164 research outputs found
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Solving inverse bioheat problems of skin tumour identification by dynamic thermography
The circumburst environment of a FRED GRB: study of the prompt emission and X-ray/optical afterglow of GRB 051111
We report a multi-wavelength analysis of the prompt emission and early
afterglow of GRB051111 and discuss its properties in the context of current
fireball models. The detection of GRB051111 by the Burst Alert Telescope
on-board Swift triggered early BVRi' observations with the 2-m robotic Faulkes
Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray
Telescope. The prompt gamma-ray emission shows a classical FRED profile. The
optical afterglow light curves are fitted with a broken power law, with
alpha_1=0.35 to alpha_2=1.35 and a break time around 12 minutes after the GRB.
Although contemporaneous X-ray observations were not taken, a power law
connection between the gamma-ray tail of the FRED temporal profile and the late
XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the
optical decay, this would represent one of the first GRBs for which the
canonical steep-shallow-normal decay typical of early X-ray afterglows has been
monitored optically. We present a detailed analysis of the intrinsic
extinction, elemental abundances and spectral energy distribution. From the
absorption measured in the low X-ray band we find possible evidence for an
overabundance of some alpha elements such as oxygen, [O/Zn]=0.7+/-0.3, or,
alternatively, for a significant presence of molecular gas. The IR-to-X-ray
Spectral Energy Distribution measured at 80 minutes after the burst is
consistent with the cooling break lying between the optical and X-ray bands.
Extensive modelling of the intrinsic extinction suggests dust with big grains
or grey extinction profiles. The early optical break is due either to an energy
injection episode or, less probably, to a stratified wind environment for the
circumburst medium.Comment: accepted to A&A on Nov. 10 (14 pages, 8 figures
The early and late-time spectral and temporal evolution of GRB 050716
We report on a comprehensive set of observations of Gamma Ray Burst 050716,
detected by the Swift satellite and subsequently followed-up rapidly in X-ray,
optical and near infra-red wavebands. The prompt emission is typical of
long-duration bursts, with two peaks in a time interval of T90 = 68 seconds (15
- 350 keV). The prompt emission continues at lower flux levels in the X-ray
band, where several smaller flares can be seen, on top of a decaying light
curve that exhibits an apparent break around 220 seconds post trigger. This
temporal break is roughly coincident with a spectral break. The latter can be
related to the extrapolated evolution of the break energy in the prompt
gamma-ray emission, and is possibly the manifestation of the peak flux break
frequency of the internal shock passing through the observing band. A possible
3 sigma change in the X-ray absorption column is also seen during this time.
The late-time afterglow behaviour is relatively standard, with an electron
distribution power-law index of p = 2 there is no noticable temporal break out
to at least 10 days. The broad-band optical/nIR to X-ray spectrum indicates a
redshift of z ~> 2 for this burst, with a host-galaxy extinction value of
E(B-V) ~ 0.7 that prefers an SMC-like extinction curve.Comment: Accepted to MNRAS. 8 pages, 5 figure
Dust extinction for an unbiased sample of GRB afterglows
In this paper we compute rest-frame extinctions for the afterglows of a
sample of gamma-ray bursts complete in redshift. The selection criteria of the
sample are based on observational high-energy parameters of the prompt emission
and therefore our sample should not be biased against dusty sight-lines. It is
therefore expected that our inferences hold for the general population of
gamma-ray bursts. Our main result is that the optical/near-infrared extinction
of gamma-ray burst afterglows in our sample does not follow a single
distribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer
from less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are
highly absorbed. The true percentage of gamma-ray burst afterglows showing high
absorption could be even higher since a fair fraction of the events without
reliable redshift measurement are probably part of this class. These events may
be due to highly dusty molecular clouds/star forming regions associated with
the gamma-ray burst progenitor or along the afterglow line of sight, and/or to
massive dusty host galaxies. No clear evolution in the dust extinction
properties is evident within the redshift range of our sample, although the
largest extinctions are at z~1.5-2, close to the expected peak of the star
formation rate. Those events classified as dark are characterized, on average,
by a higher extinction than typical events in the sample. A correlation between
optical/near-infrared extinction and hydrogen-equivalent column density based
on X-ray studies is shown although the observed NH appears to be well in excess
compared to those observed in the Local Group. Dust extinction does not seem to
correlate with GRB energetics or luminosity.Comment: 18 pages, 7 figures, 10 tables, MNRAS, in pres
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Design of constant temperature cooling device for melanoma screening by dynamic thermography
GRB host galaxies with VLT/X-Shooter: properties at 0.8 < z < 1.3
Long gamma-ray bursts (LGRBs) are associated with the death of massive stars.
Their host galaxies therefore represent a unique class of objects tracing star
formation across the observable Universe. Indeed, recently accumulated evidence
shows that GRB hosts do not differ substantially from general population of
galaxies at high (z > 2) redshifts. However, it has been long recognised that
the properties of z < 1.5 hosts, compared to general star-forming population,
are unusual. To better understand the reasons for the supposed difference in
LGRB hosts properties at z < 1.5, we obtained VLT/X- Shooter spectra of six
hosts lying in the redshift range of 0.8 < z < 1.3. Some of these hosts have
been observed before, yet we still lack well constrained information on their
characteristics such as metallicity, dust extinction and star formation rate.
We search for emission lines in the VLT/X-Shooter spectra of the hosts and
measure their fluxes. We perform a detailed analysis, estimating host average
extinction, star-formation rates, metallicities and electron densities where
possible. Measured quantities of our hosts are compared to a larger sample of
previously observed GRB hosts at z < 2. Star-formation rates and metallicities
are measured for all the hosts analyzed in this paper and metallicities are
well determined for 4 hosts. The mass-metallicity relation, the fundamental
metallicity relation and SFRs derived from our hosts occupy similar parameter
space as other host galaxies investigated so-far at the same redshift. We
therefore conclude that GRB hosts in our sample support the found discrepancy
between the properties of low-redshift GRB hosts and the general population of
star- forming galaxies.Comment: 13 pages, 6 figures, accepted for publication in MNRA
GAME: Grb and All-sky Monitor Experiment
We describe the GRB and All-sky Monitor Experiment (GAME) mission submitted
by a large international collaboration (Italy, Germany, Czech Repubblic,
Slovenia, Brazil) in response to the 2012 ESA call for a small mission
opportunity for a launch in 2017 and presently under further investigation for
subsequent opportunities. The general scientific objective is to perform
measurements of key importance for GRB science and to provide the wide
astrophysical community of an advanced X-ray all-sky monitoring system. The
proposed payload was based on silicon drift detectors (~1-50 keV), CdZnTe (CZT)
detectors (~15-200 keV) and crystal scintillators in phoswich (NaI/CsI)
configuration (~20 keV-20 MeV), three well established technologies, for a
total weight of ~250 kg and a required power of ~240 W. Such instrumentation
allows a unique, unprecedented and very powerful combination of large field of
view (3-4 sr), a broad energy energy band extending from ~1 keV up to ~20 MeV,
an energy resolution as good as ~300 eV in the 1-30 keV energy range, a source
location accuracy of ~1 arcmin. The mission profile included a launch (e.g., by
Vega) into a low Earth orbit, a baseline sky scanning mode plus pointed
observations of regions of particular interest, data transmission to ground via
X-band (4.8 Gb/orbit, Alcantara and Malindi ground stations), and prompt
transmission of GRB / transient triggers.Comment: 13 pages, 8 figures, published in International Journal of Modern
Physics
Spectrophotometric analysis of GRB afterglow extinction curves with X-shooter
In this work we use gamma-ray burst (GRB) afterglow spectra observed with the
VLT/X-shooter spectrograph to measure rest-frame extinction in GRB
lines-of-sight by modeling the broadband near-infrared (NIR) to X-ray afterglow
spectral energy distributions (SEDs). Our sample consists of nine Swift GRBs,
eight of them belonging to the long-duration and one to the short-duration
class. Dust is modeled using the average extinction curves of the Milky Way and
the two Magellanic Clouds. We derive the rest-frame extinction of the entire
sample, which fall in the range .
Moreover, the SMC extinction curve is the preferred extinction curve template
for the majority of our sample, a result which is in agreement with those
commonly observed in GRB lines-of-sights. In one analysed case (GRB 120119A),
the common extinction curve templates fail to reproduce the observed
extinction. To illustrate the advantage of using the high-quality X-shooter
afterglow SEDs over the photometric SEDs, we repeat the modeling using the
broadband SEDs with the NIR-to-UV photometric measurements instead of the
spectra. The main result is that the spectroscopic data, thanks to a
combination of excellent resolution and coverage of the blue part of the SED,
are more successful in constraining the extinction curves and therefore the
dust properties in GRB hosts with respect to photometric measurements. In all
cases but one the extinction curve of one template is preferred over the
others. We show that the modeled values of the extinction and the spectral
slope, obtained through spectroscopic and photometric SED analysis, can differ
significantly for individual events. Finally we stress that, regardless of the
resolution of the optical-to-NIR data, the SED modeling gives reliable results
only when the fit is performed on a SED covering a broader spectral region.Comment: 17 pages, 7 figures, 4 tables, accepted for publication in Astronomy
& Astrophysic
Ten per cent polarized optical emission from GRB 090102
The nature of the jets and the role of magnetic fields in gamma-ray bursts
(GRB) remains unclear. In a baryon-dominated jet only weak, tangled fields
generated in situ through shocks would be present. In an alternative model,
jets are threaded with large scale magnetic fields that originate at the
central engine and which accelerate and collimate the jets. The way to
distinguish between the models is to measure the degree of polarization in
early-time emission, however previous claims of gamma-ray polarization have
been controversial. Here we report that the early optical emission from GRB
090102 was polarized at the level of P=10+/-1%, indicating the presence of
large-scale fields originating in the expanding fireball. If the degree of
polarization and its position angle were variable on timescales shorter than
our 60-s exposure, then the peak polarization may have been larger than 10 per
cent.Comment: 16 pages, 4 figures. Published in Nature (2009), Vol. 462, p767-76
Rise and fall of the X-ray flash 080330: an off-axis jet?
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO). DOI: 10.1051/0004-6361/200911719Context. X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with a peak energy of the time-integrated spectrum, , typically below 30 keV, whereas classical GRBs have of a few hundreds of keV. Apart from and the systematically lower luminosity, the properties of XRFs, such as their duration or spectral indices, are typical of the classical GRBs. Yet, the nature of XRFs and their differences from GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. Aims. We examine in detail the case of XRF 080330 discovered by Swift at redshift 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich broadband (from NIR to UV) photometric data set we collected during this phase makes it an ideal candidate for testing the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. Methods. We present prompt -ray, early and late NIR/visible/UV and X-ray observations of the XRF 080330. We derive a spectral energy distribution from NIR to X-ray bands across the shallow/plateau phase and describe the temporal evolution of the multi-wavelength afterglow within the context of the standard afterglow model. Results. The multiwavelength evolution of the afterglow is achromatic from ~102 s to ~8104 s. The energy spectrum from NIR to X-ray is reproduced well by a simple power-law, , with = 0.790.01 and negligible rest-frame dust extinction. The light curve can be modelled by either a piecewise power-law or the combination of a smoothly broken power law with an initial rise up to ~600 s, a plateau lasting up to ~2 ks, followed by a gradual steepening to a power-law decay index of ~2 until 82 ks. At this point, a bump appears to be modelled well with a second component, while the corresponding optical energy spectrum, , reddens by = 0.260.06. Conclusions. A single-component jet viewed off-axis can explain the light curve of XRF 080330, the late-time reddening being due to the reverse shock of an energy injection episode and its being an XRF. Other possibilities, such as the optical rise marking the pre-deceleration of the fireball within a wind environment, cannot be excluded definitely, but appear to be contrived. We exclude the possibility of a dust decreasing column density being swept up by the fireball as explaining the rise of the afterglow.Peer reviewe
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