55 research outputs found
Spectral evolution in gamma-ray bursts: Predictions of the internal shock model and comparison to observations
International audienceContext. Several trends have been identified in the prompt gamma-ray burst (GRB) emission: e.g. hard-to-soft evolution, pulse width evolution with energy, time lags, and hardness-intensity and hardness-fluence correlations. Recently, Fermi has significantly extended the spectral coverage of GRB observations and improved the characterization of this spectral evolution.Aims. We want to study how internal shocks can reproduce these observations. In this model the emission comes from the synchrotron radiation of shock accelerated electrons, and the spectral evolution is governed by the evolution of the physical conditions in the shocked regions.Methods. We present a comprehensive set of simulations of a single pulse and investigate the impact of the model parameters, related to the shock microphysics and to the initial conditions in the ejecta.Results. We find general qualitative agreement between the model and the various observations used for the comparison. All these properties or relations are governed by the evolution of the peak energy and photon indices of the spectrum. In addition, we identify the conditions for quantitative agreement. We find that the best agreement is obtained for (i) steep electron slopes (p âł 2.7); (ii) microphysics parameters varying with shock conditions so that more electrons are accelerated in stronger shocks; and (iii) steep variations in the initial Lorentz factor in the ejecta. When simulating short GRBs by contracting all timescales, all other parameters being unchanged, we show that the hardness-duration correlation is reproduced, as well as the evolution with duration of the pulse properties. Finally, we investigate the signature at high energy of these different scenarios and find distinct properties â delayed onset, longer emission, and flat spectrum in some cases â suggesting that internal shocks could have a significant contribution to the prompt LAT emission.Conclusions. Spectral evolution is an important property of GRBs that is not easily reproduced in most models for the prompt emission. We find that the main observed features can be accounted for in a quantitative way within the internal shock model. However, the current uncertainties on shock acceleration in the mildly relativistic regime and relativistic ejection by compact sources prevent us from deciding if one or several of the proposed scenario are viable. By combining observations over the whole spectral range of Fermi, it may be possible in the future to identify specific signatures imprinted by this uncertain underlying physics
Ending the prompt phase in photospheric models of gamma-ray bursts
The early steep decay, a rapid decrease in X-ray flux as a function of time
following the prompt emission, is a robust feature seen in almost all gamma-ray
bursts with early enough X-ray observations. This peculiar phenomenon has often
been explained as emission from high latitudes of the last flashing shell.
However, in photospheric models of gamma-ray bursts, the timescale of
high-latitude emission is generally short compared to the duration of the steep
decay phase, and hence an alternative explanation is needed. In this paper, we
show that the early steep decay can directly result from the final activity of
the dying central engine. We find that the corresponding photospheric emission
can reproduce both the temporal and spectral evolution observed. This requires
a late-time behaviour that should be common to all GRB central engines, and we
estimate the necessary evolution of the kinetic power and the Lorentz factor.
If this interpretation is correct, observation of the early steep decay can
grant us insights into the last stages of central activity, and provide new
constraints on the late evolution of the Lorentz factor and photospheric
radius.Comment: 7 pages, 3 figures. Submitted to A&
Flares in gamma-ray burst X-ray afterglows as prompt emission from slightly misaligned structured jets
We develop a model to explain the flaring activity in gamma-ray burst X-ray
afterglows within the framework of slightly misaligned observers to structured
jets. We suggest that flares could be the manifestation of prompt dissipation
within the core of the jet, appearing to a misaligned observer in the X-ray
band because of less favorable Doppler boosting. These flares appear during the
afterglow phase because of core--observer light travel delays. In this picture,
the prompt emission recorded by this observer comes from material along their
line of sight, in the lateral structure of the jet, outside the jet's core. We
start by laying down the basic analytical framework to determine the flares
characteristics as a function of those of the gamma-ray pulse an aligned
observer would have seen. We show that, for typical flare observing times and
luminosities, there is indeed viable parameter space to explain flares in this
way. We then analytically explore this model and show that it naturally
produces flares with small width, a salient observed property of flares. We
perform fits of our model to two Swift/XRT flares representing two different
types of morphology, to show that our model can capture both. The ejection time
of the core jet material responsible of the flare is a critical parameter.
While it always remains small compared to the observed time of the flare,
confirming that our model does not require very late central engine activity,
late ejection times are strongly favored, sometimes larger than the observed
duration of the parent gamma-ray burst's prompt phase as measured by .Comment: Main text 11 pages, 4 figures and 2 tables. Submitted to MNRAS;
comments welcom
The Signature of Refreshed Shocks in the of Afterglow of GRB030329
GRB030329 displays one clear and, possibly, multiple less intense fast-rising
() jumps in its optical afterglow light curve. The decay
rate of the optical light curve remains the same before and after the photon
flux jumps. This may be the signature of energy injection into the forward and
reverse shocked material at the front of the jet. In this study, we model the
Gamma-Ray Burst (GRB) ejecta as a series of shells of material. We follow the
dynamical evolution of the ejecta as it interacts with itself (i.e., internal
shocks) and with the circumburst medium (i.e., external forward and reverse
shocks), and we calculate the emission from each shock event assuming
synchrotron emission. We confirm the viability of the model proposed by
\citet{2003Natur.426..138G} in which the jumps in the optical afterglow light
curve of GRB030329 are produced via refreshed shocks. The refreshed shocks may
be the signatures of the collisions between earlier ejected material with an
average Lorentz factor and later ejected material
with once the early material has decelerated due to
interaction with the circumburst medium. We show that even if the late material
is ejected with a spread of Lorentz factors, internal shocks naturally produce
a narrow distribution of Lorentz factors (),
which is a necessary condition to produce the observed quick rise times of the
jumps. These results imply a phase of internal shocks at some point in the
dynamical evolution of the ejecta, which requires a low magnetization in the
outflow.Comment: 11 pages, 6 figure
On the stability of thick accretion disks around black holes
Discerning the likelihood of the so-called runaway instability of thick
accretion disks orbiting black holes is an important issue for most models of
cosmic gamma-ray bursts. To this aim we investigate this phenomenon by means of
time-dependent, hydrodynamical simulations of black hole plus torus systems in
general relativity. The evolution of the central black hole is assumed to be
that of a sequence of Kerr black holes of increasing mass and spin, whose
growth rate is controlled by the transfer of mass and angular momentum from the
material of the disk spiralling in through the event horizon of the black hole.
The self-gravity of the disk is neglected. We find that when the black hole
mass and spin are allowed to increase, constant angular momentum disks undergo
a runaway instability on a dynamical timescale (a few orbital periods).
However, our simulations show that a slight increase of the specific angular
momentum of the disk outwards has a dramatic stabilizing effect. Our results,
obtained in the framework of general relativity, are in broad agreement with
earlier studies based both on stationary models and on time-dependent
simulations with Newtonian and pseudo-Newtonian gravitational potentials.Comment: 12 pages, 3 figures, accepted for publication in ApJ
Variable polarization measured in the prompt emission of GRB 041219A using IBIS on board INTEGRAL
Polarization measurements provide direct insight into the nature of
astrophysical processes. Unfortunately, only a few instruments are available
for this kind of measurements at gamma-ray energies, and the sources need to be
very bright. Gamma-Ray Bursts (GRBs) are ideal candidates due to their large
flux over limited time intervals, maximizing the available signal-to-noise
ratio. To date a few polarization measurements have been reported, claiming of
a high degree of polarization in the prompt emission of GRBs but with low
statistical evidence.
We used the IBIS telescope on board the INTEGRAL satellite to measure the
polarization of the prompt gamma-ray emission of the long and bright GRB
041219A in the 200-800 keV energy band. We find a variable degree of
polarization ranging from less than 4% over the first peak to 43+/-25% for the
whole second peak. Time resolved analysis of both peaks indicates a high degree
of polarization, and the null average polarization in the first peak can be
explained by the rapid variations observed in the polarization angle and
degree.
Our results are consistent with different models for the prompt emission of
GRBs at these energies, but they favor synchrotron radiation from a
relativistic outflow with a magnetic field which is coherent on an angular size
comparable with the angular size of the emitting region (~1/Gamma) . Indeed
this model has the best capabilities to maintain a high polarization level, and
to produce the observed variability.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
The runaway instability of thick discs around black holes. II. Non constant angular momentum discs
We present results from a comprehensive number of relativistic,
time-dependent, axisymmetric simulations of the runaway instability of
non-constant angular momentum thick discs around black holes. This second paper
extends earlier results where only constant angular momentum discs were
considered. All relevant aspects of the theory of stationary thick discs around
rotating black holes, necessary to build the initial state in our simulations,
are presented in great detail. The angular momentum of the discs is assumed to
increase outwards with the radial distance according to a power law. The main
simplifying assumptions of our approach are not to include magnetic fields and
self-gravity in the discs. Furthermore, the dynamics of the spacetime is
accounted for by computing the transfer of mass and angular momentum from the
disc to the black hole through the event horizon : the evolution of the central
black hole is assumed to follow a sequence of Kerr black holes of increasing
mass and spin. In agreement with previous results based on stationary models we
find that by allowing the mass and the spin of the black hole to grow, constant
angular momentum discs rapidly become unstable on a dynamical timescale. The
comparison with the results of paper I shows that the effect of the angular
momentum transfer from the torus to the black hole is to make constant angular
momentum discs less unstable, increasing the timescale of the instability.
However, we find that non-constant angular momentum discs are dramatically
stabilized for very small values of the angular momentum slope. Our
time-dependent simulations confirm, thus, the predictions of stationary studies
concerning the stabilizing effect of non-constant angular momentum
distributions.Comment: 36 pages, 18 figures, submitted to MNRA
"Orphan" afterglows in the Universal Structured Jet Model for gamma-ray bursts
The paucity of reliable achromatic breaks in Gamma-Ray Burst afterglow light
curves motivates independent measurements of the jet aperture. Serendipitous
searches of afterglows, especially at radio wavelengths, have long been the
classic alternative. These survey data have been interpreted assuming a
uniformly emitting jet with sharp edges (``top-hat'' jet), in which case the
ratio of weakly relativistically beamed afterglows to GRBs scales with the jet
solid angle. In this paper, we consider, instead, a very wide outflow with a
luminosity that decreases across the emitting surface. In particular, we adopt
the universal structured jet (USJ) model, that is an alternative to the top-hat
model for the structure of the jet. However, the interpretation of the survey
data is very different: in the USJ model we only observe the emission within
the jet aperture and the observed ratio of prompt emission rate to afterglow
rate should solely depend on selection effects. We compute the number and rate
of afterglows expected in all-sky snapshot observations as a function of the
survey sensitivity. We find that the current (negative) results for OA searches
are in agreement with our expectations. In radio and X-ray bands this was
mainly due to the low sensitivity of the surveys, while in the optical band the
sky-coverage was not sufficient. In general we find that X-ray surveys are poor
tools for OA searches, if the jet is structured. On the other hand, the FIRST
radio survey and future instruments like the Allen Telescope Array (in the
radio band) and especially GAIA, Pan-Starrs and LSST (in the optical band) will
have chances to detect afterglows.Comment: 10 pages, 8 figures. MNRAS accepted. Moderate revision
Detection of a Thermal Spectral Component in the Prompt Emission of GRB 100724B
Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor (GBM) find
that the spectrum is dominated by the typical Band functional form, which is
usually taken to represent a non-thermal emission component, but also includes
a statistically highly significant thermal spectral contribution. The
simultaneous observation of the thermal and non-thermal components allows us to
confidently identify the two emission components. The fact that these seem to
vary independently favors the idea that the thermal component is of
photospheric origin while the dominant non-thermal emission occurs at larger
radii. Our results imply either a very high efficiency for the non-thermal
process, or a very small size of the region at the base of the flow, both quite
challenging for the standard fireball model. These problems are resolved if the
jet is initially highly magnetized and has a substantial Poynting flux.Comment: 6 pages, 3 figures, 1 table, Accepted for publication in the
Astrophysical Journal Letters November, 23 2010 (Submitted October, 20 2010
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