385 research outputs found
Evidence for a dual population of neutron star mergers from short Gamma-Ray Burst observations
Short duration Gamma-Ray Bursts are thought to originate from the coalescence
of neutron stars in binary systems. They are detected as a brief ( 2s),
intense flash of gamma-ray radiation followed by a weaker, rapidly decreasing
afterglow. They are expected to be detected by Advanced LIGO and Virgo when
their sensitivity will be low enough. In a recent study we identified a
population of short Gamma-Ray Bursts that are intrinsically faint and nearby.
Here we provide evidence in favor of the existence of this new population that
can hardly be reproduced with a model of field neutron star binary
coalescences. We propose that these systems may be produced dynamically in
globular clusters, and may result from the merger of a black hole and a neutron
star. The advanced LIGO and Virgo observation of a high rate of NSBH mergers
compatible with the dynamical formation in globular clusters would be a
confirmation of this hypothesis and would enable for the derivation of the mass
function of black holes inside globular clusters, as well as the luminosity
function of faint short GRBs.Comment: 15 pages, 5 figures, 1 table, submitted to Ap
Revisiting coincidence rate between Gravitational Wave detection and short Gamma-Ray Burst for the Advanced and third generation
We use realistic Monte-Carlo simulations including both gravitational-wave
and short gamma-ray burst selection effects to revisit the coincident rate of
binary systems composed of two neutron stars or a neutron star and a black
hole. We show that the fraction of GW triggers that can be observed in
coincidence with sGRBs is proportional to the beaming factor at , but
increases with the distance, until it reaches 100 \% at the GW detector horizon
distance. When this is taken into account the rate is improved by a factor of
compared to the simple beaming factor correction. We provide an estimate
of the performance future GRB detectors should achieve in order to fully
exploit the potentiality of the planned third generation GW antenna Einstein
Telescope, and we propose a simple method to constrain the beaming angle of
sGRBs.Comment: 31 pages, 11 figures, 4 tables, accepted for publication in Ap
Studying the WHIM with Gamma Ray Bursts
We assess the possibility to detect and characterize the physical state of
the missing baryons at low redshift by analyzing the X-ray absorption spectra
of the Gamma Ray Burst [GRB] afterglows, measured by a micro calorimeters-based
detector with 3 eV resolution and 1000 cm2 effective area and capable of fast
re-pointing, similar to that on board of the recently proposed X-ray satellites
EDGE and XENIA. For this purpose we have analyzed mock absorption spectra
extracted from different hydrodynamical simulations used to model the
properties of the Warm Hot Intergalactic Medium [WHIM]. These models predict
the correct abundance of OVI absorption lines observed in UV and satisfy
current X-ray constraints. According to these models space missions like EDGE
and XENIA should be able to detect about 60 WHIM absorbers per year through the
OVII line. About 45 % of these have at least two more detectable lines in
addition to OVII that can be used to determine the density and the temperature
of the gas. Systematic errors in the estimates of the gas density and
temperature can be corrected for in a robust, largely model-independent
fashion. The analysis of the GRB absorption spectra collected in three years
would also allow to measure the cosmic mass density of the WHIM with about 15 %
accuracy, although this estimate depends on the WHIM model. Our results suggest
that GRBs represent a valid, if not preferable, alternative to Active Galactic
Nuclei to study the WHIM in absorption. The analysis of the absorption spectra
nicely complements the study of the WHIM in emission that the spectrometer
proposed for EDGE and XENIA would be able to carry out thanks to its high
sensitivity and large field of view.Comment: 16 pages, 16 figures, accepted for publication by Ap
Toward an optimal search strategy of optical and gravitational wave emissions from binary neutron star coalescence
Observations of an optical source coincident with gravitational wave emission
detected from a binary neutron star coalescence will improve the confidence of
detection, provide host galaxy localisation, and test models for the
progenitors of short gamma ray bursts. We employ optical observations of three
short gamma ray bursts, 050724, 050709, 051221, to estimate the detection rate
of a coordinated optical and gravitational wave search of neutron star mergers.
Model R-band optical afterglow light curves of these bursts that include a
jet-break are extrapolated for these sources at the sensitivity horizon of an
Advanced LIGO/Virgo network. Using optical sensitivity limits of three
telescopes, namely TAROT (m=18), Zadko (m=21) and an (8-10) meter class
telescope (m=26), we approximate detection rates and cadence times for imaging.
We find a median coincident detection rate of 4 yr^{-1} for the three bursts.
GRB 050724 like bursts, with wide opening jet angles, offer the most optimistic
rate of 13 coincident detections yr^{-1}, and would be detectable by Zadko up
to five days after the trigger. Late time imaging to m=26 could detect off-axis
afterglows for GRB 051221 like bursts several months after the trigger. For a
broad distribution of beaming angles, the optimal strategy for identifying the
optical emissions triggered by gravitational wave detectors is rapid response
searches with robotic telescopes followed by deeper imaging at later times if
an afterglow is not detected within several days of the trigger.Comment: 6 pages, 1 figure, Accepted for publication in MNRAS Letters (2011
April 22
Swift multi-wavelength observations of the bright flaring burst GRB051117A
We report on the temporal and spectral characteristics of the early X-ray
emission from the Gamma Ray Burst 051117A as observed by Swift. The superb
quality of the early X-ray light-curve and spectra of this source, one of the
brightest seen by the X-ray Telescope at such early times, allows an
unprecedented look at the spectral and temporal evolution of the prompt and
early afterglow emission for this GRB and allows us to place stringent limits
on the detection of lines. The X-ray light-curve at early times is
characteristic of a shot-noise process, with individual shots well-modelled by
a fast-rise and exponential decay spanning a broad range in rise-times and
decay rates. A temporal spectral analysis of the early light-curve shows that
the photon index and source intensity are highly correlated with the spectrum
being significantly harder when brighter, consistent with the movement of the
peak of the Band function to lower energies following individual flares. The
high quality spectrum obtained from the first orbit of WT mode data, enables us
to place a 3 sigma upper limit on the strength of any emission line features of
EW < 15 eV, assuming a narrow emission-line of 100 eV at the peak of the
effective area (abridged).Comment: Accepted 15/3/2007 - To appear in A&
X-ray Flashes or soft Gamma-ray Bursts? The case of the likely distant XRF 040912
In this work, we present a multi-wavelength study of XRF 040912, aimed at
measuring its distance scale and the intrinsic burst properties. We performed a
detailed spectral and temporal analysis of both the prompt and the afterglow
emission and we estimated the distance scale of the likely host galaxy. We then
used the currently available sample of XRFs with known distance to discuss the
connection between XRFs and classical Gamma-ray Bursts (GRBs). We found that
the prompt emission properties unambiguously identify this burst as an XRF,
with an observed peak energy of E_p=17+/-13 keV and a burst fluence ratio
S(2-30keV)/S(30-400keV)>1. A non-fading optical source with R~24 mag and with
an apparently extended morphology is spatially consistent with the X-ray
afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no
afterglow optical counterpart is detected down to R>25 mag (3 sigma limiting
magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected
from 3800A to 10000A, shows a single emission line at 9552A. The lack of any
other strong emission lines blue-ward of the detected one and the absence of
the Ly alpha cut-off down to 3800A are consistent with the hypothesis of the
[OII] line at redshift z=1.563+/-0.001. The intrinsic spectral properties rank
this XRF among the soft GRBs in the E_peak-E_iso diagram. Similar results were
obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903
represent a good example of instrinsic XRF(i-XRF) and are possibly associated
with a different progenitor population. This scenario may calls for a new
definition of XRFs.Comment: 10 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
On leptonic models for blazars in the Fermi era
Some questions raised by Fermi-LAT data about blazars are summarized, along
with attempts at solutions within the context of leptonic models. These include
both spectral and statistical questions, including the origin of the GeV breaks
in low-synchrotron peaked blazars, the location of the gamma-ray emission
sites, the correlations in the spectral energy distributions with luminosity,
and the difficulty of synchrotron/SSC models to fit the spectra of some TeV
blazars.Comment: 9 pages, 1 figure, in "Beamed and Unbeamed Gamma Rays from Galaxies,"
Muonio, Finland, 11-15 April, 2011, ed. R. Wagner, L. Maraschi, A. Sillanpaa,
to appear in Journal of Physics: Conference Serie
The circumburst density profile around GRB progenitors: a statistical study
According to our present understanding, long GRBs originate from the collapse
of massive stars while short bursts are due to the coalescence of compact
stellar objects. Since the afterglow evolution is determined by the circumburst
density profile, n(r), traversed by the fireball, it can be used to distinguish
between a so-called ISM profile, n(r) = const., and a free stellar wind, . Our goal is to derive the most probable circumburst density
profile for a large number of Swift-detected bursts using well-sampled
afterglow light curves in the optical and X-ray bands. We combined all publicly
available optical and Swift/X-ray afterglow data from June 2005 to September
2009 to find the best-sampled late-time afterglow light curves. After applying
several selection criteria, our final sample consists of 27 bursts, including
one short burst. The afterglow evolution was then studied within the framework
of the fireball model. We find that the majority (18) of the 27 afterglow light
curves are compatible with a constant density medium (ISM case). Only 6 of the
27 afterglows show evidence for a wind profile at late times. In particular, we
set upper limits on the wind termination-shock radius, , for GRB fireballs
which are propagating into an ISM profile and lower limits on for those
which were found to propagate through a wind medium. Observational evidence for
ISM profiles dominates in GRB afterglow studies, implying that most GRB
progenitors might have relatively small wind termination-shock radii. A smaller
group of progenitors, however, seems to be characterised by notably more
extended wind regions.Comment: A&A, accepted (Oct 26, 2010); 20 pages in journal format; 6 pages
main text, 13 pages Appendix, 1 page references, 6 tables and 2 figures;
included comments by the referee and language editor; removed grey colouring
of the table
VAST: An ASKAP Survey for Variables and Slow Transients
The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an
unprecedented opportunity to investigate the transient sky at radio
wavelengths. In this paper we present VAST, an ASKAP survey for Variables and
Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP
to enable the discovery and investigation of variable and transient phenomena
from the local to the cosmological, including flare stars, intermittent
pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar
scintillation, radio supernovae and orphan afterglows of gamma ray bursts. In
addition, it will allow us to probe unexplored regions of parameter space where
new classes of transient sources may be detected. In this paper we review the
known radio transient and variable populations and the current results from
blind radio surveys. We outline a comprehensive program based on a multi-tiered
survey strategy to characterise the radio transient sky through detection and
monitoring of transient and variable sources on the ASKAP imaging timescales of
five seconds and greater. We also present an analysis of the expected source
populations that we will be able to detect with VAST.Comment: 29 pages, 8 figures. Submitted for publication in Pub. Astron. Soc.
Australi
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