9 research outputs found
The optical rebrightening of GRB100814A: an interplay of forward and reverse shocks?
We present a wide dataset of -ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and
spectral properties of GRB100814 the best
The circumburst environment of a FRED GRB: Study of the prompt emission and X-ray/optical afterglow of GRB 051111
Aims.We report a multi-wavelength analysis of the prompt emission and early afterglow of GRB 051111 and discuss its properties in the context of current fireball models.
Methods.The detection of GRB 051111 by the Burst Alert Telescope on-board Swift triggered early BVRi' observations with the 2-m robotic Faulkes Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray Telescope.
Results.The prompt -ray emission shows a classical FRED profile. The optical afterglow light curves are fitted with a broken power law, with to and a break time around 12 min after the GRB. Although contemporaneous X-ray observations were not taken, a power law connection between the -ray tail of the FRED temporal profile and the late XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the optical decay, this would represent one of the first GRBs for which the canonical steep-shallow-normal decay typical of early X-ray afterglows has been monitored optically. We present a detailed analysis of the intrinsic extinction, elemental abundances and spectral energy distribution. From the absorption measured in the low X-ray band we find possible evidence for an overabundance of some elements such as oxygen, [O/Zn] = 0.7 0.3, or, alternatively, for a significant presence of molecular gas. The IR-to-X-ray Spectral Energy Distribution measured at 80 min after the burst is consistent with the cooling break lying between the optical and X-ray bands. Extensive modelling of the intrinsic extinction suggests dust with big grains or grey extinction profiles. The early optical break is due either to an energy injection episode or, less probably, to a stratified wind environment for the circumburst medium
PAN-CHROMATIC OBSERVATIONS OF THE RECURRENT NOVA LMC 2009a (LMC 1971b)
Nova LMC 2009a is confirmed as a recurrent nova (RN) from positional coincidence with nova LMC 1971b. The observational data set is one of the most comprehensive for any Galactic or extragalactic RN: optical and near-IR photometry from outburst until over 6 years later; optical spectra for the first 6 months, and Swift satellite ultraviolet (UV) and X-ray observations from 9 days to almost 1 year post-outburst. We find MV = â8.4 ± 0.8r ± 0.7s and expansion velocities between 1000 and 4000 km sâ1. Coronal line emission before day 9 indicates shocks in the ejecta. Strengthening of He iiλ4686 preceded the emergence of the super-soft source (SSS) in X-rays at ~63â70 days, which was initially very variable. Periodic modulations, P = 1.2 days, most probably orbital in nature, were evident in the UV and optical from day 43. Subsequently, the SSS shows an oscillation with the same period but with a delay of 0.28P. The progenitor system has been identified; the secondary is most likely a sub-giant feeding a luminous accretion disk. Properties of the SSS infer a white dwarf (WD) mass 1.1 Mâ lesssim MWD lesssim 1.3 Mâ. If the accretion occurs at a constant rate, yrâ1 is needed, consistent with nova models for an inter-eruption interval of 38 years, low outburst amplitude, progenitor position in the colorâmagnitude diagram, and spectral energy distribution at quiescence. We note striking similarities between LMC 2009a and the Galactic nova KT Eri, suggesting that KT Eri is a candidate RN
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 (zâł 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 zâČ 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
Varying linear polarisation in the dust-free GRB 210610B
Long gamma ray bursts (GRBs) are produced by the collapse of some very massive stars, which emit ultra-relativistic jets. When the jets collide with the interstellar medium they decelerate and generate the so-called afterglow emission, which has been observed to be polarised. In this work we study the polarimetric evolution of GRB 210610B afterglow, at z=1.1341. This allows to evaluate the role of geometric and/or magnetic mechanisms in the GRB afterglow polarisation. We observed GRB 210610B using imaging polarimetry with CAFOS on the 2.2 m Calar Alto Telescope and FORS2 on the 4 Ă 8.1 m Very Large Telescope. Complementary optical spectroscopy was obtained with OSIRIS on the 10.4 m Gran Telescopio Canarias. We study the GRB light-curve from X-rays to optical bands and the Spectral Energy Distribution (SED). This allows us to strongly constrain the line-of-sight extinction. Finally, we study the GRB host galaxy using optical/NIR data to fit the SED and derive its integrated properties. GRB 210610B had a bright afterglow with a negligible line-of-sight extinction. Polarimetry was obtained at three epochs: during an early plateau phase, at the time when the light curve breaks, and after the light curve steepened. We observe an initial polarisation of âŒ4% that goes to zero at the time of the break, and then increases again to âŒ2% with a change of the position angle of 54±9 deg. The spectrum show features with very low equivalent widths, indicating a small amount of material in the line-of-sight within the host. The lack of dust and the low amount of material on the line-of-sight to GRB 210610B allow us to study the intrinsic polarisation of the GRB optical afterglow. We find the GRB polarisation signals are consistent with ordered magnetic fields in refreshed shock or/and hydrodynamics-scale turbulent fields in the forward shock.</p
Anatomy of a dark burst - The afterglow of GRB 060108
We present a multiwavelength study of GRB 060108 â the 100th gamma-ray burst discovered by Swift. The X-ray flux and light curve (three segments plus a flare) detected with the X-ray Telescope are typical of Swift long bursts.
We report the discovery of a faint optical afterglow detected in deep BVRiâČ-band imaging obtained with the Faulkes Telescope North beginning 2.75 min after the burst. The afterglow is below the detection limit of the Ultraviolet/Optical Telescope within 100 s of the burst, while is evident in K-band images taken with the United Kingdom Infrared Telescope 45 min after the burst. The optical light curve is sparsely sampled. Observations taken in the R and iâČ bands can be fitted either with a single power-law decay in flux, F(t) âtâα where α= 0.43 ± 0.08, or with a two-segment light curve with an initial steep decay α1 < 0.88 ± 0.2, flattening to a slope α2⌠0.31 ± 0.12. A marginal evidence for rebrightening is seen in the iâČ band.
Deep R-band imaging obtained âŒ12 d post-burst with the Very Large Telescope reveals a faint, extended object (R⌠23.5âmag) at the location of the afterglow. Although the brightness is compatible with the extrapolation of the slow decay with index α2, significant flux is likely due to a host galaxy. This implies that the optical light curve had a break before 12 d, akin to what observed in the X-rays.
We derive the maximum photometric redshift z < 3.2 for GRB 060108. We find that the spectral energy distribution at 1000 s after the burst, from the optical to the X-ray range, is best fitted by a simple power law, FÎœâÎœâÎČ, with ÎČOX= 0.54 and a small amount of extinction. The optical to X-ray spectral index (ÎČOX) confirms GRB 060108 to be one of the optically darkest bursts detected. Our observations rule out a high redshift as the reason for the optical faintness of GRB 060108. We conclude that a more likely explanation is a combination of an intrinsic optical faintness of the burst, a hard optical to X-ray spectrum and a moderate amount of extinction in the host galaxy
Accessing the population of high-redshift Gamma Ray Bursts
Gamma Ray Bursts (GRBs) are a powerful probe of the high-redshift Universe. We present a tool to estimate the detection rate of high-z GRBs by a generic detector with defined energy band and sensitivity. We base this on a population model that reproduces the observed properties of GRBs detected by Swift, Fermi and CGRO in the hard X-ray and Îł-ray bands. We provide the expected cumulative distributions of the flux and fluence of simulated GRBs in different energy bands. We show that scintillator detectors, operating at relatively high energies (e.g. tens of keV to the MeV), can detect only the most luminous GRBs at high redshifts due to the link between the peak spectral energy and the luminosity (EpeakâLiso) of GRBs. We show that the best strategy for catching the largest number of high-z bursts is to go softer (e.g. in the soft X-ray band) but with a very high sensitivity. For instance, an imaging soft X-ray detector operating in the 0.2â5 keV energy band reaching a sensitivity, corresponding to a fluence, of âŒ10â8 erg cmâ2 is expected to detect â40âGRBsâyrâ1âsrâ1 at z â„ 5 (â3âGRBsâyrâ1âsrâ1 at z â„ 10). Once high-zâGRBs are detected the principal issue is to secure their redshift. To this aim we estimate their NIR afterglow flux at relatively early times and evaluate the effectiveness of following them up and construct usable samples of events with any forthcoming GRB mission dedicated to explore the high-z Universe
Evidence for the magnetar nature of 1E 161348-5055 in RCW 103
We report on the detection of a bright, short, structured X-ray burst coming from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift/Burst Alert Telescope (BAT) monitor, and on the follow-up campaign made with Swift/X-ray Telescope, Swift/UV/Optical Telescope, and the optical/near-infrared (NIR) Gamma-Ray burst Optical and Near-infrared Detector. The characteristics of this flash, such as duration and spectral shape, are consistent with typical short bursts observed from soft gamma repeaters. The BAT error circle at 68 per cent confidence range encloses the point-like X-ray source at the centre of the nebula, 1E 161348â5055. Its nature has been long debated due to a periodicity of 6.67 h in X-rays, which could indicate either an extremely slow pulsating neutron star, or the orbital period of a very compact X-ray binary system. We found that 20 min before the BAT trigger, the soft X-ray emission of 1E 161348â5055 was a factor of âŒ100 higher than measured 2 yr earlier, indicating that an outburst had already started. By comparing the spectral and timing characteristics of the source in the 2 yr before the outburst and after the BAT event, we find that, besides a change in luminosity and spectral shape, also the 6.67 h pulsed profile has significantly changed with a clear phase shift with respect to its low-flux profile. The UV/optical/NIR observations did not reveal any counterpart at the position of 1E 161348â5055. Based on these findings, we associate the BAT burst with 1E 161348â5055, we classify it as a magnetar, and pinpoint the 6.67 h periodicity as the magnetar spin period
Relativistic jet activity from the tidal disruption of a star by a massive black hole
Supermassive black holes have powerful gravitational fields with
strong gradients that can destroy stars that get too close1,2, producing
a bright flare in ultraviolet and X-ray spectral regions from
stellar debris that forms an accretion disk around the black hole3â7.
The aftermath of this process may have been seen several times over
the past two decades in the form of sparsely sampled, slowly fading
emission from distant galaxies8â14, but the onset of the stellar disruption
event has not hitherto been observed. Here we report
observations of a bright X-ray flare from the extragalactic transient
Swift J164449.31573451. This source increased in brightness in
the X-ray band by a factor of at least 10,000 since 1990 and by a
factor of at least 100 since early 2010. We conclude that we have
captured the onset of relativistic jet activity from a supermassive
black hole. A companion paper15 comes to similar conclusions on
the basis of radio observations. This event is probably due to the
tidal disruption of a star falling into a supermassive black hole, but
the detailed behaviour differs from current theoretical models of
such events