57 research outputs found
Testing the GRB Variability/Peak Luminosity correlation using the pseudo-redshifts of a large sample of BATSE GRBs
We test the correlation found by Reichart et al. (2001) between time
variability and peak luminosity of Gamma-Ray Bursts (GRBs). Recently Guidorzi
et al. (2005) found that this still holds for a sample of 32 GRBs with
spectroscopic redshift, although with a larger scatter than that originally
found by Reichart et al. (2001). However Guidorzi et al. (2005) also found that
a power law does not provide a good description of that. We report on the same
test performed on a sample of 551 BATSE GRBs with a significant measure of
variability assuming the pseudo-redshifts derived by Band et al. (2004) (1186
GRBs) through the anticorrelation between spectral lag and peak luminosity. We
still find a correlation between variability as defined by Reichart et al.
(2001) and peak luminosity with higher significance. However, this subsample of
BATSE GRBs show a higher scatter around the best-fitting power law than that
found by Reichart et al. (2001) in the variability/peak luminosity space. This
is in agreement with the result found by Guidorzi et al. (2005) on a sample of
32 GRBs with measured redshift. These results confirm that a power law does not
provide a satisfactory description for all the GRBs, in contrast with the
original findings by Reichart et al. (2001).Comment: MNRAS, accepted, 6 pages, 4 figure
Evidence for a long duration component in the prompt emission of short Gamma-Ray Bursts detected with BeppoSAX
A statistical study on the light curves of all the short Gamma-Ray Bursts
detected with the Gamma Ray Burst Monitor (GRBM) aboard BeppoSAX is reported.
Evidence for a very weak and long duration component associated with these
events in the two 1 s counters of the GRBM (40-700 keV and >100 keV) is found.
It starts a few tens of seconds before the burst and continues for about 30 s
after the burst. The overall hardness of this component is comparable with that
of the event itself. The detection of a signal before the onset time and the
similar hardness are consistent with an interpretation of the long duration
component in terms of prompt emission associated with short GRBs.Comment: 12 pages, 6 figures, accepted for publication in ApJ
A Python approach for GRB afterglow analysis: sAGa (Software for AfterGlow Analysis)
This technical note describes a fully self-consistent code in Python – called sAGa (Software for AfterGlow Analysis) – to cope with the complex landscape of GRB afterglows. sAGa adds up to other pre-existing broadband fitting tools in the literature and provides an independent check, emphasising the broadband study of GRB afterglows over the last two decades.
This code aims to model GRB afterglow data within a self-consistent physically grounded picture. Built adopting a Bayesian approach, all the data set, from radio to gamma-rays, is modelled. By-products are plots of spectra and light-curves, and computation of the break frequencies and normalisations as a function of the shock microphysical parameters, such as the power-law index of the electron energy distribution, the fractions of the blastwave energy delivered to relativistic electrons and magnetic fields, and other parameters such as the kinetic energy of the explosion and the density of the circumburst medium (CBM).
Dust extinction of optical along the sightline and scintillation in radio frequencies are also accounted for.
sAGa has been successfully tested on the broadband data of the afterglows of GRB120521C, GRB090423, and GRB050904. Our results are consistent with those reported in the literature within 2 sigma. Moreover, the values of the power-law index of the electron energy distribution obtained from sAGa analysis are compatible with the inferences based on the
lines of reasoning based on the observation of the optical/X-ray spectra
A numerical jet model for the prompt emission of gamma-ray bursts
Gamma-ray bursts (GRBs) are known to be highly collimated events, and are
mostly detectable when they are seen on-axis or very nearly on-axis. However,
GRBs can be seen from off-axis angles, and the recent detection of a short GRB
associated to a gravitational wave event has conclusively shown such a
scenario. The observer viewing angle plays an important role in the observable
spectral shape and the energetic of such events. We present a numerical model
which is based on the single-pulse approximation with emission from a top-hat
jet and has been developed to investigate the effects of the observer viewing
angle. We assume a conical jet parametrized by a radius ,
half-opening angle , a comoving-frame emissivity law and an
observer viewing angle , and then study the effects for the
conditions and . We present results considering a smoothly broken power-law
emissivity law in jet comoving frame, albeit the model implementation easily
allows to consider other emissivity laws. We find that the relation (Amati relation) is naturally obtained
from pure relativistic kinematic when and it results . Using data
from literature for a class of well-know sub-energetic GRBs, we show that their
position in the plane is consistent with event
observed off-axis. The presented model is developed as a module to be
integrated in spectral fitting software package XSPEC and can be used by the
scientific community.Comment: 11 pages, 8 figures, accepted for publication in MNRA
Broad band turbulent spectra in gamma-ray burst light curves
Broad band power density spectra offer a window to understanding turbulent
behavior in the emission mechanism and, at the highest frequencies, in the
putative inner engines powering long GRBs. We describe a chirp search method
which steps aside Fourier analysis for signal detection in the Poisson
noise-dominated 2 kHz sampled BeppoSAX light curves. An efficient numerical
implementation is described in operations, where is the
number of chirp templates and is the length of the light curve time series,
suited for embarrassingly parallel processing. For detection of individual
chirps of duration s, the method is one order of magnitude more
sensitive in SNR than Fourier analysis. The Fourier-chirp spectra of GRB 010408
and GRB 970816 show a continuation of the spectral slope up to 1 kHz of
turbulence identified in low frequency Fourier analysis. The same continuation
is observed in an ensemble averaged spectrum of 40 bright long GRBs. An outlook
on a similar analysis of upcoming gravitational wave data is included
Prospects for multi-messenger extended emission from core-collapse supernovae in the Local Universe
Multi-messenger emissions from SN1987A and GW170817/GRB170817A suggest a
Universe rife with multi-messenger transients associated with black holes and
neutron stars. For LIGO-Virgo, soon to be joined by KAGRA, these observations
promise unprecedented opportunities to probe the central engines of
core-collapse supernovae (CC-SNe) and gamma-ray bursts. Compared to neutron
stars, central engines powered by black hole-disk or torus systems may be of
particular interest to multi-messenger observations by the relatively large
energy reservoir of angular momentum, up to 29\% of total mass in the
Kerr metric. These central engines are expected from relatively massive stellar
progenitors and compact binary coalescence involving a neutron star. We review
prospects of multi-messenger emission by catalytic conversion of by a
non-axisymmetric disk or torus. Observational support for this radiation
process is found in a recent identification of in Extended Emission to GW170817 at a significance of
4.2\, concurrent with GRB170817A. A prospect on similar emissions from
nearby CC-SNe justifies the need for all-sky blind searches of long duration
bursts by heterogeneous computing.Comment: 96 pages, 20 figure
Power density spectrum of nonstationary short-lived light curves
The power density spectrum of a light curve is often calculated as the
average of a number of spectra derived on individual time intervals the light
curve is divided into. This procedure implicitly assumes that each time
interval is a different sample function of the same stochastic ergodic process.
While this assumption can be applied to many astrophysical sources, there
remains a class of transient, highly nonstationary and short-lived events, such
as gamma-ray bursts, for which this approach is often inadequate. The power
spectrum statistics of a constant signal affected by statistical (Poisson)
noise is known to be a chi2(2) in the Leahy normalisation. However, this is no
more the case when a nonstationary signal is also present. As a consequence,
the uncertainties on the power spectrum cannot be calculated based on the
chi2(2) properties, as assumed by tools such as XRONOS powspec. We generalise
the result in the case of a nonstationary signal affected by uncorrelated white
noise and show that the new distribution is a non-central chi2(2,lambda), whose
non-central value lambda is the power spectrum of the deterministic function
describing the nonstationary signal. Finally, we test these results in the case
of synthetic curves of gamma-ray bursts. We end up with a new formula for
calculating the power spectrum uncertainties. This is crucial in the case of
nonstationary short-lived processes affected by uncorrelated statistical noise,
for which ensemble averaging does not make any physical sense.Comment: 11 pages, 5 figures, accepted to MNRA
Imaging performance above 150 keV of the wide field monitor on board the ASTENA concept mission
A new detection system for X-/Gamma-ray broad energy passband detectors for
astronomy has been developed. This system is based on Silicon Drift Detectors
(SDDs) coupled with scintillator bars; the SDDs act as a direct detector of
soft (<30 keV) X-ray photons, while hard X-/Gamma-rays are stopped by the
scintillator bars and the scintillation light is collected by the SDDs. With
this configuration, it is possible to build compact, position sensitive
detectors with unprecedented energy passband (2 keV - 10/20 MeV). The X and
Gamma-ray Imaging Spectrometer (XGIS) on board the THESEUS mission, selected
for Phase 0 study for M7, exploits this innovative detection system. The Wide
Field Monitor - Imager and Spectrometer (WFM-IS) of the ASTENA (Advanced
Surveyor of Transient Events and Nuclear Astrophysics) mission concept consists
of 12 independent detection units, also based on this new technology. For the
WFM-IS, a coded mask provides imaging capabilities up to 150 keV, while above
this limit the instrument will act as a full sky spectrometer. However, it is
possible to extend imaging capabilities above this limit by alternatively
exploiting the Compton kinematics reconstruction or by using the information
from the relative fluxes measured by the different cameras. In this work, we
present the instrument design and results from MEGAlib simulations aimed at
evaluating the effective area and the imaging performances of the WFM-IS above
150 keV
A flash of polarized optical light points to an aspherical ‘cow’
The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool telescope RINGO3 observations of AT 2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached ∼ 7% at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT 2018cow at early times. A second 'bump' in the polarization is observed at blue wavelengths at ∼ 12 days. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period (<1 d), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be ∼ 10%.</p
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