The Swift Gamma Ray Burst satellite routinely provides prompt positions for GRBs and their afterglows on timescales of a few hundred seconds. However, with a pointing accuracy of only a few ...arcminutes, and a systematic uncertainty on the star-tracker solutions to the World Coordinate System of 3–4 arcsec, the precision of the early XRT positions is limited to 3–4 arcsec at best. This is significant because operationally, the XRT detects >95% of all GRBs, while the UVOT detects only the optically brightest bursts, ~30% of all bursts detected by BAT; thus early and accurate XRT positions are important because for the majority of bursts they provide the best available information for the initial ground-based follow-up campaigns. Here we describe an autonomous way of producing more accurate prompt XRT positions for GRBs and their afterglows, based on UVOT astrometry and a detailed mapping between the XRT and UVOT detectors. The latter significantly reduces the dominant systematic error – the star-tracker solution to the World Coordinate System. This technique, which is limited to times when there is significant overlap between UVOT and XRT PC-mode data, provides a factor of 2 improvement in the localisation of XRT refined positions on timescales of less than a few hours. Furthermore, the accuracy achieved is superior to astrometrically corrected XRT PC mode images at early times (for up to 24 h), for the majority of bursts, and is comparable to the accuracy achieved by astrometrically corrected X-ray positions based on deep XRT PC-mode imaging at later times.
Context. Our knowledge of the intrinsic properties of short duration Gamma-Ray Bursts has relied, so far, only upon a few cases for which the estimate of the distance and an extended, multiwavelength ...monitoring of the afterglow have been obtained. Aims. We carried out multiwavelength observations of the short GRB 061201 aimed at estimating its distance and studying its properties. Methods.We performed a spectral and timing analysis of the prompt and afterglow emission and discuss the results in the context of the standard fireball model. Results. A clear temporal break was observed in the X-ray light curve about 40 min after the burst trigger. We find that the spectral and timing behaviour of the X-ray afterglow is consistent with a jet origin of the observed break, although the optical data can not definitively confirm this and other scenarios are possible. No underlying host galaxy down to R ~ 26 mag was found after fading of the optical afterglow. Thus, no secure redshift could be measured for this burst. The nearest galaxy is at $z = 0.111$ and shows evidence of star formation activity. We discuss the association of GRB 061201 with this galaxy and with the ACO S 995 galaxy cluster, from which the source is at an angular distance of 17'' and 8.5', respectively. We also test the association with a possible undetected, positionally consistent galaxy at z ~ 1. In all these cases, in the jet interpretation, we find a jet opening angle of 1–2 degrees.
We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at $t_{\rm peak} \sim 210$ s after the BAT trigger, a ...flux variation of $\delta F / F \sim 6$ in a very short time $\delta t / t_{\rm peak} \ll 1$ was observed. Its measured fluence in the 0.2-10 keV band was ~$1.4 \times 10^{-8}$ erg cm-2, which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210 s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ~0.5 after ~4400 s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (~10-7 erg cm-2 in the 15-350 keV band, $E_{\rm iso} \sim 10^{51}$ erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy $E_{\rm p} < 15$ keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.
Science with the Cherenkov Telescope Array The Cta Consortium, The Cta Consortium
The Astrophysical journal. Supplement series,
01/2019, Letnik:
240, Številka:
2
eBook, Journal Article, Book
Recenzirano
Odprti dostop
This book summarizes the science to be carried out by the upcoming Cherenkov Telescope Array, a major ground-based gamma-ray observatory that will be constructed over the next six to eight years. The ...major scientific themes, as well as core program of key science projects, have been developed by the CTA Consortium, a collaboration of scientists from many institutions worldwide. CTA will be the major facility in high-energy and very high-energy photon astronomy over the next decade and beyond. CTA will have capabilities well beyond past and present observatories. Thus, CTA's science program is expected to be rich and broad and will complement other major multiwavelength and multimessenger facilities. This book is intended to be the primary resource for the science case for CTA and it thus will be of great interest to the broader physics and astronomy communities. The electronic version (e-book) is available in open access.
Context.The prompt emission of gamma-ray bursts is mostly thought to be produced in internal shocks of relativistic shells emitted by the progenitor at different times, whereas the late multi-band ...afterglow is interpreted as the synchrotron emission of electrons swept up by the fireball expanding through the surrounding interstellar medium. The short timescale variability observed in flares superimposed on the X-ray/optical afterglow of several bursts, recently made possible by Swift, has been interpreted as evidence for prolonged activity of the inner engine through internal shocks. Yet, it is not clear whether this applies to all the observed bursts and, in particular, whether the bursts exhibiting single γ-ray pulses with no short timescale variability at late times could also be entirely interpreted as external shocks. Aims.We present prompt γ-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt γ-ray pulse as well as for the early and late multi-band afterglow of GRB 070311. Methods.We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. Results.The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the γ-ray burst and possibly accompanied by a faint γ-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3 $\times$ 104 and 2 $\times$ 105 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt γ-ray pulse and an underlying power law. Conclusions.This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock.
Line Searches in Swift X-Ray Spectra Hurkett, C. P; Vaughan, S; Osborne, J. P ...
The Astrophysical journal,
05/2008, Letnik:
679, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Prior to the launch of the Swift mission several X-ray line detections were reported in gamma-ray burst afterglow spectra. To date, these pre-Swift era results have not been conclusively confirmed. ...The most contentious issue in this area is the choice of statistical method used to evaluate the significance of these features. In this paper we compare three different methods already extant in the literature for assessing the significance of possible line features and discuss their relative advantages and disadvantages. The methods are demonstrated by application to observations of 40 bursts from the archive of Swift XRT at early times (<a few kiloseconds posttrigger in the rest frame of the burst). Based on this thorough analysis we found no strong evidence for emission lines. For each of the three methods we have determined detection limits for emission- line strengths in bursts with spectral parameters typical of the Swift- era sample. We also discuss the effects of the current calibration status on emission-line detection.
We test the gamma-ray burst (GRB) correlation between temporal variability and peak luminosity of the γ-ray profile on a homogeneous sample of 36 Swift/Burst Alert Telescope (BAT) GRBs with firm ...redshift determination. This is the first time that this correlation can be tested on a homogeneous data sample. The correlation is confirmed, as long as the six GRBs with low luminosity (<5 × 1050 erg s−1 in the rest-frame 100-1000 keV energy band) are ignored. We confirm that the considerable scatter of the correlation already known is not due to the combination of data from different instruments with different energy bands, but it is intrinsic to the correlation itself. Thanks to the unprecedented sensitivity of Swift/BAT, the variability/peak luminosity correlation is tested on low-luminosity GRBs. Our results show that these GRBs are definite outliers.
We present an analysis of all BeppoSAX observations of NGC 4151. This source was observed 5 times from 1996 to 2001 with durations ranging from a day to four days. The intrinsic continuum (described ...as a cut-off power law) is absorbed at low energies by a complex system: a cold patchy absorber plus a warm uniform screen photoionised by the central continuum. We find that this “dual absorber” is the main driver of the observed variability, up to a factor of eight, at 3 keV. In particular the covering fraction of the cold absorber changes on time scales of the order of a day, supporting its association with the broad-line region. The column density of the warm gas varies on a longer time scale (months to year). Some of the small amplitude spectral variability above 10 keV can be explained with an intrinsic variation (with variation of the photon index $\Delta\Gamma \sim 0.2$). The flux below 1 keV remains constant confirming an extended origin. Its spectrum is reproduced by a combination of a thermal component (with temperature $kT=0.15$ keV) and a power law with the same slope as the intrinsic continuum, but with an intensity of a few percent. A Compton reflection component is significantly detected in 1996 (averaged value of $\Omega/2\pi \sim0.4$, with the solid angle Ω covered by the reflecting medium), with intensity decreasing on a time scale of a year, and it desappears in 2000 and 2001. The long time scale of variations argues for an association with an optically thick torus at a distance of a few light years. An iron line was detected in all spectra. Its energy is consistent with fluorescence by cold iron. We find that the line is variable. Its behaviour is reproduced by a variable component proportional to the level of the reflection flux plus a constant component. The flux of the latter is consistent with the extended line emission observed by Chandra. We conclude that the first component likely arises from the torus and the second is produced in the extended narrow-line region.
Aims.The aim of this paper is to study the Gamma Ray Burst (GRB) environment through the analysis of the optical absorption features due to the gas surrounding the GRB. Methods.To this purpose we ...analyze high resolution spectroscopic observations ($R=20\,000$–45 000, corresponding to 14 km s-1 at 4200 Å and 6.6 km s-1 at 9000 Å) of the optical afterglow of GRB050730, obtained with UVES@VLT ~4 h after the GRB trigger. Results.The spectrum shows that the ISM of the GRB host galaxy at $z = 3.967$ is complex, with at least five components contributing to the main absorption system. We detect strong C II*, Si II*, O I* and Fe II* fine structure absorption lines associated to the second and third component. Conclusions.For the first three components we derive information on the relative distance from the site of the GRB explosion. Component 1, which has the longest wavelength, highest positive velocity shift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as C IV and O VI, suggesting that this component is very close to the GRB site. From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10–100 times smaller than that of component 3. We evaluated the mean metallicity of the $z=3.967$ system obtaining values ≈10-2 of the solar metallicity or less. However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISM closer to the GRB site. Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values. The mean C/Fe ratio agrees well with that expected by single star-formation event models. Interestingly the C/Fe of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site.
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