprised of four 550 mm aperture refractive telescopes observing the polarization of the cosmic microwave background (CMB) at 30/40, 95, 150 and 220/270 GHz with over 30,000 detectors. We present an overview of the receiver, detailing the optics, thermal, mechanical, and magnetic shielding design. BICEP Array follows BICEP3's modular focal plane concept, and upgrades to 6" wafer to reduce fabrication with higher detector count per module. The first receiver at 30/40 GHz is expected to start observing at the South Pole during the 2019-20 season. By the end of the planned BICEP Array program, we project $\sigma(r) \sim 0.003$, assuming current modeling of polarized Galactic foreground and depending on the level of delensing that can be achieved with higher resolution maps from the South Pole Telescope., Comment: 15 pages, 13 figures

BICEP3 is a 520mm aperture on-axis refracting telescope observing the polarization of the cosmic microwave background (CMB) at 95GHz in search of the B-mode signal originating from inflationary gravitational waves. BICEP3's focal plane is populated with modularized tiles of antenna-coupled transition edge sensor (TES) bolometers. BICEP3 was deployed to the South Pole during 2014-15 austral summer and has been operational since. During the 2016-17 austral summer, we implemented changes to optical elements that lead to better noise performance. We discuss this upgrade and show the performance of BICEP3 at its full mapping speed from the 2017 and 2018 observing seasons. BICEP3 achieves an order-of-magnitude improvement in mapping speed compared to a Keck 95GHz receiver. We demonstrate $6.6\mu K\sqrt{s}$ noise performance of the BICEP3 receiver., Comment: 12 pages, 9 figures, Published in Proc. SPIE. Presented at SPIE Astronomical Telescopes and Instrumentation Conference 10708: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI, June 2018

We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$ erg/s exceeding those of superluminous supernovae (SNe), before declining as $\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at $E\ge 10$ keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with $v_{sh}$~0.1c with a dense environment ($\dot M\sim10^{-3}-10^{-4}\,M_{\odot}yr^{-1}$ for $v_w=1000$ km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released $\sim10^{50}-10^{51.5}$ erg over $\sim10^3-10^5$ s and resides within low-mass fast-moving material with equatorial-polar density asymmetry ($M_{ej,fast}\lesssim0.3\,\rm{M_{\odot}}$). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations., Comment: 26 pages + tables. Submitted

We present the first observations of a Type I superluminous supernova (SLSN) at $\gtrsim 1000$ days after maximum light. We observed SN 2015bn using the Hubble Space Telescope Advanced Camera for Surveys in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at $>700$ days of $0.19 \pm 0.03$ mag (100 d)$^{-1}$, much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of $^{56}$Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same [O I] and [Ca II] lines as seen at $\sim300-400$ days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates $10^{-2.7} \lesssim \dot{M}/v_{10} \lesssim 10^{-1.1}$ M$_\odot$ yr$^{-1}$ (where $v_{10}$ is the wind velocity in units of 10 km s$^{-1}$). The optical light curve is consistent with $L \propto t^{-4}$, which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at $\sim 0.01$ cm$^2$ g$^{-1}$, consistent with photon-matter pair-production over a broad $\sim$GeV-TeV range. This suggests the magnetar spectral energy distribution, and hence the 'missing energy' leaking from the ejecta, may peak in this range., Accepted for publication in ApJL, updated to match accepted version

We report deep Chandra X-ray Observatory (CXO), Hubble Space Telescope (HST), and Karl J. Jansky Very Large Array (VLA) observations of the binary neutron star event GW170817 at t < 160 days after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency νc is above the X-ray band and the synchrotron frequency νm is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons $N(\gamma )\propto {\gamma }^{-p}$ accelerated by a shock launched by a neutron star (NS)–NS merger to date. We find p = 2.17 ± 0.01, which indicates that radiation from ejecta with Γ ~ 3–10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the non-thermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal short gamma-ray burst (SGRB) directed away from our line of sight. Observations at t ≤ 200 days are unlikely to settle the debate, as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

We present new observations of the binary neutron star merger GW170817 at $\Delta t\approx 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $\gtrsim5\sigma$ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until $\sim 1000$ days post-merger., Comment: Accepted to ApJL. Updated version includes new VLA observations extending through 2018 June 2

The energy source powering the extreme optical luminosity of hydrogen-stripped Superluminous Supernovae (SLSNe-I) is not known, but recent studies have highlighted the case for a central engine. Radio and/or X-ray observations are best placed to track the fastest ejecta and probe the presence of outflows from a central engine. We compile all the published radio observations of SLSNe-I to date and present three new observations of two new SLSNe-I. None were detected. Through modeling the radio emission, we constrain the sub-parsec environments and possible outflows in SLSNe-I. In this sample we rule out on-axis collimated relativistic jets of the kind detected in Gamma-Ray Bursts (GRBs). We constrain off-axis jets with opening angles of 5\arcdeg\ (30\arcdeg) to energies of $\rm{E_k, Accepted by ApJ, 14 pages, 6 figures

The joint detection of gravitational waves and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of physical processes: heavy element synthesis via the $r$-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the "standard siren" technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC4993. In this paper we measure the surface brightness fluctuation (SBF) distance to NGC4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel on the Hubble Space Telescope (HST). For the preferred F110W passband we derive a distance modulus of $m{-}M=33.05\pm0.08\pm0.10$ mag, or a linear distance $d=40.7\pm1.4\pm1.9$ Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC4993 implies a Hubble constant $H_0=71.9\pm 7.1~km~s^{-1}~Mpc^{-1}$. A comparison of our result to the GW-inferred value of $H_0$ indicates a binary orbital inclination of $i\,{\gtrsim}\,137~\deg$. The SBF technique can be applied to early-type host galaxies of BNS mergers to ${\sim\,}100$ Mpc with HST and possibly as far as ${\sim\,}300$ Mpc with the James Webb Space Telescope, thereby helping to break the inherent distance-inclination degeneracy of the GW signals at distances where many future BNS mergers are likely to be detected., Comment: ApJ Letters in press

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational-wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the Southern Astrophysical Research and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly fading blue component (T ≈ 5500 K at 1.5 days) that quickly reddens; spectra later than ≳4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ∼7900 Å; at t ≲ 4.5 days. The colors, rapid evolution, and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light r-process nuclei with atomic mass number A ≲ 140. This indicates a sightline within θobs ≲ 45° of the orbital axis. Comparison to models suggests ∼0.03 M o of blue ejecta, with a velocity of . The required lanthanide fraction is ∼10-4, but this drops to, La lista completa de autores que integran el documento puede consultarse en el archivo, Instituto de Astrofísica de La Plata

We present a comprehensive comparison of the properties of the radio through X-ray counterpart of GW170817 and the properties of short-duration gamma-ray bursts (GRBs). For this effort, we utilize a sample of 36 short GRBs spanning a redshift range of $z \approx 0.12-2.6$ discovered over 2004-2017. We find that the counterpart to GW170817 has an isotropic-equivalent luminosity that is $\approx 3000$ times less than the median value of on-axis short GRB X-ray afterglows, and $\gtrsim10^{4}$ times less than that for detected short GRB radio afterglows. Moreover, the allowed jet energies and particle densities inferred from the radio and X-ray counterparts to GW170817 and on-axis short GRB afterglows are remarkably similar, suggesting that viewing angle effects are the dominant, and perhaps only, difference in their observed radio and X-ray behavior. From comparison to previous claimed kilonovae following short GRBs, we find that the optical and near-IR counterpart to GW170817 is comparatively under-luminous by a factor of $\approx 3-5$, indicating a range of kilonova luminosities and timescales. A comparison of the optical limits following short GRBs on $\lesssim 1$ day timescales also rules out a "blue" kilonova of comparable optical isotropic-equivalent luminosity in one previous short GRB. Finally, we investigate the host galaxy of GW170817, NGC4993, in the context of short GRB host galaxy stellar population properties. We find that NGC4993 is superlative in terms of its large luminosity, old stellar population age, and low star formation rate compared to previous short GRB hosts. Additional events within the Advanced LIGO/VIRGO volume will be crucial in delineating the properties of the host galaxies of NS-NS mergers, and connecting them to their cosmological counterparts., 11 Pages, 4 Figures, 1 Table. ApJL, In Press. Keywords: GW170817, LVC

We report the discovery of rising X-ray emission from the binary neutron star (BNS) merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave source. Observations acquired with the Chandra X-ray Observatory (CXO) at t~2.3 days post merger reveal no significant emission, with L_x, 7 Pages, 4 Figures, ApJL, In Press. Keywords: GW170817, LVC

The localization of the repeating FRB 121102 to a low-metallicity dwarf galaxy at $z=0.193$, and its association with a quiescent radio source, suggests the possibility that FRBs originate from magnetars, formed by the unusual supernovae in such galaxies. We investigate this via a comparison of magnetar birth rates, the FRB volumetric rate, and host galaxy demographics. We calculate average volumetric rates of possible millisecond magnetar production channels such as superluminous supernovae (SLSNe), long and short gamma-ray bursts (GRBs), and general magnetar production via core-collapse supernovae. For each channel we also explore the expected host galaxy demographics using their known properties. We determine for the first time the number density of FRB emitters (the product of their volumetric birthrate and lifetime), $R_{\rm FRB}\tau\approx 10^4$Gpc$^{-3}$, assuming that FRBs are predominantly emitted from repetitive sources similar to FRB 121102 and adopting a beaming factor of 0.1. By comparing rates we find that production via rare channels (SLSNe, GRBs) implies a typical FRB lifetime of $\approx$30-300 yr, in good agreement with other lines of argument. The total energy emitted over this time is consistent with the available energy stored in the magnetic field. On the other hand, any relation to magnetars produced via normal core-collapse supernovae leads to a very short lifetime of $\approx$0.5yr, in conflict with both theory and observation. We demonstrate that due to the diverse host galaxy distributions of the different progenitor channels, many possible sources of FRB birth can be ruled out with $\lesssim 10$ host galaxy identifications. Conversely, targeted searches of galaxies that have previously hosted decades-old SLSNe and GRBs may be a fruitful strategy for discovering new FRBs and related quiescent radio sources, and determining the nature of their progenitors.

We present detailed multi-frequency, multi-epoch radio observations of GRB 140304A at $z=5.283$ from 1 to 86 GHz and 0.45 d to 89 d. The radio and mm data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multi-wavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to $E_{\rm K, iso}\approx4.9\times10^{54}\,$erg, $A_* \approx 2.6\times10^{-2}$, $\epsilon_{\rm e}\approx2.5\times10^{-2}$, $\epsilon_{\rm B}\approx5.9\times10^{-2}$, $p\approx2.6$, and $\theta_{\rm jet}\approx1.1^{\circ}$, yielding a beaming corrected $\gamma$-ray and kinetic energy, $E_{\gamma}\approx2.3\times10^{49}\,$erg and $E_{\rm K}\approx9.5\times10^{50}\,$erg, respectively. We model the excess radio emission as due to a combination of a late-time reverse shock (RS) launched by a shell collision, which also produces a re-brightening in the X-rays at $\approx0.26\,$d, and either a standard RS or diffractive interstellar scintillation. Under the standard RS interpretation, we invoke consistency arguments between the forward and reverse shocks to derive a deceleration time, $t_{\rm dec}\approx100\,$s, the ejecta Lorentz factor, $\Gamma(t_{\rm dec})\approx300$, and a low RS magnetization, $R_{\rm B}\approx0.6$. Our observations highlight both the power of radio observations in capturing RS emission and thus constraining the properties of GRB ejecta and central engines, and the challenge presented by interstellar scintillation in conclusively identifying RS emission in GRB radio afterglows., Comment: 24 pages, 17 figures, 7 tables; submitted to ApJ