1,298 research outputs found
A complete sample of bright Swift short Gamma-Ray Bursts
We present a carefully selected sample of short gamma-ray bursts (SGRBs)
observed by the Swift satellite up to June 2013. Inspired by the criteria we
used to build a similar sample of bright long GRBs (the BAT6 sample), we
selected SGRBs with favorable observing conditions for the redshift
determination on ground, ending up with a sample of 36 events, almost half of
which with a redshift measure. The redshift completeness increases up to about
70% (with an average redshift value of z = 0.85) by restricting to those events
that are bright in the 15-150 keV Swift Burst Alert Telescope energy band. Such
flux-limited sample minimizes any redshift-related selection effects, and can
provide a robust base for the study of the energetics, redshift distribution
and environment of the Swift bright population of SGRBs. For all the events of
the sample we derived the prompt and afterglow emission in both the observer
and (when possible) rest frame and tested the consistency with the correlations
valid for long GRBs. The redshift and intrinsic X-ray absorbing column density
distributions we obtain are consistent with the scenario of SGRBs originated by
the coalescence of compact objects in primordial binaries, with a possible
minor contribution (~10%-25%) of binaries formed by dynamical capture (or
experiencing large natal kicks). This sample is expected to significantly
increase with further years of Swift activity.Comment: 17 pages, 8 figures, 7 tables. Accepted for publication in MNRA
Optical and X-ray Rest-frame Light Curves of the BAT6 sample
We present the rest-frame light curves in the optical and X-ray bands of an
unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the
BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest
level of completeness in redshift ( 95%), allowing us to compute the
rest-frame X-ray and optical light curves for 55 and 47 objects, respectively.
We compute the X-ray and optical luminosities accounting for any possible
source of absorption (Galactic and intrinsic) that could affect the observed
fluxes in these two bands. We compare the behaviour observed in the X-ray and
in the optical bands to assess the relative contribution of the emission during
the prompt and afterglow phases. We unarguably demonstrate that the GRBs
rest-frame optical luminosity distribution is not bimodal, being rather
clustered around the mean value Log(L) = 29.9 0.8 when estimated at
a rest frame time of 12 hr. This is in contrast with what found in previous
works and confirms that the GRB population has an intrinsic unimodal luminosity
distribution. For more than 70% of the events the rest-frame light curves in
the X-ray and optical bands have a different evolution, indicating distinct
emitting regions and/or mechanisms. The X-ray light curves normalised to the
GRB isotropic energy (E), provide evidence for X-ray emission still
powered by the prompt emission until late times ( hours after the burst
event). On the other hand, the same test performed for the E-normalised optical light curves shows that the optical emission is a
better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table
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
A magnetar powering the ordinary monster GRB 130427A?
We present the analysis of the extraordinarily bright Gamma-Ray Burst (GRB)
130427A under the hypothesis that the GRB central engine is an
accretion-powered magnetar. In this framework, initially proposed to explain
GRBs with precursor activity, the prompt emission is produced by accretion of
matter onto a newly-born magnetar, and the observed power is related to the
accretion rate. The emission is eventually halted if the centrifugal forces are
able to pause accretion. We show that the X-ray and optical afterglow is well
explained as the forward shock emission with a jet break plus a contribution
from the spin-down of the magnetar. Our modelling does not require any
contribution from the reverse shock, that may still influence the afterglow
light curve at radio and mm frequencies, or in the optical at early times. We
derive the magnetic field ( G) and the spin period (
ms) of the magnetar and obtain an independent estimate of the minimum
luminosity for accretion. This minimum luminosity results well below the prompt
emission luminosity of GRB 130427A, providing a strong consistency check for
the scenario where the entire prompt emission is the result of continuous
accretion onto the magnetar. This is in agreement with the relatively long spin
period of the magnetar. GRB 130427A was a well monitored GRB showing a very
standard behavior and, thus, is a well-suited benchmark to show that an
accretion-powered magnetar gives a unique view of the properties of long GRBs.Comment: 5 pages, 1 figure, accepted for publication in MNRAS Letter
HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?
The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrain
the source of the hard ionizing continuum that powers the HeII emission. We
have assembled a sample of 277 galaxies with a high quality spectroscopic
redshift at 2<z<4.6 from the VVDS survey, and we have identified 39 HeII1640A
emitters. We study their spectral properties, measuring the fluxes, equivalent
widths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 show
HeII in emission, with rest frame equivalent widths EW0~1-7A, equally
distributed between galaxies with Lya in emission or in absorption. We find 11
high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13
high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, and
an additional 12 possible HeII emitters. The properties of the individual broad
emitters are in agreement with expectations from a W-R model. On the contrary,
the properties of the narrow emitters are not compatible with such model,
neither with predictions of gravitational cooling radiation produced by gas
accretion. Rather, we find that the EW of the narrow HeII line emitters are in
agreement with expectations for a PopIII star formation, if the episode of star
formation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1
only is enough to sustain the observed HeII flux. We conclude that narrow HeII
emitters are either powered by the ionizing flux from a stellar population rare
at z~0 but much more common at z~3, or by PopIII star formation. As proposed by
Tornatore et al. (2007), incomplete ISM mixing may leave some small pockets of
pristine gas at the periphery of galaxies from which PopIII may form, even down
to z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD in
PopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the value
predicted by Tornatore et al. (2007).Comment: accepted for publication in A&
The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts
A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently
selected by Salvaterra et al. (2011). The sample has a high level of
completeness in redshift (91%). We derive here the intrinsic absorbing X-ray
column densities of these GRBs making use of the Swift X-ray Telescope data.
This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is
consistent with the distribution of the column densities derived from the total
sample of GRBs with redshift. We find a mild increase of the intrinsic column
density with redshift. This can be interpreted as due to the contribution of
intervening systems along the line of sight. Making use of the spectral index
connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the
relation of the intrinsic column density and the GRB `darkness'. We find that
there is a very tight correlation between dark GRBs and high X-ray column
densities. This clearly indicates that the dark GRBs are formed in a metal-rich
environment where dust must be present.Comment: MNRAS, 6 pages, 3 figures, 1 tabl
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