1,155 research outputs found
The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics
Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. The flares were sometimes intense, had rapid rise and
decay phases, and occurred late relative to the ``prompt'' phase. Some
remarkable flares are observed with fluence comparable to the prompt GRB
fluence. Many GRBs have several flares, which are sometimes overlapping. Short,
intense, repetitive, and late flaring can be most easily understood within the
context of the standard fireball model with the internal engine that powers the
prompt GRB emission in an active state at late times. However, other models for
flares have been proposed. Flare origin can be investigated by comparing the
flare spectra to that of the afterglow and the initial prompt emission. In this
work, we have analyzed all significant X-ray flares from the first 110 GRBs
observed by Swift. From this sample 33 GRBs were found to have significant
X-ray flares, with 77 flares that were detected above the 3 level. In
addition to temporal analysis presented in a companion paper, a variety of
spectral models have been fit to each flare. In some cases, we find that the
spectral fits favor a Band function model, which is more akin to the prompt
emission than to that of an afterglow. We find that the average fluence of the
flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is
approximately a factor of ten below the average prompt GRB fluence. These
results, when combined with those presented in the companion paper on temporal
properties of flares, supports the hypothesis that most X-ray flares are
late-time activity of the internal engine that spawned the initial GRB; not an
afterglow related effect.Comment: accepted by ApJ; 39 pages with 14 figures and 7 table
Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift
We show that all X-ray decay curves of GRBs measured by Swift can be fitted
using one or two components both of which have exactly the same functional form
comprised of an early falling exponential phase followed by a power law decay.
The 1st component contains the prompt gamma-ray emission and the initial X-ray
decay. The 2nd component appears later, has a much longer duration and is
present for ~80% of GRBs. It most likely arises from the external shock which
eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the
initial X-ray decay of the 1st component fades more slowly than the 2nd and
dominates at late times to form an afterglow but it is not clear what the
origin of this emission is.
The temporal decay parameters and gamma/X-ray spectral indices derived for
107 GRBs are compared to the expectations of the standard fireball model
including a search for possible "jet breaks". For ~50% of GRBs the observed
afterglow is in accord with the model but for the rest the temporal and
spectral indices do not conform to the expected closure relations and are
suggestive of continued, late, energy injection. We identify a few possible jet
breaks but there are many examples where such breaks are predicted but are
absent.
The time, T_a, at which the exponential phase of the 2nd component changes to
a final powerlaw decay afterglow is correlated with the peak of the gamma-ray
spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that
this time is in some way related to optically observed break times measured for
pre-Swift bursts.Comment: submitted to Ap
Automated hippocampal segmentation in 3D MRI using random undersampling with boosting algorithm
The afterglow and host galaxy of GRB 090205: evidence for a Ly-alpha emitter at z=4.65
Gamma-ray bursts have been proved to be detectable up to distances much
larger than any other astrophysical object, providing the most effective way,
complementary to ordinary surveys, to study the high redshift universe. To this
end, we present here the results of an observational campaign devoted to the
study of the high-z GRB 090205. We carried out optical/NIR spectroscopy and
imaging of GRB 090205 with the ESO-VLT starting from hours after the event up
to several days later to detect the host galaxy. We compared the results
obtained from our optical/NIR observations with the available Swift high-energy
data of this burst. Our observational campaign led to the detection of the
optical afterglow and host galaxy of GRB 090205 and to the first measure of its
redshift, z=4.65. Similar to other, recent high-z GRBs, GRB 090205 has a short
duration in the rest-frame with T_{90,rf}=1.6 s, which suggests the possibility
that it might belong to the short GRBs class. The X-ray afterglow of GRB 090205
shows a complex and interesting behaviour with a possible rebrightening at
500-1000s from the trigger time and late flaring activity. Photometric
observations of the GRB 090205 host galaxy argue in favor of a starburst galaxy
with a stellar population younger than ~ 150 Myr. Moreover, the metallicity of
Z > 0.27 Z_Sun derived from the GRB afterglow spectrum is among the highest
derived from GRB afterglow measurement at high-z, suggesting that the burst
occurred in a rather enriched envirorment. Finally, a detailed analysis of the
afterglow spectrum shows the existence of a line corresponding to Lyman-alpha
emission at the redshift of the burst. GRB 090205 is thus hosted in a typical
Lyman-alpha emitter (LAE) at z=4.65. This makes the GRB 090205 host the
farthest GRB host galaxy, spectroscopically confirmed, detected to date.Comment: Accepted for publication in Astronomy and Astrophysics; 8 pages, 7
figure
Simultaneous Swift and REM monitoring of the blazar PKS0537-441 in 2005
The blazar PKS0537-441 has been observed by Swift between the end of 2004 and
November 2005. The BAT monitored it recurrently for a total of 2.7 Ms, and the
XRT and UVOT pointed it on seven occasions for a total of 67 ks, making it one
of the AGNs best monitored by Swift. The automatic optical and infrared
telescope REM has monitored simultaneously the source at all times. In
January-February 2005 PKS0537-441 has been detected at its brightest in optical
and X-rays: more than a factor of 2 brighter in X-rays and about a factor 60
brighter in the optical than observed in December 2004. The July 2005
observation recorded a fainter X-ray state. The simultaneous optical state,
monitored by both Swift UVOT and REM, is high, and in the VRI bands it is
comparable to what was recorded in early January 2005, before the outburst. In
November 2005, the source subsided both in X-rays and optical to a quiescent
state, having decreased by factors of ~4 and ~60 with respect to the
January-February 2005 outburst, respectively. Our monitoring shows an overall
well correlated optical and X-ray decay. On the shorter time scales (days or
hours), there is no obvious correlation between X-ray and optical variations,
but the former tend to be more pronounced, opposite to what is observed on
monthly time scales. The widely different amplitude of the long term
variability in optical and X-rays is very unusual and makes this observation a
unique case study for blazar activity. The spectral energy distributions are
interpreted in terms of the synchrotron and inverse Compton mechanisms within a
jet where the plasma radiates via internal shocks and the dissipation depends
on the distance of the emitting region from the central engine (abridged).Comment: 24 pages, 7 figures, 3 tables, in press in the Ap
The Palermo Swift-BAT hard X-ray catalogue III. Results after 54 months of sky survey
We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by
analysing data acquired in the first 54 months of the Swift mission. Using our
software dedicated to the analysis of data from coded mask telescopes, we
analysed the BAT survey data in three energy bands (15-30 keV, 15-70 keV,
15-150 keV), obtaining a list of 1256 detections above a significance threshold
of 4.8 standard deviations. The identification of the source counterparts is
pursued using two strategies: the analysis of field observations of soft X-ray
instruments and cross-correlation of our catalogue with source databases.The
survey covers 50% of the sky to a 15--150 keV flux limit of 1.0 x 10^-11 erg
s^-1 cm^-2 and 9.2 x 10^-12 erg s^-1 cm^-2 for |b| 10
degrees, respectively. The Second Palermo Swift-BAT hard X-ray catalogue
includes 1079 (86%) hard X-ray sources with an associated counterpart (26 with
a double association and 2 with a triple association) and 177 BAT excesses
(14%) that still lack a counterpart. The distribution of the BAT sources among
the different object classes consists of 19% Galactic sources, 57%
extragalactic sources, and 10% sources with a counterpart at softer energies
whose nature has not yet been determined. About half of the BAT associated
sources lack a counterpart in the ROSAT catalogues. This suggests that either
moderate or strong absorption may be preventing their detection in the ROSAT
energy band. The comparison of our BAT catalogue with the Fermi Large Area
Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48
blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries,
and 4 pulsars/supernova remnants. This small number of correspondences
indicates that different populations make the sky shine in these two different
energy bands
A Study of the X-Ray Emission of Magnetic Cataclysmic Variable Ae Aquarii
We report results from analysis of the X-ray observations of AE Aqr, made
with Ginga in June 1988 and with ASCA in October 1995. Pulsations are detected
clearly with a sinusoidal pulse profile with periods of s
(Ginga) and s (ASCA)\@. The pulse amplitude is relatively
small and the modulated flux remains nearly constant despite a factor of 3
change in the average flux during the flare. We reproduce the time-averaged
spectrum in the 0.4 -- 10 keV energy band by a thermal emission model with a
combination of two different temperatures: kT keV
and kT keV\@. There is no significant difference
between the quiescent and flare energy spectra, although a hint of spectral
hardening is recognized during the flare. We interpret these observational
results with a model in which AE Aqr is in a propeller stage. Based on this
propeller scenario, we suggest that the X-ray emission is originated from
magnetospheric radiation.Comment: 24 pages, 7 Postscript figures, AAS LaTex, To appear in the Nov 1
issue of Ap
GRB 081028 and its late-time afterglow re-brightening
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical SocietySwift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB 081028. A rising phase is likely present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the γ-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB 081028 comparing its properties against a sample of Swift long GRBs with secure Eγ,iso measurements.Peer reviewe
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