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In the Swift era, rapid multiband observations led to the discovery of the optical afterglows and host galaxies of short gamma-ray bursts (GRBs). In spite of these recent advancements, the number of short GRBs with secure detections in the optical is still fairly small. We present photometric observations of the short/hard GRB 051227, GRB 061006 and GRB 071227. We discuss our findings in light of the current models of short GRB progenitors. [ABSTRACT FROM AUTHOR]

The X-ray light curves of hundreds of bursts are now available, thanks to the X-ray Telescope on board the Swift satellite, on time scales from ∼ 1 minute up to weeks and in some cases months from the burst explosion. These data allow us to investigate the physics of the highly relativistic fireball outflow and its interaction with the circumburst environment. Here we review the main results of the XRT observations, with particular regard to the evolution of the X-ray light curves in the early phases. Unexpectedly, they are characterised by different slopes, with a very steep decay in the first few hundred of seconds, followed by a flatter decay and, a few thousand of seconds later, by a somewhat steeper decay. Often strong flare activity up to few hours after the burst explosion is also seen. These flares, most likely, are still related to the central engine activity, that last much longer than expected and it is still dominating the X-ray light curve well after the prompt phase, up to a few thousand of seconds. The real afterglow emission (external shock) is dominating the X-ray light curve only after the flatter phase ends. The flatter phase is probably the combination of late-prompt emission and afterglow emission. When the late-prompt emission ends the light curve steepens again. Some flare activity can still be detected during these later phases. Finally, even the late evolution of the XRT light curves is puzzling, in particular many of them do not show a “jet-break”. There are various possibilities to explain these observations (e.g. time evolution of the microphysical parameters, structured jet). However, a clear understanding of the formation and evolution of the jet and of the afterglow emission is still lacking. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present optical observations of the short/hard gamma-ray burst GRB 050709, the first such event with an identified optical counterpart. The object is located inside a galaxy at redshift z = 0.1606 ± 0.0002 but slightly off-center. Late-time monitoring places strong limits on any supernova simultaneous with the GRB. The host galaxy is not of early type: it has blue colours and a star formation rate of ∼ 2–3.5 M⊙ yr-1. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

During the first year of operations of the Swift observatory, the X-ray Telescope has made a number of discoveries concerning the nature of X-ray afterglows of both long and short GRBs. We highlight the key findings, which include rapid declines at early times, a standard template of afterglow light curve shapes, common flaring, and the discovery of the first short GRB afterglow. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The Swift X-ray Telescope autonomously refines the Burst Alert Telescope positions (∼1–4′ uncertainty) to better than 5 arcsec, within 5 seconds of target acquisition by the observatory for typical bursts. The results of the rapid positioning capability of the XRT are presented here for both known sources and newly discovered GRBs, demonstrating the ability to automatically utilise one of two integration times according to the burst brightness, and to correct the position for alignment offsets caused by the fast pointing performance and variable thermal environment of the satellite as measured by the Telescope Alignment Monitor. We present an evaluation of the position accuracy for both the onboard centroiding software and the ground software for the calibration targets and show that a significant improvement in position accuracy is obtained if the boresight detector position is optimised relative to the spacecraft pointing. Finally, we present an updated catalogue of Swift GRB X-ray positions obtained in Photon Counting Mode using the improved, calibrated boresight. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the GRB 050326 prompt and afterglow emission. The combined capabilities of Swift (which sampled the light curve for a relatively long time span) and XMM (which ensured a large statistics) allowed to obtain a thorough characterization of the afterglow properties. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than the Galactic value. A few studies based on BeppoSAX and XMM-Newton found evidence of higher than Galactic values for the column density in a number of GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by Swift up to April 15, 2005. We observed a large number of GRBs with an excess of column density. Our sample, together with previous determinations of the intrinsic column densities for GRBs with known redshift, provides evidence for a distribution of absorption consistent with that predicted for randomly occurring GRB within molecular clouds. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present observations of XRF 050406, an X-ray flash with a relatively low fluence (∼ 10-7 erg cm-2 in the 15–350 keV band), a soft spectrum (photon index Γγ = 2.65), no significant flux above ∼ 50 keV and a peak energy Ep < 15 keV. XRF 050406 is the first burst detected by Swift clearly showing a flare in its X-ray light curve. The flare peaks 210 s after the BAT trigger, presents a flux variation δF/F ∼ 6 in a timescale δt/tpeak < 1 and a measured fluence of 1–15% of the prompt one. 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 more shallow slope with a decay index of ∼ 0.5 after ∼ 4400 s, also supporting continued central engine activity. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

XRF 050416A was discovered by the Swift Burst Alert Telescope and re-pointed with the Swift narrow field instruments only 64.5 s after the burst onset. The 15 – 150 keV BAT average spectrum has a photon index of Γ ∼ 3.0 which classifies the bursts as an X-ray flash. The afterglow X-ray emission was monitored by the Swift X-Ray Telescope up to 74 days after the burst. The X-ray light curve shows a decay with three different phases: an initial steep decay with a decay slope of ∼ 2.4 (phase A), then, starting at ∼172 s from the burst onset, a second phase with a flat decay slope of ∼ 0.44 (phase B), and finally, after ∼1450 s from the burst onset, a third long-lasting phase with a decay slope of ∼ 0.88 (phase C). We find evidence of spectral evolution from a softer emission in the phase A of the afterglow decay, with Γ ∼ 3.0, to a harder emission with Γ ∼ 2.0 in the phases B and C. A redshift of 0.6535 was measured for the source. The spectra show intrinsic absorption in the host galaxy of ∼ 6.8 × 1021 cm-2. The consistency of the phase A photon index with the BAT photon index suggests that the initial fast decaying phase of the XRT afterglow might be the low energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve suggests very low collimation of the expanding fireball. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Swift discovered the high redshift (z=6.29) GRB 050904 with the Burst Alert Telescope and began observing with its narrow field instruments only 161 s after the burst onset. GRB 050904 was a long, multi-peaked, bright GRB with a presence of flaring activity lasting up to 1–2 hours after the burst onset. The spectral energy distribution shows a clear softening trend along the burst evolution with a photon index decreasing from -1.2 up to -1.9. The observed variability is more dramatic than the typical Swift afterglow, the amplitude and rise/fall times of the flares are consistent with the behavior of nearby (z ≤ 1) long GRBs and suggest the interpretation of the BAT and XRT data as a single continuous observation of long lasting prompt emission. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The Swift X-ray Telescope (XRT) is designed to make astrometric, spectroscopic and photometric observations of the X-ray emission from Gamma-ray bursts and their afterglows, in the energy band 0.2–10 keV. Here we report the results of the analysis of Swift XRT Point Spread Function (PSF) as measured in the first four months of the mission during the instrument calibration phase. The analysis includes the study of the PSF of different point-like sources both on-axis and off-axis with different spectral properties. We compare the in-flight data with the expectations from the on-ground calibration. On the basis of the calibration data we built an analytical model to reproduce the PSF as a function of the energy and the source position within the detector which can be applied in the PSF correction calculation for any extraction region geometry. All the results of this study are implemented in the standard public software. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present the current on-orbit calibration and operational plans for the Swift XRT. The XRT is a largely autonomous instrument and requires very little manual commanding for normal operations. A detailed calibration plan is being developed to verify the instrumental performance on-orbit, including effective area, point spread function, vignetting, spectroscopic performance, and timing accuracy. Operational plans include regular calibration measurements using on-board calibration sources as well as periodic calibration observations using celestial targets. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We observed three blazars in an active state with BeppoSAX. For ON231 and BL Lac, we detected simultaneously both the synchrotron and the Compton component. Fast time variability was present in both sources, only for the synchrotron component. PKS 2005-489 was observed in a very high state and the 0.1-200 keV X-ray spectrum is well accounted for by the sole synchrotron emission. No fast time variability was detected. The SED of the three sources are well described by pure SSC models. [ABSTRACT FROM AUTHOR]

We present the X-ray variability properties of the TeV blazars PKS 2155-304 and MKN421 as observed with BeppoSAX and ASCA. The minimum timescales of ∼ 1000s are suggested. For PKS 2155-304 we found that the soft X-ray lags (relative to higher energies) are inversely correlated with the source intensity, i.e. the lag is longer when the source is fainter. In April 1998 a flare of MKN 421 was detected, during which higher energy X-rays lagged lower energy ones by ∼ 2500s. The origin of these lags is briefly discussed. [ABSTRACT FROM AUTHOR]

© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the first results obtained from a systematic search of periodicities carried out over two samples of X-ray sources: (i) those detected in the public ROSAT HRI fields, and (ii) a selected number of candidate High Mass X-ray Binaries in the Small Magellanic Cloud observed by BeppoSAX. [ABSTRACT FROM AUTHOR]

We present the preliminary results of a long BeppoSAX observation of the BL Lac object Mkn501 carried out in June 1999. The source was fainter than found during the BeppoSAX pointings of 1997 and 1998, but is still detected with a good signal-to-noise ratio up to ∼40 keV. The X-ray spectrum in the energy range 0.1-40 keV, produced through synchrotron radiation, is steeper than in the previous years, it is clearly curved, and peaks (in vF[sub v]) at 0.5 keV. This energy is much lower than those at which the synchrotron component was found to peak in 1997 and 1998. Some intraday variability suggests that activity of the source on small time scales accompanies the large long time scales changes of brightness and spectrum. [ABSTRACT FROM AUTHOR]

We present BeppoSAX observations of the γ-ray emitting quasars 0836+710, 1510-089 and 2230+114. All the objects have been detected in the PDS up to 100 keV and have extremely fiat power-law spectra above 2 keV (α[sub χ =0.3-0.5). 0836+710 shows absorption higher than the galactic value and marginal evidence for the presence of the redshifted 6.4 keV Iron line. 1510-089 shows a spectral break around I keV, with the low energy spectrum steeper (α[sub l]=1.6) than the high energy power-law (α[sub h]=0.3). The data are discussed in the light of current Inverse Compton models for the high energy emission. [ABSTRACT FROM AUTHOR]

The Swift Gamma-Ray Burst Explorer is designed to make prompt multiwavelength observations of Gamma-Ray Bursts (GRBs) and their afterglows. The X-ray Telescope (XRT) provides key capabilities that permit Swift to determine GRB positions with several arcsecond accuracy within 100 seconds of the burst onset. We present an overview of the XRT and its capabilities. The XRT is designed to observe GRB afterglows covering 7 orders of magnitude in flux in the 0.2-10 kev band, with completely autonomous operation. Accurate GRB positions are determined within seconds of target acquisition and are sent to the ground for distribution over the GCN. The XRT can also measure redshifts of GRBs if they have Fe line emission or other spectral features. [ABSTRACT FROM AUTHOR]

The Swift XRT [4] uses a 12 shell grazing incidence mirror developed for the JET-X telescope and an open electrode CCD detector developed for XMM-Newton [9]. Thc mirror has recently undergone re-characterization at the Panter facility and thc flight detector is currently being calibrated at thc University of Leicester. We present key performance characteristics of the mirror/detector combination, in particular, thc point spread function (PSF) and centroiding capability and thc impact of background events, as well as the telescope effective area and spectral response. [ABSTRACT FROM AUTHOR]

We present the 2nd Palermo Swift-BAT Catalogue, obtained using 54 months of BAT survey data. The catalog contains 1258 BAT source candidates, of which 1049 have been associated with a known counterpart. [ABSTRACT FROM AUTHOR]