The Swift satellite lives up to its name, revealing cosmic explosions as they happen

Gamma-ray Bursts are the most powerful objects in the Universe. Discovered in the 1960’s as brief flashes of gamma-radiation, we now know they emit across the entire electromagnetic spectrum, are located in distant galaxies and comprise two distinct populations, one of which may originate in the deaths of massive stars. The launch of the Swift satellite in 2004 has brought a flurry of new discoveries, advancing our understanding of these sources and the galaxies that host them. We highlight a number of important results from the Swift era thus far

Calibration of EFOSC2 Broadband Linear Imaging Polarimetry

The European Southern Observatory Faint Object Spectrograph and Camera v2 is one of the workhorse instruments on ESO’s New Technology Telescope, and is one of the most popular instruments at La Silla observatory. It is mounted at a Nasmyth focus, and therefore exhibits strong, wavelength and pointing-direction-dependent instrumental polarisation. In this document, we describe our efforts to calibrate the broadband imaging polarimetry mode, and provide a calibration for broadband B, V, and R filters to a level that satisfies most use cases (i.e. polarimetric calibration uncertainty ~0.1%). We make our calibration codes public. This calibration effort can be used to enhance the yield of future polarimetric programmes with the European Southern Observatory Faint Object Spectrograph and Camera v2, by allowing good calibration with a greatly reduced number of standard star observations. Similarly, our calibration model can be combined with archival calibration observations to post-process data taken in past years, to form the European Southern Observatory Faint Object Spectrograph and Camera v2 legacy archive with substantial scientific potential

Can X-ray emission powered by a spinning-down magnetar explain some gamma-ray burst light-curve features?

Long-duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray emission arises from internal shocks in the jet or magnetized outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetized millisecond pulsar or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission becomes fairly constant followed by a steep decay when the magnetar collapses. We assume that the collapse of a massive star to a magnetar can launch the initial jet. By automatically fitting the X-ray light curves of all GRBs observed by the Swift satellite, we identified a subset of bursts which have a feature in their light curves which we call an internal plateau – unusually constant emission followed by a steep decay – which may be powered by a magnetar. We use the duration and luminosity of this internal plateau to place limits on the magnetar spin period and magnetic field strength, and find that they are consistent with the most extreme predicted values for magnetars

On the nature of late X-ray flares in Swift gamma-ray bursts

Context. Previously detected in only a few gamma-ray bursts (GRBs), X-ray flares are now observed in $\sim$50% of Swift GRBs, though their origins remain unclear. Most flares are seen early on in the afterglow decay, while some bursts exhibit flares at late times of 104 to 105 s, which may have implications for flare models. Aims. We investigate whether a sample of late time ($\gtrsim$ $1 \times 10^4$ s) flares are different from previous samples of early time flares, or whether they are merely examples on the tail of the early flare distribution. Methods. We examine the X-ray light curves of Swift bursts for late flares and compare the flare and underlying temporal power-law properties with those of early flares, and the values of these properties predicted by the blast wave model. Results. The burst sample shows late flare properties consistent with those of early flares, where the majority of the flares can be explained by either internal or external shock, though in a few cases one origin is favoured over the other. The underlying power-laws are mainly consistent with the normal decay phases of the afterglow. Conclusions. If confirmed by the ever growing sample of late time flares, this would imply that, in some cases, prolonged activity out to a day or a restarting of the central engine is required

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

The Swift Burst Analyser I. BAT and XRT spectral and flux evolution of gamma ray bursts

Context: Gamma ray burst models predict the broadband spectral evolution and the temporal evolution of the energy flux. In contrast, standard data analysis tools and data repositories provide count-rate data, or use single flux conversion factors for all of the data, neglecting spectral evolution. Aims: We produce Swift BAT and XRT light curves in flux units, where the spectral evolution is accounted for. Methods: We have developed software to use the hardness ratio information to track spectral evolution of GRBs, and thus to convert the count-rate light curves from the BAT and XRT instruments on Swift into accurate, evolution-aware flux light curves. Results: The Swift Burst Analyser website (http://www.swift.ac.uk/burst_analyser) contains BAT, XRT and combined BAT-XRT flux light curves in three energy regimes for all GRBs observed by the Swift satellite. These light curves are automatically built and updated when data become available, are presented in graphical and plain-text format, and are available for download and use in research

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

GRB 090618: Detection of thermal X-ray emission from a bright gamma-ray burst

GRB 090618 was an extremely bright burst, detected across the electromagnetic spectrum. It has a redshift of 0.54 and a supernova (SN) was identified in ground-based photometry. We present a thorough analysis of the prompt and early afterglow emission using data from Swift, Fermi Gamma-ray Burst Monitor and ROTSE, in which we track the evolution of the synchrotron spectral peak during the prompt emission and through the steep decay phase. We find evidence of a thermal X-ray component alongside the expected non-thermal power-law continuum. Such a component is rare among gamma-ray bursts (GRBs), with firm data for only GRBs 060218 and 100316D so far, and could potentially originate from an SN shock breakout, although there remains doubt regarding this explanation for any of the bursts. However, in contrast to these other Swift GRB–SNe with similar thermal signatures, GRB 090618 is a much more ‘typical’ burst: GRB–SNe 060218 and 100316D were both low-luminosity events, with long durations and low peak energies, while GRB 090618 was more representative of the wider population of long GRBs in all of these areas. It has been argued, based both on theory and observations, that most long GRBs should be accompanied by SNe. If this thermal X-ray component is related to the SN, its detection in GRB 090618, a fairly typical burst in many ways, may prove an important development in the study of the GRB–SN connection

A case of mistaken identity? GRB 060912A and the nature of the long-short GRB divide

We investigate the origin of the GRB 060912A, which has observational properties that make its classification as either a long or short burst ambiguous. Short-duration gamma-ray bursts (SGRBs) are thought to have typically lower energies than long-duration bursts, can be found in galaxies with populations of all ages and are likely to originate from different progenitors to the long-duration bursts. However, it has become clear that duration alone is insufficient to make a distinction between the two populations in many cases, leading to a desire to find additional discriminators of burst type. GRB 060912A had a duration of 6 s and occurred only ∼10 arcsec from a bright, low-redshift (z= 0.0936) elliptical galaxy, suggesting that this may have been the host, which would favour it being a short burst. However, our deep optical imaging and spectroscopy of the location of GRB 060912A using the Very Large Telescope (VLT) shows that GRB 060912A more likely originates in a distant star-forming galaxy at z= 0.937, and is most likely a long burst. This demonstrates the risk in identifying bright, nearby galaxies as the hosts of given gamma-ray bursts (GRBs) without further supporting evidence. Further, it implies that, in the absence of secure identifications, ‘host’ type, or more broadly discriminators that rely on galaxy redshifts, may not be good indicators of the true nature of any given GRB

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