arXiv subjects

G. Tagliaferri

Publications and source records attributed to G. Tagliaferri.

At least 289 records · Page 16Linked to original sources

arXiv2005

The dust-scattered X-ray halo around Swift GRB 050724

This paper discusses the X-ray halo around the Swift gamma-ray burst GRB 050724 (z=0.258), detected by the Swift X-Ray Telescope. The halo, which forms a ring around the fading X-ray source, expands to a radius of 200" within 8 ks of the burst exactly as expected for small-angle X-ray scattering by Galactic dust along the line of sight to a cosmologically distant GRB. The expansion curve and radial profile of the halo constrain the scattering dust to be concentrated at a distance of D = 139 +/- 9 pc (from Earth) in a cloud/sheet of thickness delta-D < 22 pc. The halo was observed only out to scattering angles of 200", for which the scattering is dominated by the largest grains, with a maximum size estimated to be a_max ~ 0.4-0.5 um. The scattering-to-extinction ratio was estimated to be tau_scat/A_V > 0.022; this is a lower limit to the true value because contribution from smaller grains, which scatter to larger angles, was not directly observed. The line-of-sight to the GRB passes close to the Ophiuchus molecular cloud complex, which provides a plausible site for the scattering dust.

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arXiv2005

The X-ray Telescope on board Swift: status and main results

The X-ray Telescope (XRT), on board the {\it Swift} satellite, provides: automated source detection and position with few arcsecond accuracy within few seconds from target acquisition; CCD spectroscopy and imaging capability (0.2-10 keV), with the capability of detecting a milliCrab source in about 10 seconds; automatic adjusting of the CCD readout mode to optimize the science return as the source fades. Swift main scientific goal is the study of gamma-ray burst (GRBs). XRT can observe GRB afterglows over several orders of magnitude in flux. The first results obtained during the first ten months of operation confirm that XRT is fully compliant with the requirements and is providing excellent results. In particular it is detecting a very steep decay in the early X-ray light curve of many afterglows. Often there are also strong flares superimposed to the X-ray light curve, probably related to the continued internal engine activity. XRT is also localising for the first time the X-ray counterparts to short bursts.

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arXiv2005

XRT Light curves: Morphology, Flares and Energy

Following a brief introduction we show that the observations obtained so far with the Swift satellite begin to shed light over a variety of problems that were left open following the excellent performance and related discoveries of the Italian - Dutch Beppo SAX satellite. The XRT light curves show common characteristics that are reasonably understood within the framework of the fireball model. Unforeseen flares are however detected in a large fraction of the GRB observed and the energy emitted by the brightest ones may be as much as 85% of the total soft X ray emission measured by XRT. These characteristics seems to be common to long and short bursts.

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arXiv2005

Swift XRT Observations of X-ray Flares in GRB Afterglows

The Swift XRT has been observing GRB afterglows since December 23, 2004. Three-quarters of these observations begin within 300 s of the burst onset, providing an unprecendented look at the behavior of X-ray emission from GRB afterglows in the first few hours after the burst. While most of the early afterglows have smoothly declining lightcurves, a substantial fraction has large X-ray flares on short time-scales. We suggest that these flares provide support for models with extended central engine activity producing late-time internal shocks.

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arXiv2005

The XMM-Newton view of gamma-ray loud active nuclei

Notwithstanding the big efforts devoted to the investigation of the mechanisms responsible for the high-energy (E>100 MeV) gamma-ray emission in active galactic nuclei (AGN), the definite answer is still missing. The X-ray energy band (0.4-10 keV) is crucial for this type of study, since both synchrotron and inverse Compton emission can contribute to the formation of the continuum. Within an ongoing project aimed at the investigation of the gamma-ray emission mechanism acting in the AGN detected by the EGRET telescope onboard CGRO, we firstly focused on the sources for which X-ray and optical/UV data are available in the XMM-Newton public archive. The preliminary results are outlined here.

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arXiv2005

Swift observations of the X-ray bright GRB 050315

This paper discusses Swift observations of the gamma-ray burst GRB 050315 (z=1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t^-5) for ~200 s and several breaks. However, both the prompt hard X-ray/gamma-ray emission (observed by the BAT) and the first ~ 300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt gamma-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray rich GRB. The prompt (BAT) spectrum was a steep down to 15 keV, and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline the X-ray afterglow did not fade for ~1.2*10^4 s, after which time it decayed with a temporal index of alpha ~ 0.7, followed by a second break at ~2.5*10^5 s to a slope of alpha ~ 2. The apparent `plateau' in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break then the jet opening angle was theta_0 ~ 5 deg, and implying E_gamma > 10^50 erg.

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arXiv2005

The in-flight spectroscopic performance of the Swift XRT CCD camera

The Swift X-ray Telescope (XRT) focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 144 eV FWHM at 6.5 keV. We describe the CCD calibration program based on celestial and on-board calibration sources, relevant in-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Loss of temperature control motivated a laboratory program to re-optimize the CCD substrate voltage, we describe the small changes in the CCD response that would result from use of a substrate voltage of 6V.

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arXiv2005

GRB 050904 at redshift 6.3: observations of the oldest cosmic explosion after the Big Bang

We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is 3.4x10^53 erg in the rest-frame 110-1100 keV energy band. Despite the high redshift, both the prompt and the afterglow emission are not peculiar with respect to other GRBs. We find a break in the J-band light curve at t_b = 2.6 +- 1.0 d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy E_gamma = (4-12)x10^51 erg. This limit shows that GRB 050904 is consistent with the Amati and Ghirlanda relations. This detection is consistent with the expected number of GRBs at z > 6 and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.

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arXiv2005

GRB 050117: Simultaneous Gamma-ray and X-ray Observations with the Swift Satellite

The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-ray follow-up to a newly detected GRB on 2005 January 17, within 193 seconds of the burst trigger by the Swift Burst Alert Telescope. While the burst was still in progress, the X-ray Telescope obtained a position and an image for an un-catalogued X-ray source; simultaneous with the gamma-ray observation. The XRT observed flux during the prompt emission was 1.1 x 10^{-8} ergs cm^{-2} s^{-1} in the 0.5-10 keV energy band. The emission in the X-ray band decreased by three orders of magnitude within 700 seconds, following the prompt emission. This is found to be consistent with the gamma-ray decay when extrapolated into the XRT energy band. During the following 6.3 hours, the XRT observed the afterglow in an automated sequence for an additional 947 seconds, until the burst became fully obscured by the Earth limb. A faint, extremely slowly decaying afterglow, alpha=-0.21$, was detected. Finally, a break in the lightcurve occurred and the flux decayed with alpha<-1.2$. The X-ray position triggered many follow-up observations: no optical afterglow could be confirmed, although a candidate was identified 3 arcsecs from the XRT position.

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arXiv2005

Swift XRT Observations of the Afterglow of GRB 050319

Swift discovered the high redshift GRB 050319 with the Burst Alert Telescope and began observing with its narrow field instruments only 225 s after the burst onset. The afterglow X-ray emission was monitored by the XRT up to 28 days after the burst. The light curve shows a decay with three different phases, each characterized by a distinct slope: an initial steep decay with a power law index of ~ 5.5, a second phase characterized by a flat decay slope of \~ 0.54, and a third phase with a decay slope of ~ 1.14. During the first phase the spectral energy distribution is softer than in the following two phases and the photon index is consistent with the GRB prompt spectrum. The extrapolation of the BAT light curve to the XRT band suggests that the initial fast decaying phase of the XRT afterglow might be the low energy tail of the prompt emission. The second break in the afterglow light curve occurs about 27000 s after the burst. The spectral energy distribution before and after the second break does not change and it can be tentatively interpreted as a jet break or the end of a delayed or continuous energy injection phase.

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arXiv2005

Swift XRT Observations of the Breaking X-ray Afterglow of GRB 050318

We report the results of Swift X-Ray Telescope (XRT) observations of GRB 050318. This event triggered the Burst Alert Telescope (BAT) aboard Swift and was followed-up with XRT and UVOT for 11 consecutive orbits starting from 54 minutes after the trigger. A previously unknown fading X-ray source was detected and accurately monitored. The source was found to decrease in intensity with time and a clear temporal break occurring at ~18000 s after the trigger was observed. The X-ray light curve was found to be consistent with a broken power-law with decay indices -1.17 +/- 0.08 and -2.10 (+0.22) (-0.24) before and after the break. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with energy index of -1.09 +/-0.09. No evidence of spectral evolution was found. We compare these results with those obtained with UVOT and separately reported and refine the data analysis of BAT. We discuss our results in the framework of a collimated fireball model and a synchrotron radiation emission mechanism. Assuming the GRB redshift derived from the farthest optical absorption complex (z = 1.44), the event is fully consistent with the E_p-E_iso correlation.

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arXiv2005

SIMBOL-X, a formation flying-mission for hard X-ray astrophysics

SIMBOL-X is a hard X-ray mission, operating in the ~ 0.5-70 keV range, which is proposed by a consortium of European laboratories in response to the 2004 call for ideas of CNES for a scientific mission to be flown on a formation flying demonstrator. Relying on two spacecrafts in a formation flying configuration, SIMBOL-X uses for the first time a ~ 30 m focal length X-ray mirror to focus X-rays with energy above 10 keV, resulting in a two orders of magnitude improvement in angular resolution and sensitivity in the hard X-ray range with respect to non focusing techniques. The SIMBOL-X revolutionary instrumental capabilities will allow to elucidate outstanding questions in high energy astrophysics, related in particular to the physics of accretion onto compact objects, to the acceleration of particles to the highest energies, and to the nature of the Cosmic X-Ray background. The mission, which has gone through a thorough assessment study performed by CNES, is expected to start a competitive phase A in autumn 2005, leading to a flight decision at the end of 2006, for a launch in 2012. The mission science objectives, the current status of the instrumentation and mission design, as well as potential trade-offs are presented in this paper.

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arXiv2005

Prompt and afterglow early X-ray phases in the comoving frame. Evidence for Universal properties?

We analyze the Swift XRT light curves and spectra of the gamma-ray bursts (GRBs) for which the redshift has been measured. The sample consists of seven GRBs. The soft X-ray light curves of all these GRBs are separated into at least two morphological classes: 1) those starting off with a very steep light curve decay and 2) those showing a rather mild initial decay. This initial decay is followed by a flattening and by a further steepening. During these transitions the soft X-ray spectrum of these GRBs remains constant within the observational errors (except for one case, GRB050319). For the first time we are able to exploit the early light curve of GRB afterglows in the comoving frame. Besides the temporal and spectral behavior we find that the energy of the afterglow emitted in the (rest frame) time interval 20-200 s and 1300-12600 s after the trigger correlates with the mean energy of the prompt emission, hinting at a close link between the two. Averaging over the bursts that have a rather well sampled light curve and starting immediately after the prompt phase, the energy emitted in the soft X-rays is about 3% of the energy emitted during the prompt phase, except for GRB050315, where the soft X-ray emission is as high as 14% of the prompt emission.

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arXiv2005

The Swift X-ray Telescope

The Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of Gamma-Ray Bursts (GRBs) and GRB afterglows. The X-ray Telescope (XRT) enables Swift to determine GRB positions with a few arcseconds accuracy within 100 seconds of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of > 120 cm^2 at 1.5 keV, field of view of 23.6 x 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2x10^-14 erg cm^-2 s^-1 in 10^4 seconds. The instrument is designed to provide automated source detection and position reporting within 5 seconds of target acquisition. It can also measure the redshifts of GRBs with Fe line emission or other spectral features. The XRT operates in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source intensity fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days or weeks.

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arXiv2005

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright X-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid X-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

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arXiv2005

GRB050223: A faint Gamma-Ray Burst discovered by Swift

GRB050223 was discovered by the Swift Gamma-Ray Burst Explorer on 23 February 2005 and was the first Gamma-Ray Burst to be observed by both Swift and XMM-Newton. At the time of writing (May 2005), it has one of the faintest GRB afterglows ever observed. The spacecraft could not slew immediately to the burst, so the first X-ray and optical observations occurred approximately 45 minutes after the trigger. Although no optical emission was found by any instrument, both Swift and XMM-Newton detected the fading X-ray afterglow. Combined data from both of these observatories show the afterglow to be fading monotonically as 0.99 +0.15/-0.12 over a time frame between 45 minutes to 27 hours post-burst. Spectral analysis, allowed largely by the higher through-put of XMM-Newton, implies a power-law with a slope of Gamma=1.75 +0.19/-0.18 and shows no evidence for absorption above the Galactic column of 7 x 10^20 cm^-2. From the X-ray decay and spectral slopes, a low electron power-law index of p = 1.3-1.9 is derived; the slopes also imply that a jet-break has not occured up to 27 hours after the burst. The faintness of GRB050223 may be due to a large jet opening or viewing angle or a high redshift.

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arXiv2005

An unexpectedly rapid decline in the X-ray afterglow emission of long gamma-ray bursts

Long gamma-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which gives rise to a highly relativistic jet. Internal inhomogeneities in the expanding flow give rise to internal shock waves that are believed to produce the gamma-rays we see. As the jet travels further outward into the surrounding circumstellar medium `external' shocks give rise to the afterglow emission seen in the X-ray, optical and radio bands. Here we report on the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

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arXiv2005

Open issues in gamma-ray bursts: polarimetry and dark GRBs

We review some open problems in the physics of afterglows, namely their polarization properties and the existence of dark/faint bursts. Polarization studies yield precious insights in the physical structure and dynamical evolution of GRB jets, revealing their magnetization properties and their energy profile. Polarimetric observations of GRB 020813 already allowed to exclude a homogeneous jet for this event. We then present observations of faint/dark bursts, showing that some of them may be obscured by dust, while others are possibly just intrinsically dim.

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