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The bright, blue, rapidly evolving AT 2018cow is a well-studied peculiar extragalactic transient. Despite an abundance of multiwavelength data, there still is no consensus on the nature of the event. We present our analysis of three epochs of Hubble Space Telescope (HST) observations spanning the period from 713 to 1474 d post-burst, paying particular attention to uncertainties of the transient photometry introduced by the complex background in which AT 2018cow resides. Photometric measurements show evident fading in the UV and more subtle but significant fading in the optical. During the last HST observation, the transient's optical/UV colours were still bluer than those of the substantial population of compact, young, star-forming regions in the host of AT 2018cow, suggesting some continued transient contribution to the light. However, a compact source underlying the transient would substantially modify the resulting spectral energy distribution, depending on its contribution in the various bands. In particular, in the optical filters, the complex, diffuse background poses a problem for precise photometry. An underlying cluster is expected for a supernova occurring within a young stellar environment or a tidal-disruption event (TDE) within a dense older one. While many recent works have focused on the supernova interpretation, we note the substantial similarity in UV light-curve morphology between AT 2018cow and several tidal disruption events around supermassive black holes. Assuming AT 2018cow arises from a TDE-like event, we fit the late-time emission with a disc model and find M BH = 103.2 ± 0.8 M⊙. Further observations are necessary to determine the late-time evolution of the transient and its immediate environment. [ABSTRACT FROM AUTHOR]

We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles. We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, θ c. We fit model and observed afterglow light curves with a smoothly-broken power-law function with a free-parameter κ that describes the sharpness of the break. We find that the jet break is sharper (κ is greater) when lateral spreading is included than in the absence of lateral spreading. For profiles with a sharp-edged core, the sharpness parameter has a broad range of 0.1 ≲ κ ≲ 4.6, whereas profiles with a smooth-edged core have a narrower range of 0.1 ≲ κ ≲ 2.2 when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within θ c, whereas for smooth-edged jets, κ is more strongly dependent on the size of θ c. Using a sample of 20 GRBs, we find 9 candidate smooth-edged jet structures and 8 candidate sharp-edged jet structures, while the remaining 3 are consistent with either. The shape of the jet break, as measured by the sharpness parameter κ , can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time. [ABSTRACT FROM AUTHOR]

Energy injection into the external shock system that generates the afterglow to a gamma-ray burst (GRB) can result in a rebrightening of the emission. Here we investigate the off-axis view of a rebrightened refreshed shock afterglow. We find that the afterglow light curve, when viewed from outside of the jet opening angle, could be characterized by a slow rise, or long plateau, with a maximum flux determined by the total system energy. Using the broadband afterglow data for GRB 170817A, associated with the gravitational-wave-detected binary neutron star merger GW170817, we show that a refreshed shock model with a simple top-hat jet can reproduce the observed afterglow features. We consider two refreshed shock models: a single episode of energy injection, and an episode of continuous energy injection. The best-fit model parameters give a jet opening angle for our first and second models, respectively, of θj = 5.° and 6.° , an inclination to the line of sight ι = 16.° and 17.° , an initial on-axis isotropic equivalent kinetic energy and erg, and a total/final, on-axis isotropic equivalent refreshed shock energy and erg. The first model fitting prefers an initial bulk Lorentz factor Γ0,1 < 60, with a comparatively low central value of Γ0,1 = 19.5, indicating that, in this case, the on-axis jet could have been a "failed GRB." Alternatively, our second model is consistent with a bright GRB for an on-axis observer, with. Due to the low Lorentz factor and/or the jet opening angles at θj ∼ ι/3, both models are unable to reproduce the γ-ray emission observed in GRB 170817A, which would therefore require an alternative explanation such as cocoon shock breakout. [ABSTRACT FROM AUTHOR]

We report the detection of the radio afterglow of a long gamma-ray burst (GRB) 111005A at 5-345 GHz, including very long baseline interferometry observations with a positional error of 0.2 mas. The afterglow position is coincident with the disc of a galaxy ESO 58049 at z = 0.01326 (∼1″ from its centre), which makes GRB 111005A the second-closest GRB known to date, after GRB 980425. The radio afterglow of GRB 111005A was an order of magnitude less luminous than those of local low-luminosity GRBs, and obviously less luminous than those of cosmological GRBs. The radio flux was approximately constant and then experienced an unusually rapid decay a month after the GRB explosion. Similarly to only two other GRBs, we did not find the associated supernovae (SNe), despite deep near- and mid-infrared observations 1-9 days after the GRB explosion, reaching ∼20 times fainter than other SNe associated with GRBs. Moreover, we measured a twice-solar metallicity for the GRB location. The low y-ray and radio luminosities, rapid decay, lack of a SN, and super-solar metallicity suggest that GRB 111005A represents a rare class of GRB that is different from typical core-collapse events. We modelled the spectral energy distribution of the GRB 111005A host finding that it is a moderately star-forming dwarf galaxy, similar to the host of GRB 980425. The existence of two local GRBs in such galaxies is still consistent with the hypothesis that the GRB rate is proportional to the cosmic star formation rate (SFR) density, but suggests that the GRB rate is biased towards low SFRs. Using the far-infrared detection of ESO 580-49, we conclude that the hosts of both GRBs 111005A and 980425 exhibit lower dust content than what would be expected from their stellar masses and optical colors. [ABSTRACT FROM AUTHOR]

We present new Jansky Very Large Array observations of five pre-Swift gamma-ray bursts for which an ultraluminous [star formation rate (SFR) >100 M⊙ yr-1] dusty host galaxy had previously been inferred from radio or submillimetre observations taken within a few years after the burst. In four of the five cases, we no longer detect any source at the host location to limits much fainter than the original observations, ruling out the existence of an ultraluminous galaxy hosting any of these gamma-ray bursts (GRBs). We continue to detect a source at the position of GRB 980703, but it is much fainter than it was a decade ago and the inferred radio SFR (~80 M⊙) is relatively modest. The radio flattening at 200-1000 d observed in the light curve of this GRB may have been caused by a decelerating counterjet oriented 180 deg away from the viewer, although an unjetted wind model can also explain the data. Our results eliminate all well-established ultraluminous infrared galaxies (ULIRGs) among the pre-Swift host population. They also rule out all cases for which an unobscured GRB was found in a galaxy dominated by heavily obscured star formation. When GRBs do occur in ULIRGs, the afterglow is almost always observed to be heavily obscured, consistent with the large dust opacities and high dust covering fractions characteristic of these systems. [ABSTRACT FROM AUTHOR]

While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using a deep Hubble Space Telescope/Advanced Camera for Surveys data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo a fields which sample diverse substructures. In agreement with our previous analysis of colour- 3 magnitude diagram morphologies, we find the resultant behaviours can be broadly separated a into two categories. The SFHs of 'disc-like' fields indicate that most of their mass has formed 33 since z ~ 1, with one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' " fields to be on average 1.5 Gyr older, with <10 per cent of their stellar mass formed within § the last 5 Gyr. These fields are also characterized by an age-metallicity relation showing rapid a chemical enrichment to solar metallicity by z = 1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all 2 the fields studied here. The presence of these young stars in our most remote fields suggests 5 that they have not formed in situ but have been kicked-out from the thin disc through disc heating in the recent past. [ABSTRACT FROM AUTHOR]

For gamma-ray burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: 1 h, 1 d, 3 d and 1 week after the burst, thanks to the afterglow remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe II, and from metastable levels of Fe II and Ni II at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni II 4F9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the ultraviolet radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems. [ABSTRACT FROM AUTHOR]

The brightness of gamma-ray burst (GRB) afterglows and their occurrence in young, blue galaxies make them excellent probes to study star-forming regions in the distant universe. We here elucidate dust extinction properties in the early universe through the analysis of the afterglows of all known z > 6 GRBs: GRB 090423, 080913, and 050904, at z = 8.2, 6.69, and 6.295, respectively. We gather all available optical and near-infrared photometry, spectroscopy, and X-ray data to construct spectral energy distributions (SEDs) at multiple epochs. We then fit the SEDs at all epochs with a dust-attenuated power law or broken power law. We find no evidence for dust extinction in GRB 050904 and GRB 090423, with possible evidence for a low level of extinction in GRB 080913. We compare the high redshift GRBs to a sample of lower redshift GRB extinctions and find a lack of even moderately extinguished events (AV ~ 0.3) above z ≳ 4. In spite of the biased selection and small number statistics, this result hints at a decrease in dust content in star-forming environments at high redshifts. [ABSTRACT FROM AUTHOR]

We present the results from our Swift/VLT legacy survey, a VLT Large Programme aimed at characterizing the host galaxies of a homogeneously selected sample of Swift gamma-ray bursts (GRBs). The immediate goals are to determine the host luminosity function, study the effects of reddening, determine the fraction of Lyα emitters in the hosts, and obtain redshifts for targets without a reported one. We have carefully selected a sample, obeying strict and well-defined criteria: 69 targets in total. Among the results is a large optical detection rate, the lack of extremely red objects (only one possible case in the sample), and 15 new GRB redshifts with the mean redshift of the host sample assessed to be 〈z〉 ≳ 2.2. [ABSTRACT FROM PUBLISHER]

ABSTRACT GRB 090423 is the most distant spectroscopically confirmed source observed in the Universe. Using observations at 37.5 GHz, we place constraints on molecular gas emission in the CO (3-2) line from its host galaxy and immediate environs. The source was not detected either in line emission or in the rest-frame 850-μm continuum, yielding an upper limit of S8mm= 9.3 μJy and M(H2) <4.3 × 109 M⊙ (3σ), applying standard conversions. This implies that the host galaxy of GRB 090423 did not possess a large reservoir of warm molecular gas but was rather modest either in star formation rate or in mass. It suggests that this was not an extreme starburst and hence that gamma ray bursts at high redshift trace relatively modest star formation rates, in keeping with the behaviour seen at lower redshifts. We do, however, identify a millimetre emission-line source in the field of GRB 090423. Plausible interpretations include a CO (1-0) emitting galaxy at z= 2.1, CO (2-1) at z= 5.2 and CO (3-2) at z= 8.3. Efforts to identify a counterpart for the molecular line emitter and to further characterize this source are continuing. [ABSTRACT FROM AUTHOR]

There is increasing evidence of a local population of short duration gamma-ray bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible luminosity functions (LFs) for both neutron star binary mergers and giant flares from soft gamma repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall Burst and Transient Source Experiment (BATSE) sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best-fitting parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population. [ABSTRACT FROM AUTHOR]

The same as Figure 1 in the published article but with the time for black crosses and circles found using the correct Equation (2). 1 Copyright of Astrophysical Journal is the property of IOP Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. Erratum: GRB170817A as a Refreshed Shock Afterglow Viewed Off-axis (2020, ApJ, 899, 105) Gavin P. Lamb 1, Andrew J. Levan 2, and Nial R. Tanvir 1 1 School of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK; gpl6@le.ac.uk 2 Department of Astrophysics, Radboud University, 6525 AJ Nijmegen, The Netherlands Received 2021 March 4; published 2021 April 7 Equation (2) in the published article is incorrect. The comparison points shift slightly within Figure 1 of the published article, and are updated in Figure 1 here with the values found using Equation (2). [Extracted from the article]

We present simultaneous optical and near-infrared (NIR) spectroscopy of 19 Swift γ-ray burst (GRB) host galaxies observed with the VLT/X-shooter with the aim of measuring their redshifts. Galaxies were selected from The Optically Unbiased GRB Host (TOUGH) survey (15 of the 19 galaxies) or because they hosted GRBs without a bright optical afterglow. Here we provide emission-line redshifts for 13 of the observed galaxies with brightnesses between F606W > 27 mag and R = 22.9 mag (median ). The median redshift is for all hosts and for the TOUGH hosts. Our new data significantly improve the redshift completeness of the TOUGH survey, which now stands at 77% (53 out of 69 GRBs). They furthermore provide accurate redshifts for nine prototype dark GRBs (e.g., GRB 071021 at z = 2.452 and GRB 080207 at z = 2.086), which are exemplary of GRBs where redshifts are challenging to obtain via afterglow spectroscopy. This establishes X-shooter spectroscopy as an efficient tool for redshift determination of faint, star-forming, high-redshift galaxies such as GRB hosts. It is hence a further step toward removing the bias in GRB samples that is caused by optically dark events, and provides the basis for a better understanding of the conditions in which GRBs form. The distribution of column densities as measured from X-ray data (NH, X), for example, is closely related to the darkness of the afterglow and skewed toward low NH, X values in samples that are dominated by bursts with bright optical afterglows. [ABSTRACT FROM AUTHOR]

Long-duration gamma-ray bursts (GRBs) are powerful tracers of star-forming galaxies. We have defined a homogeneous subsample of 69 Swift GRB-selected galaxies spanning a very wide redshift range. Special attention has been devoted to making the sample optically unbiased through simple and well-defined selection criteria based on the high-energy properties of the bursts and their positions on the sky. Thanks to our extensive follow-up observations, this sample has now achieved a comparatively high degree of redshift completeness, and thus provides a legacy sample, useful for statistical studies of GRBs and their host galaxies. In this paper, we present the survey design and summarize the results of our observing program conducted at the ESO Very Large Telescope (VLT) aimed at obtaining the most basic properties of galaxies in this sample, including a catalog of R and Ks magnitudes and redshifts. We detect the host galaxies for 80% of the GRBs in the sample, although only 42% have Ks-band detections, which confirms that GRB-selected host galaxies are generally blue. The sample is not uniformly blue, however, with two extremely red objects detected. Moreover, galaxies hosting GRBs with no optical/NIR afterglows, whose identification therefore relies on X-ray localizations, are significantly brighter and redder than those with an optical/NIR afterglow. This supports a scenario where GRBs occurring in more massive and dusty galaxies frequently suffer high optical obscuration. Our spectroscopic campaign has resulted in 77% now having redshift measurements, with a median redshift of 2.14 ± 0.18. TOUGH alone includes 17 detected z > 2 Swift GRB host galaxies suitable for individual and statistical studies—a substantial increase over previous samples. Seven hosts have detections of the Lyα emission line and we can exclude an early indication that Lyα emission is ubiquitous among GRB hosts, but confirm that Lyα is stronger in GRB-selected galaxies than in flux-limited samples of Lyman break galaxies. [ABSTRACT FROM AUTHOR]

We report the discovery by the Swift hard X-ray monitor of the transient source Swift J2058.4+0516 (Sw J2058+05). Our multi-wavelength follow-up campaign uncovered a long-lived (duration ≳ months), luminous X-ray (LX, iso ≈ 3 × 1047 erg s–1) and radio (νLν, iso ≈ 1042 erg s–1) counterpart. The associated optical emission, however, from which we measure a redshift of 1.1853, is relatively faint, and this is not due to a large amount of dust extinction in the host galaxy. Based on numerous similarities with the recently discovered GRB 110328A/Swift J164449.3+573451 (Sw J1644+57), we suggest that Sw J2058+05 may be the second member of a new class of relativistic outbursts resulting from the tidal disruption of a star by a supermassive black hole. If so, the relative rarity of these sources (compared with the expected rate of tidal disruptions) implies that either these outflows are extremely narrowly collimated (θ < 1°) or only a small fraction of tidal disruptions generate relativistic ejecta. Analogous to the case of long-duration gamma-ray bursts and core-collapse supernovae, we speculate that rapid spin of the black hole may be a necessary condition to generate the relativistic component. Alternatively, if powered by gas accretion (i.e., an active galactic nucleus (AGN)), Sw J2058+05 would seem to represent a new mode of variability in these sources, as the observed properties appear largely inconsistent with known classes of AGNs capable of generating relativistic jets (blazars, narrow-line Seyfert 1 galaxies). [ABSTRACT FROM AUTHOR]

The two closest gamma-ray bursts so far detected (GRBs 980425 and 060218) were both under-luminous, spectrally soft, long-duration bursts with smooth, single-peaked light curves. Only of the order of 100 GRBs have measured redshifts, and there are, for example, 2704 GRBs in the BATSE (Burst and Transient Source Experiment) catalogue alone. It is therefore plausible that other nearby GRBs have been observed but not identified as being relatively nearby. Here we search for statistical correlations between BATSE long-duration GRBs and galaxy samples with recession velocities selected from two catalogues of nearby galaxies. We also examine the correlations using burst subsamples restricted to those with properties similar to the two known nearby bursts. Our results show correlation of the entire long-GRB sample to remain consistent with zero out to the highest radii considered whereas a subsample selected to be low fluence, spectrally soft, with smooth single-peaked light curves (177 bursts), demonstrates increased correlation with galaxies within ≈155 Mpc. The measured correlation (28 ± 16 per cent of the sample) suggests that BATSE observed between two and nine long-duration GRBs per year similar to, and from within similar distances to, GRBs 980425 and 060218. This implies an observed local rate density (to BATSE limits) of 700 ± 360 Gpc−3 yr−1 within 155 Mpc. [ABSTRACT FROM AUTHOR]

Recent progress on the nature of short-duration gamma-ray bursts has shown that a fraction of them originate in the local Universe. These systems may well be the result of giant flares from soft gamma-repeaters (highly magnetized neutron stars commonly known as magnetars). However, if these neutron stars are formed via the core collapse of massive stars then it would be expected that the bursts should originate from predominantly young stellar populations, while correlating the positions of BATSE short bursts with structure in the local Universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars formed via the core collapse of massive stars, they also form via accretion-induced collapse following the merger of two white dwarfs, one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via white dwarf–white dwarf (WD–WD) mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of production of magnetars by core collapse is proportional to the star formation rate, the rate of production via WD–WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD–WD mergers may produce soft gamma-repeater giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short-duration burst population at higher redshift. [ABSTRACT FROM AUTHOR]