120 research outputs found
Prompt thermal emission in gamma-ray bursts
GRB spectra appear non-thermal, but recent observations of a few bursts with
Fermi GBM have confirmed previous indications from BATSE of the presence of an
underlying thermal component. Photospheric emission is indeed expected when the
relativistic outflow emerging from the central engine becomes transparent to
its own radiation, with a quasi-blackbody spectrum in absence of additional
sub-photospheric dissipation. However, its intensity strongly depends on the
acceleration mechanism - thermal or magnetic - of the flow. We aim to compute
the thermal and non-thermal emissions produced by an outflow with a variable
Lorentz factor, where the power injected at the origin is partially thermal
(fraction epsilon_th) and partially magnetic (fraction 1-epsilon_th). The
thermal emission is produced at the photosphere, and the non-thermal emission
in the optically thin regime. Apart from the value of epsilon_th, we want to
test how the other model parameters affect the observed ratio of the thermal to
non-thermal emission. If the non-thermal emission is made by internal shocks,
we self-consistently obtained the light curves and spectra of the thermal and
non-thermal components for any distribution of the Lorentz factor in the flow.
If the non-thermal emission results from magnetic reconnection we were unable
to produce a light curve and could only compare the respective non-thermal and
thermal spectra. In the different considered cases, we varied the model
parameters to see when the thermal component in the light curve and/or spectrum
is likely to show up or, on the contrary, to be hidden. We finally compared our
results to the proposed evidence for the presence of a thermal component in GRB
spectra. Focussing on GRB 090902B and GRB 10072B, we showed how these
observations can be used to constrain the nature and acceleration mechanism of
GRB outflows.Comment: 11 pages, 5 figures, accepted for publication in A&
Origin of the bright prompt optical emission in the naked eye burst
The huge optical brightness of GRB 080319B (the "Naked Eye Burst) makes this
event really challenging for models of the prompt GRB emission. In the
framework of the internal shock model, we investigate a scenario where the
dominant radiative process is synchrotron emission and the high optical flux is
due to the dynamical properties of the relativistic outflow: if the initial
Lorentz factor distribution in the jet is highly variable, many internal shocks
will form within the outflow at various radii. The most violent shocks will
produce the main gamma-ray component while the less violent ones will
contribute at lower energy, including the optical range.Comment: 3 pages, 1 figure, proceedings of the conference "Deciphering the
Ancient Universe with Gamma-Ray Bursts", Kyoto, Japan. Editors: N. Kawai, S.
Nagatak
Do Fermi-LAT observations imply very large Lorentz factors in GRB outflows ?
Recent detections of GeV photons in a few GRBs by Fermi-LAT imply huge bulk
Lorentz factors to avoid a large gamma gamma optical depth at high energy.
Estimates can be as high as Gamma ~ 1000 in the most extreme cases. This puts
severe constraints on models of the central engine and the jet acceleration in
GRBs. These estimates are however obtained from a simplified single zone model.
We present here a more realistic calculation which takes into account the time,
space and direction dependent photon field existing in an outflow with several
relativistically moving emitting zones. The formalism is general and can be
applied to many models of the prompt GRB emission. We present results obtained
for a numerical implementation in the framework of the internal shock model. We
show that (i) the minimum Lorentz factor Gamma_min in bright LAT GRBs is
reduced by a factor ~ 2-3 compared to previous estimates if the GeV and MeV
emission are produced in the same region, and by an additional factor ~ 2-8 if
the GeV emission is produced at larger radii. We provide an improved
approximate formula for Gamma_min which is in good agreement with our numerical
results and can be directly applied to LAT GRB data; (ii) a delayed GeV onset
can be due to the time evolution of the opacity. As an illustration of these
two first results, we present a synthetic GRB that reproduces most features of
GRB 080916C with a mean Lorentz factor of ~ 340, an optically thin regime for
gamma gamma opacity at 3GeV in bin 'b', a variability timescale of 0.5s in the
MeV lightcurve and a delayed onset of ~ 5s of the GeV emission; (iii) the gamma
gamma opacity can smooth the short timescale variability in the GeV lightcurve.
This last result implies that the observed variability at high energy is not
necessarily a good test to distinguish between an internal and an external
origin for the GeV emission in GRBs. [abridged]Comment: 23 pages, 11 figure
Accounting for the XRT early steep decay in models of the prompt GRB emission
The Swift-XRT observations of the early X-ray afterglow of GRBs show that it
usually begins with a steep decay phase. A possible origin of this early steep
decay is the high latitude emission that subsists when the on-axis emission of
the last dissipating regions in the relativistic outflow has been switched-off.
We wish to establish which of various models of the prompt emission are
compatible with this interpretation. We successively consider internal shocks,
photospheric emission, and magnetic reconnection and obtain the typical decay
timescale at the end of the prompt phase in each case. Only internal shocks
naturally predict a decay timescale comparable to the burst duration, as
required to explain XRT observations in terms of high latitude emission. The
decay timescale of the high latitude emission is much too short in photospheric
models and the observed decay must then correspond to an effective and generic
behavior of the central engine. Reconnection models require some ad hoc
assumptions to agree with the data, which will have to be validated when a
better description of the reconnection process becomes available.Comment: 4 pages, 2 figure
Evidence for a Photospheric Component in the Prompt Emission of the Short GRB120323A and its Effects on the GRB Hardness-Luminosity Relation
The short GRB 120323A had the highest flux ever detected with the Fermi/GBM.
Here we study its remarkable spectral properties and their evolution using two
spectral models: (i) a single emission component scenario, where the spectrum
is modeled by the empirical Band function, and (ii) a two component scenario,
where thermal (Planck-like) emission is observed simultaneously with a
non-thermal component (a Band function). We find that the latter model fits the
integrated burst spectrum significantly better than the former, and that their
respective spectral parameters are dramatically different: when fit with a Band
function only, the Epeak of the event is unusually soft for a short GRB, while
adding a thermal component leads to more typical short GRB values. Our
time-resolved spectral analysis produces similar results. We argue here that
the two-component model is the preferred interpretation for GRB 120323A, based
on: (i) the values and evolution of the Band function parameters of the two
component scenario, which are more typical for a short GRB, and (ii) the
appearance in the data of a significant hardness-intensity correlation,
commonly found in GRBs, when we employee two-component model fits; the
correlation is non-existent in the Band-only fits. GRB 110721A, a long burst
with an intense photospheric emission, exhibits the exact same behavior. We
conclude that GRB 120323A has a strong photospheric emission contribution,
first time observed in a short GRB. Magnetic dissipation models are difficult
to reconcile with these results, which instead favor photospheric thermal
emission and fast cooling synchrotron radiation from internal shocks. Finally,
we derive a possibly universal hardness-luminosity relation in the source frame
using a larger set of GRBs L,i=(1.59+/-0.84).10^50 (Epeak,i)^(1.33+/-0.07)
erg/s), which could be used as a possible redshift estimator for cosmology.Comment: 27 pages, 13 figures, Accepted by ApJ (April, 7th 2013
How universal is the fractional-quantum-Hall edge Luttinger liquid?
This article reports on our microscopic investigations of the edge of the
fractional quantum Hall state at filling factor . We show that the
interaction dependence of the wave function is well described in an
approximation that includes mixing with higher composite-fermion Landau levels
in the lowest order. We then proceed to calculate the equal time edge Green
function, which provides evidence that the Luttinger exponent characterizing
the decay of the Green function at long distances is interaction dependent. The
relevance of this result to tunneling experiments is discussed.Comment: 5 page
Was the "naked burst" GRB 050421 really naked ?
A few long gamma-ray bursts such as GRB 050421 show no afterglow emission
beyond the usual initial steep decay phase. It has been suggested that these
events correspond to "naked" bursts that occur in a very low density
environment. We reconsider this possibility in the context of various scenarios
for the origin of the afterglow. In the standard model where the afterglow
results from the forward shock as well as in the alternative model where the
afterglow comes from the reverse shock, we aim to obtain constraints on the
density of the environment, the microphysics parameters, or the Lorentz factor
of the ejecta, which are imposed by the absence of a detected afterglow. For
the two models we compute the afterglow evolution for different values of the
external density (uniform or wind medium) and various burst parameters. We then
compare our results to the Swift data of GRB 050421, which is the best example
of a long burst without afterglow. In the standard model we show that
consistency with the data imposes that the external density does not exceed
1E-5 cm-3 or that the microphysics parameters are very small with epsilon_e <~
1E-2 and epsilon_B <~ 1E-4. If the afterglow is caused by the reverse shock, we
find that its contribution can be strongly reduced if the central source has
mainly emitted fast-moving material (with less than 10 - 30 % of the kinetic
energy at Gamma<100 and was located in a dense environment. The two considered
scenarios therefore lead to opposite constraints on the circumburst medium. The
high-density environment, favored by the reverse shock model, better
corresponds to what is expected if the burst progenitor was a massive star.Comment: 6 pages, 3 figures, 1 table, to appear in A&
The origin of the late rebrightening in GRB 080503
GRB 080503, detected by Swift, belongs to the class of bursts whose prompt
phase consists of an initial short spike followed by a longer soft tail. It did
not show any transition to a regular afterglow at the end of the prompt
emission but exhibited a surprising rebrightening after one day. We aim to
explain this rebrightening with two different scenarios - refreshed shocks or a
density clump in the circumburst medium - and two models for the origin of the
afterglow, the standard one where it comes from the forward shock, and an
alternative one where it results from a long-lived reverse shock. We computed
afterglow light curves either using a single-zone approximation for the shocked
region or a detailed multizone method that more accurately accounts for the
compression of the material. We find that in several of the considered cases
the detailed model must be used to obtain a reliable description of the shock
dynamics. The density clump scenario is not favored. We confirm previous
results that the presence of the clump has little effect on the forward shock
emission, except if the microphysics parameters evolve when the shock enters
the clump. Moreover, we find that the rebrightening from the reverse shock is
also too weak when it is calculated with the multi-zone method. On the other
hand, in the refreshed-shock scenario both the forward and reverse shock models
provide satisfactory fits of the data under some additional conditions on the
distribution of the Lorentz factor in the ejecta and the beaming angle of the
relativistic outflow.Comment: 8 pages, 4 figures, accepted for publication in A&
Inverse Compton cooling in Klein-Nishina regime and GRB prompt spectrum
Synchrotron radiation mechanism, when electrons are accelerated in a
relativistic shock, is known to have serious problems to explain the observed
gamma-ray spectrum below the peak for most Gamma-Ray Bursts (GRBs); the
synchrotron spectrum below the peak is much softer than observed spectra.
Recently, the possibility that electrons responsible for the radiation cool via
Inverse Compton, but in the Klein-Nishina regime, has been proposed as a
solution to this problem. We provide an analytical study of this effect and
show that it leads to a hardening of the low energy spectrum but not by enough
to make it consistent with the observed spectra for most GRBs (this is assuming
that electrons are injected continuously over a time scale comparable to the
dynamical time scale, as is expected for internal shocks of GRBs). In
particular, we find that it is not possible to obtain a spectrum with
\alpha>-0.1 (f_{\nu} \propto \nu^{\alpha}) whereas the typical observed value
is \alpha\sim0. Moreover, extreme values for a number of parameters are
required in order that \alpha\sim-0.1: the energy fraction in magnetic field
needs to be less than about 10^{-4}, the thermal Lorentz factor of electrons
should be larger than 10^6, and the radius where gamma-rays are produced should
be not too far away from the deceleration radius. These difficulties suggest
that the synchrotron radiation mechanism in internal shocks does not provide a
self-consistent solution when \alpha>-0.2.Comment: 10 pages (single column), 2 figures, MNRAS in pres
A detailed spectral study of GRB 041219A and its host galaxy
GRB 041219A is one of the longest and brightest gamma-ray bursts (GRBs) ever
observed. It was discovered by the INTEGRAL satellite, and thanks to a
precursor happening about 300 s before the bulk of the burst, ground based
telescopes were able to catch the rarely-observed prompt emission in the
optical and in the near infrared bands. Here we present the detailed analysis
of its prompt gamma-ray emission, as observed with IBIS on board INTEGRAL, and
of the available X-ray afterglow data collected by XRT on board Swift. We then
present the late-time multi-band near infrared imaging data, collected at the
TNG, and the CFHT, that allowed us to identify the host galaxy of the GRB as an
under-luminous, irregular galaxy of about 5x10^9 M_Sun at best fit redshift of
z=0.31 -0.26 +0.54. We model the broad-band prompt optical to gamma-ray
emission of GRB 041219A within the internal shock model. We were able to
reproduce the spectra and light curve invoking the synchrotron emission of
relativistic electrons accelerated by a series of propagating shock waves
inside a relativistic outflow. On the other hand, it is less easy to
simultaneously reproduce the temporal and spectral properties of the infrared
data.Comment: 12 pages, 9 figures, accepted for publication in MNRAS, Figure 5 in
reduced qualit
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