astro-grb

Bunyo Hatsukade, Kotaro Kohno, Akira Endo, Tomoka Tosaki, Kouji Ohta, Seiichi Sakamoto, Nobuyuki Kawai, Juan R. Cortes, Kouichiro Nakanishi, Takeshi Okuda, Kazuyuki Muraoka, Takeshi Sakai, Paul M. Vreeswijk, Hajime Ezawa, Nobuyuki Yamaguchi, Kazuhisa Kamegai, Ryohei Kawabe

We report on a deep search for CO(J=3-2) line emission from the host galaxy
of GRB 980425 with the Atacama Submillimeter Telescope Experiment (ASTE). We
observed five points of the galaxy covering the entire region. After combining
all of the spectra, we obtained a global spectrum with the rms noise level of
3.3 mK in T_mb scale at a velocity resolution of 10 km s^-1. No significant
emission was detected, though we find a marginal emission feature in the
velocity range corresponding to the redshift of the galaxy. We derive 3 sigma
upper limits on the global properties: the velocity-integrated CO(3-2)
intensity of I_CO(3-2) < 0.26 K km s^-1 by adopting a velocity width of 67 km
s^-1; the H_2 column density of N(H_2) < 3 x 10^20 cm^-2; the molecular gas
mass of M(H_2) < 3 x 10^8 M_sun, by assuming a CO line luminosity to H_2
molecular gas mass conversion factor of X_CO = 5.0 x 10^20 cm^-2 (K km
s^-1)^-1; and the star formation rate of SFR < 0.1 M_sun yr^-1, based on the
Schmidt law. The SFR is consistent with the previous results of H_alpha and
mid-IR observations, thereby suggesting that there is no significant obscured
star formation in the host galaxy of GRB 980425. This result implies that there
is a variety of GRB hosts with regard to the presence of obscured star
formation.

http://arxiv.org/abs/0704.3654

R. L. C. Starling (1,2), A. J. van der Horst (2), E. Rol (1), R. A. M. J. Wijers (2), C. Kouveliotou (3), K. Wiersema (2), P. A. Curran (2), P. Weltevrede (2) ((1) Leicester, (2) Amsterdam, (3) NASA/MSFC)

We constrain blastwave parameters and the circumburst media of a subsample of
BeppoSAX Gamma-Ray Bursts. For this sample we derive the values of the injected
electron energy distribution index, p, and the density structure index of the
circumburst medium, k, from simultaneous spectral fits to their X-ray, optical
and nIR afterglow data. The spectral fits have been done in count space and
include the effects of metallicity, and are compared with the previously
reported optical and X-ray temporal behaviour. Assuming the fireball model, we
can find a mean value of p for the sample as a whole of 2.04 +0.02/-0.03. A
statistical analysis of the distribution demonstrates that the p values in this
sample are inconsistent with a single universal value for p at the 3-sigma
level or greater, which has significant implications for particle acceleration
models. This approach provides us with a measured distribution of circumburst
density structures rather than considering only the cases of k=0 (homogeneous)
and k=2 (wind-like). We find five GRBs for which k can be well constrained, and
in four of these cases the circumburst medium is clearly wind-like. The fifth
source has a value of 0<k<1, consistent with a homogeneous circumburst medium.

http://arxiv.org/abs/0704.3718

C. Thompson (CITA), P. Meszaros (PSU), M.J. Rees (IOA Cambridge)

We present an interpretation of the phenomenological relations between the
spectral peak, isotropic luminosity and duration of long gamma ray bursts that
have been discovered by Amati et al., Ghirlanda et al., Firmani et al., and
Liang & Zhang. In our proposed model, a jet undergoes internal dissipation
which prevents its bulk Lorentz factor from exceeding 1/theta (theta being the
jet opening angle) until it escapes from the core of its progenitor star, at a
radius of order 1e10 cm; dissipation may continue at larger radii. The
dissipated radiation will be partially thermalized, and we identify its thermal
peak (Doppler boosted by the outflow) with E_pk. The radiation comes, in
effect, from within the jet photosphere. The non-thermal, high energy part of
the GRB emission arises from Comptonization of this radiation by relativistic
electrons and positrons outside the effective photosphere. This model can
account naturally not only for the surprisingly small scatter in the various
claimed correlations, but also for the normalization, as well as the slopes. It
then has further implications for the jet energy, the limiting jet Lorentz
factor, and the relation of the energy, opening angle and burst duration to the
mass and radius of the stellar stellar progenitor. The observed relation
between pulse width and photon frequency can be reproduced by inverse-Compton
emission at ~ 1e14 cm from the engine, but there are significant constraints on
the energy distribution and isotropy of the radiating particles.

http://arxiv.org/abs/astro-ph/0608282

Xiang-Yu Wang, Zhuo Li, Eli Waxman, Peter Meszaros

Thermal X-ray emission which is simultaneous with the prompt gamma-rays has
been detected for the first time from a supernova connected with a gamma-ray
burst (GRB), namely GRB060218/SN2006aj. It has been interpreted as arising from
the breakout of a mildly relativistic, radiation-dominated shock from a dense
stellar wind surrounding the progenitor star. There is also evidence for the
presence of a mildly relativistic ejecta in GRB980425/SN1998bw, based on its
X-ray and radio afterglow. Here we study the process of repeated bulk Compton
scatterings of shock breakout thermal photons by the mildly relativistic
ejecta. During the shock breakout process, a fraction of the thermal photons
would be repeatedly scattered between the pre-shock material and the shocked
material as well as the mildly relativistic ejecta and, as a result, the
thermal photons get boosted to increasingly higher energies. This bulk motion
Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes,
which could account for the prompt gamma-ray burst emission in low-luminosity
supernova-connected GRBs, such as GRB060218. A Monte Carlo code has been
developed to simulate this repeated scattering process, which confirms that a
significant fraction of the thermal photons get “accelerated” to form a
nonthermal component, with a dominant luminosity. This interpretation for the
prompt nonthermal emission of GRB060218 may imply that either the usual
internal shock emission from highly relativistic jets in these low-luminosity
GRBs is weak, or alternatively, that there are no highly relativistic jets in
this peculiar class of bursts.

http://arxiv.org/abs/astro-ph/0608033

C. Thompson (CITA), P. Meszaros (PSU), M.J. Rees (IOA Cambridge)

We present an interpretation of the phenomenological relations between the
spectral peak, isotropic luminosity and duration of long gamma ray bursts that
have been discovered by Amati et al., Ghirlanda et al., Firmani et al., and
Liang & Zhang. In our proposed model, a jet undergoes internal dissipation
which prevents its bulk Lorentz factor from exceeding 1/theta (theta being the
jet opening angle) until it escapes from the core of its progenitor star, at a
radius of order 1e10 cm; dissipation may continue at larger radii. The
dissipated radiation will be partially thermalized, and we identify its thermal
peak (Doppler boosted by the outflow) with E_pk. The radiation comes, in
effect, from within the jet photosphere. The non-thermal, high energy part of
the GRB emission arises from Comptonization of this radiation by relativistic
electrons and positrons outside the effective photosphere. This model can
account naturally not only for the surprisingly small scatter in the various
claimed correlations, but also for the normalization, as well as the slopes. It
then has further implications for the jet energy, the limiting jet Lorentz
factor, and the relation of the energy, opening angle and burst duration to the
mass and radius of the stellar stellar progenitor. The observed relation
between pulse width and photon frequency can be reproduced by inverse-Compton
emission at ~ 1e14 cm from the engine, but there are significant constraints on
the energy distribution and isotropy of the radiating particles.

http://arxiv.org/abs/astro-ph/0608282

G. Ghirlanda (1), Z. Bosnjak (2,1), G. Ghisellini (1), F. Tavecchio (1), C. Firmani (1,3) ((1) INAF-Osservatorio Astronomico di Brera; (2) Institut d'Astrophysique de Paris; (3) U.N.A.M. - Mexico)

We study 7 Gamma Ray Bursts (GRBs), detected both by the BATSE instrument,
on-board the Compton Gamma Ray Observatory, and by the Wide Field Camera (WFC),
on-board BeppoSAX. These bursts have measured spectroscopic redshifts and are a
sizeable fraction of the bursts defining the correlation between the peak
energy E_peak (i.e. the peak of the vFv spectrum) and the total prompt
isotropic energy E_iso (the so called “Amati” relation). Recent theoretical
interpretations of this correlation assume that black-body emission dominates
the time resolved spectra of GRBs, even if, in the time integrated spectrum,
its presence may be hidden by the change of its temperature and by the dilution
of a possible non-thermal power law component. We perform a time resolved
spectral analysis, and show that the sum of a power-law and a black-body gives
acceptable fits to the time dependent spectra within the BATSE energy range,
but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cutoff
power-law plus a black-body is consistent with the WFC data, but the black-body
component contributes a negligible fraction of the total flux. On the contrary,
we find that fitting the spectra with a Band model or a simple cutoff power-law
model yields an X-ray flux and spectral slope which well matches the WFC
spectra.

http://arxiv.org/abs/0704.3438

Li-Xin Li (MPA)

Because of the limit in the number of gamma-ray bursts (GRBs) with available
redshifts and spectra, all current investigations on the correlation among GRB
variables use burst samples with redshifts that span a very large range. The
evolution and selection effects have thus been ignored, which might have
important influence on the results. In this Letter, we divide the 48
long-duration GRBs in Amati (2006, 2007) into four groups with redshift from
low to high, each group contains 12 GRBs. Then we fit each group with the Amati
relation $\log E_\iso = a + b \log E_\p$, and check if the parameters $a$ and
$b$ evolve with the GRB redshift. We find that $a$ and $b$ vary with the mean
redshift of the GRBs in each group systematically and significantly.
Monte-Carlo simulations show that there is only $\sim 4$ percent of chance that
the variation is caused by the selection effect arising from the fluence limit.
Hence, our results may indicate that GRBs evolve strongly with the cosmological
redshift.

http://arxiv.org/abs/0704.3128

T.Mineo, V.Mangano, S.Covino, G.Cusumano, V.La Parola, E.Troja, P.Roming, D.N.Burrows, S.Campana, M.Capalbi, G.Chincarini, N.Gehrels, P.Giommi, J.E.Hill, F.Marshall, A.Moretti, P.O'Brien, M.Page, M.Perri, P.Romano, B.Sbarufatti, G.Sato, G.Tagliaferri

We present a detailed analysis of the prompt and afterglow emission of GRB
050410 and GRB 050412 detected by Swift for which no optical counterpart was
observed. The 15-150 keV energy distribution of the GRB 050410 prompt emission
shows a peak energy at 53 keV. The XRT light curve of this GRB decays as a
power law with a slope of alpha=1.06+/-0.04. The spectrum is well reproduced by
an absorbed power law with a spectral index Gamma_x=2.4+/-0.4 and a low energy
absorption N_H=4(+3;-2)x10^21 cm^(-2) which is higher than the Galactic value.
The 15-150 keV prompt emission in GRB 050412 is modelled with a hard
(Gamma=0.7+/-0.2) power law. The XRT light curve follows a broken power law
with the first slope alpha_1=0.7+/-0.4, the break time T_break=254(-41;+79) s
and the second slope alpha_2=2.8(-0.8;+0.5). The spectrum is fitted by a power
law with spectral index Gamma_x=1.3+/-0.2 which is absorbed at low energies by
the Galactic column. The GRB 050410 afterglow reveals the expected
characteristics of the third component of the canonical Swift light curve.
Conversely, a complex phenomenology was detected in the GRB 050412 because of
the presence of the very early break. The light curve in this case can be
interpreted as being the last peak of the prompt emission. The two bursts
present tight upper limits for the optical emission, however, neither of them
can be clearly classified as dark. For GRB 050410, the suppression of the
optical afterglow could be attributed to a low density interstellar medium
surrounding the burst. For GRB 050412, the evaluation of the darkness is more
difficult due to the ambiguity in the extrapolation of the X-ray afterglow
light curve.

http://arxiv.org/abs/0704.3191

N. Mirabal, J. P. Halpern, P. T. O'Brien

We present the optical identification and spectroscopy of the host galaxy of
GRB 050826 at a redshift z = 0.296 +/- 0.001. Image subtraction among
observations obtained on three consecutive nights, reveals a fading object 5
hours after the burst, confirming its identification as the optical afterglow
of this event. Deep imaging shows that the optical afterglow is offset by 0.4
arcseconds (~1.76 kpc) from the center of its irregular host galaxy, which is
typical for long-duration gamma-ray bursts. Combining these results with X-ray
measurements acquired by the Swift XRT instrument, we find that GRB 050826
falls entirely within the subluminous, subenergetic group of long gamma-ray
bursts at low redshift (z < 0.3). The results are discussed in the context of
models that possibly account for this trend, including the nature of the
central engine, the evolution of progenitor properties as a function of
redshift, and incompleteness in current gamma-ray burst samples.

http://arxiv.org/abs/0704.3069

Dimitrios Giannios, Henk C. Spruit (Max Planck Institute for Astrophysics)

We explore the observational appearance of the photosphere of an
ultrarelativistic flow with internal dissipation of energy as predicted by the
magnetic reconnection model. Previous study of the radiative transfer in the
photospheric region has shown that gradual dissipation of energy results in a
hot photosphere. There, inverse Compton scattering of the thermal radiation
advected with the flow leads to powerful photospheric emission with spectral
properties close to those of the observed prompt GRB emission. Here, we build
on that study by calculating the spectra for a large range of the
characteristics of the flow. An accurate fitting formula is given that provides
the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy
range (in the central engine frame) as a function of the basic physical
parameters of the flow. It facilitates the direct comparison of the model
predictions with observations, including the variability properties of the
lightcurves. We verify that the model naturally accounts for the observed
clustering in peak energies of the E*f(E) spectrum. In this model, the Amati
relation indicates a tendency for the most luminous bursts to have more energy
per baryon. If this tendency also holds for individual GRB pulses, the model
predicts the observed narrowing of the width of pulses with increasing photon
energy.

http://arxiv.org/abs/astro-ph/0611385