The Swift X-ray telescope BURROWS, David N; HILL, J. E; CHINCARINI, G ...
Space science reviews,
10/2005, Letnik:
120, Številka:
3-4
Journal Article
Recenzirano
he 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 s of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM-Newton/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of > 120 cm^sup 2^ at 1.5 keV, field of view of 23.6 × 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2×10^sup -14^ erg cm^sup -2^ s^sup -1^ in 10^sup 4^ s. The instrument is designed to provide automated source detection and position reporting within 5 s 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.PUBLICATION ABSTRACT
We study the properties of the population of optically dark events present in a carefully selected complete sample of bright Swift long gamma-ray bursts. The high level of completeness in redshift of ...our sample (52 objects out of 58) allows us to establish the existence of a genuine dark population, and we are able to estimate the maximum fraction of dark burst events (∼30 per cent) expected for the whole class of long gamma-ray burst. The redshift distribution of this population of dark bursts is similar to that of the whole sample. Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux) of the subclass of dark bursts is slightly higher than the average luminosity of the non-dark events. At the same time, the prompt properties do not differ and the optical flux of dark events is at the lower tail of the optical flux distribution, corrected for Galactic absorption. All these properties suggest that dark bursts events generate in much denser environments with respect to normal bright events. We can therefore exclude the high-z and the low-density scenarios and conclude that the major cause of the origin of optically dark events is the dust extinction.
We present the first systematic investigation of the morphological and timing properties of flares in GRBs observed by Swift XRT. We consider a large sample drawn from all GRBs detected by Swift, ...INTEGRAL, and HETE-2 prior to 2006 January 31, which had an XRT follow-up and which showed significant flaring. Our sample of 33 GRBs includes long and short, at low and high redshift, and a total of 69 flares. The strongest flares occur in the early phases, with a clear anticorrelation between the flare peak intensity and the flare time of occurrence. Fitting each X-ray flare with a Gaussian model, we find that the mean ratio of the width and peak time is < Delta t/t> similar to 0.13 plus or minus 0.10, albeit with a large scatter. Late flares at times >2000 s have long durations, Delta t > 300 s, and can be very energetic compared to the underlying continuum. We further investigated whether there is a clear link between the number of pulses detected in the prompt phase by BAT and the number of X-ray flares detected by XRT, finding no correlation. However, we find that the distribution of intensity ratios between successive BAT prompt pulses and that between successive XRT flares is the same, an indication of a common origin for gamma-ray pulses and X-ray flares. All evidence indicates that flares are indeed related to the workings of the central engine and, in the standard fireball scenario, originate from internal shocks rather than external shocks. While all flares can be explained by long-lasting engine activity, 29/69 flares may also be explained by refreshed shocks. However, 10 can only be explained by prolonged activity of the central engine.
Aims.
The sample of serendipitous sources detected in all
Swift
-XRT images pointing at gamma ray bursts (GRBs) constitutes the largest existing medium-deep survey of the X-ray sky. To build such ...dataset we analysed all
Swift
X-ray images centred on GRBs and observed over a period of 15 years using automatic tools that do not require any expertise in X-ray astronomy. Besides presenting a new large X-ray survey and a complete sample of blazars, this work aims to be a step in the direction of achieving the ultimate goal of the Open Universe Initiative, which is to enable non-expert people to benefit fully from space science data, possibly extending the potential for scientific discovery, which is currently confined within a small number of highly specialised teams, to a much larger population.
Methods.
We used the
Swift
_deepsky Docker container encapsulated pipeline to build the largest existing flux-limited and unbiased sample of serendipitous X-ray sources.
Swift
_deepsky runs on any laptop or desktop computer with a modern operating system. The tool automatically downloads the data and the calibration files from the archives, runs the official
Swift
analysis software, and produces a number of results including images, the list of detected sources, X-ray fluxes, spectral energy distribution data, and spectral slope estimations.
Results.
We used our source list to build the LogN-LogS of extra-galactic sources, which perfectly matches that estimated by other satellites. Combining our survey with multi-frequency data, we selected a complete radio-flux-density-limited sample of high energy peaked blazars (HBL). The LogN-LogS built with this data set confirms that previous samples are incomplete below ∼20 mJy.
Until recently, X-ray flares during the afterglow of gamma-ray bursts (GRBs) were a rarely detected phenomenon; thus, their nature is unclear. During the afterglow of GRB 050502B, the largest X-ray ...flare ever recorded rose rapidly above the afterglow light curve detected by the Swift X-Ray Telescope. The peak flux of the flare was >500 times that of the underlying afterglow, and it occurred >12 minutes after the nominal prompt burst emission. The fluence of this X-ray flare, (1.0 c 0.05) x 10 super(-6) ergs cm super(-2) in the 0.2-10.0 keV energy band, exceeded the fluence of the nominal prompt burst. The spectra during the flare were significantly harder than those measured before and after the flare. Later in time, there were additional flux increases detected above the underlying afterglow, as well as a break in the afterglow light curve. All evidence presented below, including spectral and, particularly, timing information during and around the giant flare, suggests that this giant flare was the result of internal dissipation of energy due to late central engine activity, rather than an afterglow-related effect. We also find that the data are consistent with a second central engine activity episode, in which the ejecta is moving slower than that of the initial episode, causing the giant flare and then proceeding to overtake and refresh the afterglow shock, thus causing additional activity at even later times in the light curve.
Rapid N H changes in NGC 4151 Puccetti, S; Fiore, F; Risaliti, G ...
Monthly notices of the Royal Astronomical Society,
05/2007, Letnik:
377, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We have analysed the two longest (elapsed time ≳3 d) BeppoSAX observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for spectral variability on time-scales from a few tens of ...kiloseconds to years. We found in both cases highly significant spectral variability below ≈6 keV down to the shortest time-scales investigated. These variations can be naturally explained in terms of variations in the low energy cut-off due to obscuring matter along the line of sight. If the cut-off is modelled by two neutral absorption components, one fully covering the source and the second covering only a fraction of the source, the shortest time-scale of variability of a few days constrains the location of the obscuring matter to within 3.4 × 104 Schwarzschild radii from the central X-ray source. This is consistent with the distance of the broad emission-line region, as inferred from reverberation mapping, and difficult to reconcile with the parsec scale dusty molecular torus of Krolik & Begelman. We have also explored a more complex absorption structure, namely the presence of an ionized absorber. Although the behaviour of the ionization parameter is nicely consistent with the expectations, the results are not completely satisfactory from the statistical point of view.
The overall absorption during the 2001 December observation is lower than in all other historical observations with similar 2-10 keV flux. This suggests that absorption variability plays a crucial role in the observed flux variability of this source.
Rapid NH changes in NGC 4151 Puccetti, S.; Fiore, F.; Risaliti, G. ...
Monthly notices of the Royal Astronomical Society,
20/May , Letnik:
377, Številka:
2
Journal Article
Recenzirano
We have analysed the two longest (elapsed time ≳3 d) BeppoSAX observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for spectral variability on time-scales from a few tens of ...kiloseconds to years. We found in both cases highly significant spectral variability below ≈6 keV down to the shortest time-scales investigated. These variations can be naturally explained in terms of variations in the low energy cut-off due to obscuring matter along the line of sight. If the cut-off is modelled by two neutral absorption components, one fully covering the source and the second covering only a fraction of the source, the shortest time-scale of variability of a few days constrains the location of the obscuring matter to within 3.4 × 104 Schwarzschild radii from the central X-ray source. This is consistent with the distance of the broad emission-line region, as inferred from reverberation mapping, and difficult to reconcile with the parsec scale dusty molecular torus of Krolik & Begelman. We have also explored a more complex absorption structure, namely the presence of an ionized absorber. Although the behaviour of the ionization parameter is nicely consistent with the expectations, the results are not completely satisfactory from the statistical point of view. The overall absorption during the 2001 December observation is lower than in all other historical observations with similar 2–10 keV flux. This suggests that absorption variability plays a crucial role in the observed flux variability of this source.
Bright X-ray Flares in Gamma-Ray Burst Afterglows Burrows, D. N.; Romano, P.; Falcone, A. ...
Science (American Association for the Advancement of Science),
09/2005, Letnik:
309, Številka:
5742
Journal Article
Recenzirano
Odprti dostop
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.
Short duration ($\la $2 s) Gamma-ray bursts (GRBs) have been a mystery since their discovery. Until May 2005 very little was known about short GRBs, but this situation has changed rapidly in the last ...few months since the Swift and HETE-2 satellites have made it possible to discover X-ray and optical counterparts to these sources. Positional associations indicate that short GRBs arise in close-by galaxies ($z<0.7$). Here we report on a detailed study of the short GRB 050724 X-ray afterglow. This burst shows strong flaring variability in the X-ray band. It clearly confirms early suggestions of X-ray activity in the 50–100 s time interval following the GRB onset seen with BATSE. Late flare activity is also observed. These observations support the idea that flares are related to the inner engine for short GRBs, as well as long GRBs.
GRB 060614 is a remarkable gamma-ray burst (GRB) observed by Swift with puzzling properties, which challenge current progenitor models. In particular, the lack of any bright supernova (SN) down to ...very strict limits and the vanishing spectral lags during the whole burst are typical of short GRBs, strikingly at odds with the long (102 s) duration of this event. Here we present detailed spectral and temporal analysis of the Swift observations of GRB 060614. We show that the burst presents standard optical, ultraviolet and X-ray afterglows, detected beginning 4 ks after the trigger. An achromatic break is observed simultaneously in the optical and X-ray bands, at a time consistent with the break in the R-band light curve measured by the VLT. The achromatic behaviour and the consistent post-break decay slopes make GRB 060614 one of the best examples of a jet break for a Swift burst. The optical and ultraviolet afterglow light curves have also an earlier break at 29.7 ± 4.4 ks, marginally consistent with a corresponding break at 36.6 ± 2.4 ks observed in the X-rays. In the optical, there is strong spectral evolution around this break, suggesting the passage of a break frequency through the optical/ultraviolet band. The very blue spectrum at early times suggests this may be the injection frequency, as also supported by the trend in the light curves: rising at low frequencies, and decaying at higher energies. The early X-ray light curve (from 97 to 480 s) is well interpreted as the X-ray counterpart of the burst extended emission. Spectral analysis of the BAT and XRT data in the ~80 s overlap time interval show that the peak energy of the burst has decreased to as low as 8 keV at the beginning of the XRT observation. Spectral analysis of following XRT data shows that the peak energy of the burst continues to decrease through the XRT energy band and exits it at about 500 s after the trigger. The average peak energy Ep of the burst is likely below the BAT energy band (<24 keV at the 90% confidence level) but larger than 8 keV. The initial group of peaks observed by BAT (~5 s) is however distinctly harder than the rest of the prompt emission, with a peak energy of about 300 keV as measured by Konus Wind. Considering the time-averaged spectral properties, GRB 060614 is consistent with the $E_{\rm iso}-E_{\rm p}^{\rm rest}$, $E_{\gamma}-E_{\rm p}^{\rm rest}$, and $L_{\rm p,iso}-E_{\rm p}^{\rm rest}$ correlations.
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