Implications for the origin of short gamma-ray bursts from their observed positions around their host galaxies

We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them with the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence, the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts, the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies. One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is misidentified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters

Constraining the molecular gas in the environs of a z ~ 8 gamma-ray burst host galaxy

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

Dust reddening and extinction curves toward gamma-ray bursts at z > 4

Context. Dust is known to be produced in the envelopes of asymptotic giant branch (AGB) stars, the expanded shells of supernova (SN) remnants, and in situ grain growth within the interstellar medium (ISM), although the corresponding efficiency of each of these dust formation mechanisms at different redshifts remains a topic of debate. During the first Gyr after the Big Bang, it is widely believed that there was not enough time to form AGB stars in high numbers, hence the dust at this epoch is expected to be purely from SNe or subsequent grain growth in the ISM. The time period corresponding to z ~ 5−6 is thus expected to display the transition from SN-only dust to a mixture of both formation channels as is generally recognized at present. Aims. Here we aim to use afterglow observations of gamma-ray bursts (GRBs) at redshifts larger than z > 4 to derive host galaxy dust column densities along their line of sight and to test if a SN-type dust extinction curve is required for some of the bursts. Methods. We performed GRB afterglow observations with the seven-channel Gamma-Ray Optical and Near-infrared Detector (GROND) at the 2.2 m MPI telescope in La Silla, Chile (ESO), and we combined these observations with quasi-simultaneous data gathered with the XRT telescope on board the Swift satellite. Results. We increase the number of measured AV values for GRBs at z > 4 by a factor of ~2–3 and find that, in contrast to samples at mostly lower redshift, all of the GRB afterglows have a visual extinction of AV < 0.5 mag. Analysis of the GROND detection thresholds and results from a Monte Carlo simulation show that although we partly suffer from an observational bias against highly extinguished sight-lines, GRB host galaxies at 4 < z < 6 seem to contain on average less dust than at z ~ 2. Additionally, we find that all of the GRBs can be modeled with locally measured extinction curves and that the SN-like dust extinction curve, as previously found toward GRB 071025, provides a better fit for only two of the afterglow SEDs. However, because of the lack of highly extinguished sight lines and the limited wavelength coverage we cannot distinguish between the different scenarios. For the first time we also report a photometric redshift of zphot = 7.88-0.94+0.75 for GRB 100905A, making it one of the most distant GRBs known to date

Soft gamma repeaters and short gamma ray bursts: Making magnetars from WD-WD mergers

Recent progress on the nature of short duration Gamma Ray Bursts (GRBs) 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 (SGRs) believed to be magnetars (neutron stars with extremely large magnetic fields ⩾10^14 G). If these magnetars are formed via the core collapse of massive stars, then it would be expected that the bursts should originate from predominantly young stellar populations. However, 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 forming 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 WD‐WD mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of magnetar production 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 SGR 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 GRB population at low redshift

Polarimetry of the transient relativistic jet of GRB 110328/Swift J164449.3+573451

We present deep infrared (Ks-band) imaging polarimetry and radio (1.4- and 4.8-GHz) polarimetry of the enigmatic transient Swift J164449.3+573451. This source appears to be a short-lived jet phenomenon in a galaxy at redshift z= 0.354, activated by a sudden mass accretion on to the central massive black hole, possibly caused by the tidal disruption of a star. We aim to find evidence for this scenario through linear polarimetry, as linear polarization is a sensitive probe of jet physics, source geometry and the various mechanisms giving rise to the observed radiation. We find a formal Ks-band polarization measurement of Plin= 7.4 ± 3.5 per cent (including systematic errors). Our radio observations show continuing brightening of the source, which allows sensitive searches for linear polarization as a function of time. We find no evidence of linear polarization at radio wavelengths of 1.4 and 4.8 GHz at any epoch, with the most sensitive 3σ limits as deep as 2.1 per cent. These upper limits are in agreement with expectations from scenarios in which the radio emission is produced by the interaction of a relativistic jet with a dense circumsource medium. We further demonstrate how polarization properties can be used to derive properties of the jet in Swift J164449.3+573451, exploiting the similarities between this source and the afterglows of gamma-ray bursts

GRB070125 and the environments of spectral-line poor afterglow absorbers

GRB 070125 is among the most energetic bursts detected and the most extensively observed so far. Nevertheless, unresolved issues are still open in the literature on the physics of the afterglow and on the gamma-ray burst (GRB) environment. In particular, GRB 070125 was claimed to have exploded in a galactic halo environment, based on the uniqueness of the optical spectrum and the non-detection of an underlying host galaxy. In this work we collect all publicly available data and address these issues by modelling the near-infrared to X-ray spectral energy distribution (SED) and studying the high signal-to-noise ratio Very Large Telescope/FOcal Reducer/low dispersion Spectrograph afterglow spectrum in comparison with a larger sample of GRB absorbers. The SED reveals a synchrotron cooling break in the ultraviolet, low equivalent hydrogen column density and little reddening caused by a Large Magellanic Cloud type or Small Magellanic Cloud type extinction curve. From the weak Mg ii absorption at z= 1.5477 in the spectrum, we derived log N(Mg ii) = 12.96+0.13− 0.18 and upper limits on the ionic column density of several metals. These suggest that the GRB absorber is most likely a Lyman limit system with a 0.03 < Z < 1.3 Z⊙ metallicity. The comparison with other GRB absorbers places GRB 070125 at the low end of the absorption-line equivalent width distribution, confirming that weak spectral features and spectral-line poor absorbers are not so uncommon in afterglow spectra. Moreover, we show that the effect of photoionization on the gas surrounding the GRB, combined with a low N(H i) along a short segment of the line of sight within the host galaxy, can explain the lack of spectral features in GRB 070125. Finally, the non-detection of an underlying galaxy is consistent with a faint GRB host galaxy, well within the GRB host brightness distribution. Thus, the possibility that GRB 070125 is simply located in the outskirts of a gas-rich, massive star-forming region inside its small and faint host galaxy seems more likely than a gas-poor, halo environment origin

A multiwavelength study of the relativistic tidal disruption canidate Sw J2058+05 at late times

Swift J2058.4+0516 (Sw J2058+05, hereafter) has been suggested as the second member (after Sw J1644+57) of the rare class of tidal disruption events accompanied by relativistic ejecta. Here we report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of Sw J2058+05 from 3 months to 3 yr post-discovery in order to study its properties and compare its behavior with that of Sw J1644+57. Our main results are as follows. (1) The long-term X-ray light curve of Sw J2058+05 shows a remarkably similar trend to that of Sw J1644+57. After a prolonged power-law decay, the X-ray flux drops off rapidly by a factor of ≳160 within a span of Δt/t ≤ 0.95. Associating this sudden decline with the transition from super-Eddington to sub-Eddington accretion, we estimate the black hole mass to be in the range of 104−6 M⊙. (2) We detect rapid (≲500 s) X-ray variability before the dropoff, suggesting that, even at late times, the X-rays originate from close to the black hole (ruling out a forward-shock origin). (3) We confirm using HST and VLBA astrometry that the location of the source coincides with the galaxy's center to within ≲400 pc (in projection). (4) We modeled Sw J2058+05's ultraviolet/optical/infrared spectral energy distribution with a single-temperature blackbody and find that while the radius remains more or less constant at a value of 63.4±4.5 AU (∼1015 cm) at all times during the outburst, the blackbody temperature drops significantly from ∼ 30,000 K at early times to a value of ∼ 15,000 K at late times (before the X-ray dropoff). Our results strengthen Sw J2058+05's interpretation as a tidal disruption event similar to Sw J1644+57. For such systems, we suggest the rapid X-ray dropoff as a diagnostic for black hole mass

The host of GRB 060206: Kinematics of a distant galaxy

Context. GRB afterglow spectra are sensitive probes of interstellar matter along the line-of-sight in their host galaxies, as well as in intervening galaxies. The rapid fading of GRBs makes it very difficult to obtain spectra of sufficient resolution and S/N to allow for these kinds of studies. Aims. We investigate the state and properties of the interstellar medium in the host of GRB 060206 at z= 4.048 with a detailed study of groundstate and finestructure absorption lines in an early afterglow spectrum. This allows us to derive conclusions on the nature and origin of the absorbing structures and their connection to the host galaxy and/or the GRB. Methods. We used early (starting 1.6 h after the burst) WHT/ISIS optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 detecting a range of metal absorption lines and their finestructure transitions. Additional information is provided by the afterglow lightcurve. The resolution and wavelength range of the spectra and the bright afterglow have facilitated a detailed study and fitting of the absorption line systems in order to derive column densities. We also used deep imaging to detect the host galaxy and probe the nature of an intervening system at z = 1.48 seen in absorption in the afterglow spectra. Results. We detect four discrete velocity systems in the resonant metal absorption lines, best explained by shells within and/or around the host created by starburst winds. The finestructure lines have no less than three components with strengths decreasing from the redmost components. We therefore suggest that the finestructure lines are best explained as being produced by UV pumping from which follows that the redmost component is the one closest to the burst where \ion{N}{v} was detected as well. The host is detected in deep HST imaging with F814WAB = 27.48 $\pm$ 0.19 mag and a 3$\sigma$ upper limit of H = 20.6 mag (Vega) is achieved. A candidate counterpart for the intervening absorption system is detected as well, which is quite exceptional for an absorber in the sightline towards a GRB afterglow. The intervening system shows no temporal evolution as claimed by Hao et al. (2007, ApJ, 659, 99), which we prove from our WHT spectra taken before and Subaru spectra taken during those observations

The extreme, red afterglow of GRB 060923A: Distance or dust?

Gamma-ray bursts (GRBs) are powerful probes of the early Universe, but locating and identifying very distant GRBs remain challenging. We report here the discovery of the K-band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high-z burst (z≳ 11). However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely z≲ 2.8) and therefore that dust is the likely cause of the very red-afterglow colour. This being the case, it is one of the few instances so far found of a GRB afterglow with high-dust extinction

The early- and late-time spectral and temporal evolution of ORB 050716

We report on a comprehensive set of observations of gamma-ray burst 050716, detected by the Swift satellite and subsequently followed-up rapidly in X-ray, optical and near-infrared (NIR) wavebands. The prompt emission is typical of long-duration bursts, with two peaks in a time interval of T90= 68 s (15–350 keV). The prompt emission continues at lower flux levels in the X-ray band, where several smaller flares can be seen on the top of a decaying light curve that exhibits an apparent break around 220 s post-trigger. This temporal break is roughly coincident with a spectral break. The latter can be related to the extrapolated evolution of the break energy in the prompt γ-ray emission, and is possibly the manifestation of the peak flux break frequency of the internal shock passing through the observing band. A possible 3σ change in the X-ray absorption column is also seen during this time. The late-time afterglow behaviour is relatively standard, with an electron distribution power-law index of p= 2; there is no notable temporal break out to at least 10 d. The broad-band optical/NIR to X-ray spectrum indicates a redshift of z≳ 2 for this burst, with a host-galaxy extinction value of EB−V≈ 0.7 that prefers a small magellanic cloud (SMC)-like extinction curve