astro-grb

Luc Dessart, Christian Ott, Adam Burrows, Stefan Rosswog, Eli Livne

We present VULCAN/2D multi-group flux-limited-diffusion radiation
hydrodynamics simulations of binary neutron star (BNS) mergers, using the Shen
equation of state, covering ~100 ms, and starting from azimuthal-averaged 2D
slices obtained from 3D SPH simulations of Rosswog & Price for 1.4 Msun
(baryonic) neutron stars with no initial spins, co-rotating spins, and
counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a
multi-angle neutrino-transport solver. We find polar-enhanced neutrino
luminosities, dominated by $\bar{\nu}_e$ and “$\nu_\mu$'' neutrinos at peak,
although $\nu_e$ emission may be stronger at late times. We obtain typical peak
neutrino energies for $\nu_e$, $\bar{\nu}_e$, and “$\nu_\mu$'' of ~12, ~16,
and ~22 MeV. The super-massive neutron star (SMNS) formed from the merger has a
cooling timescale of ~1 s. Charge-current neutrino reactions lead to the
formation of a thermally-driven bipolar wind with <$\dot{M}$> ~10$^{-3}$
Msun/s, baryon-loading the polar regions, and preventing any production of a
GRB prior to black-hole formation. The large budget of rotational free energy
suggests magneto-rotational effects could produce a much greater polar mass
loss. We estimate that ~10$^{-4}$ Msun of material with electron fraction in
the range 0.1-0.2 become unbound during this SMNS phase as a result of neutrino
heating. We present a new formalism to compute the $\nu_i\bar{\nu}_i$
annihilation rate based on moments of the neutrino specific intensity computed
with our multi-angle solver. Cumulative annihilation rates, which decay as
$t^{-1.8}$, decrease over our 100 ms window from a few 10$^{50}$ to ~10$^{49}$
erg/s, equivalent to a few 10$^{54}$ to ~10$^{53}$ $e^-e^+$ pairs per second.

http://arxiv.org/abs/0806.4380

Atish Kamble, Kuntal Misra, D. Bhattacharya, Ram Sagar

The afterglow of GRB 050401 presents several novel and interesting features :
[1] An initially faster decay in optical band than in X-rays. [2] A break in
the X-ray light curve after $\sim$ 0.06 day with an unusual slope after the
break. [3] The X-ray afterglow does not show any spectral evolution across the
break while the R band light curve does not show any break.

We have modeled the observed multi-band evolution of the afterglow of GRB
050401 as originating in a two component jet, interpreting the break in X-ray
light curve as due to lateral expansion of a narrow collimated outflow which
dominates the X-ray emission. The optical emission is attributed to a wider jet
component. Our model reproduces all the observed features of multi-band
afterglow of GRB 050401.

We present optical observations of GRB 050401 using the 104-cm Sampurnanand
Telescope at ARIES, Nainital. Results of the analysis of multi-band data are
presented and compared with GRB 030329, the first reported case of double jet.

http://arxiv.org/abs/0806.4270

Yoshiki Kodama (1), Daisuke Yonetoku (1), Toshio Murakami (1), Sachiko Tanabe (1), Ryo Tsutsui (2), Takashi Nakamura (2) ((1) Kanazawa University, (2) Kyoto University)

We calibrated the peak energy-peak luminosity relation of GRBs (so called
Yonetoku relation) using 33 events with the redshift $z < 1.62$ without
assuming any cosmological models. The luminosity distances to GRBs are
estimated from those of large amount of Type Ia supernovae with $z<1.755$. This
calibrated Yonetoku relation can be used as a new cosmic distance ladder toward
higher redshifts. We determined the luminosity distances of 30 GRBs in $1.8 < z
< 5.6$ using the calibrated relation and plotted the likelihood contour in
$(\Omega_m,\Omega_\Lambda)$ plane. We obtained $(\Omega_m, \Omega_{\Lambda})=
(0.37^{+0.14}_{-0.11}, 0.63^{+0.11}_{-0.14})$ for a flat universe. Since our
method is free from the circularity problem, we can say that our universe in
$1.8 < z < 5.6$ is compatible with the so called concordance cosmological model
derived for $z < 1.8$. This suggests that the time variation of the dark energy
is small or zero up to $z\sim 6$.

http://arxiv.org/abs/0802.3428

R. Vavrek, L. G. Balázs, A. Mészáros, I. Horváth, Z. Bagoly

We studied the complete randomness of the angular distribution of gamma-ray
bursts (GRBs) detected by BATSE. Since GRBs seem to be a mixture of objects of
different physical nature we divided the BATSE sample into 5 subsamples
(short1, short2, intermediate, long1, long2) based on their durations and peak
fluxes and studied the angular distributions separately. We used three methods,
Voronoi tesselation, minimal spanning tree and multifractal spectra to search
for non-randomness in the subsamples. To investigate the eventual
non-randomness in the subsamples we defined 13 test-variables (9 from the
Voronoi tesselation, 3 from the minimal spanning tree and one from the
multifractal spectrum). Assuming that the point patterns obtained from the
BATSE subsamples are fully random we made Monte Carlo simulations taking into
account the BATSE's sky-exposure function. The MC simulations enabled us to
test the null hypothesis i.e. that the angular distributions are fully random.
We tested the randomness by binomial test and introducing squared Euclidean
distances in the parameter space of the test-variables. We concluded that the
short1, short2 groups deviate significantly (99.90%, 99.98%) from the fully
randomness in the distribution of the squared Euclidean distances but it is not
the case at the long samples. At the intermediate group the squared Euclidean
distances also give significant deviation (98.51%).

http://arxiv.org/abs/0806.3932

Rong-Feng Shen (1), Richard Willingale (2), Pawan Kumar (1), Paul T. O'Brien (2), Phil A. Evans (2) ((1) UT Austin, (2) U. of Leicester)

A dust scattering model was recently proposed to explain the shallow X-ray
decay (plateau) observed prevalently in Gamma-Ray Burst (GRB) early afterglows.
In this model the plateau is the scattered prompt X-ray emission by the dust
located close (about 10 to a few hundred pc) to the GRB site. In this paper we
carefully investigate the model and find that the scattered emission undergoes
strong spectral softening with time, due to the model's essential ingredient
that harder X-ray photons have smaller scattering angle thus arrive earlier,
while softer photons suffer larger angle scattering and arrive later. The model
predicts a significant change, i.e., $\Delta \beta \sim 2 - 3$, in the X-ray
spectral index from the beginning of the plateau toward the end of the plateau,
while the observed data shows close to zero softening during the plateau and
the plateau-to-normal transition phase. The scattering model predicts a big
difference between the harder X-ray light curve and the softer X-ray light
curve, i.e., the plateau in harder X-rays ends much earlier than in softer
X-rays. This feature is not seen in the data. The large scattering optical
depths of the dust required by the model imply strong extinction in optical,
$A_V \gtrsim $ 10, which contradicts current findings of $A_V= 0.1 - 0.7$ from
optical and X-ray afterglow observations. We conclude that the dust scattering
model can not explain the X-ray plateaus.

http://arxiv.org/abs/0806.3541

M. Nysewander, A.S. Fruchter, A. Pe'er

We present a comparative study of the observed properties of the optical and
X-ray afterglows of short- and long-duration $\gamma$-ray bursts (GRBs). Using
a large sample of 37 short GRBs and 421 long GRBs, we find a strong correlation
between afterglow brightness measured after 11 hours and the energy release in
the prompt emission, measured in both the optical (R band) and X-ray flux and
E$_{ISO}$ : $F_{R,X} \propto {E_{ISO}}^{\alpha}$, with $\alpha \simeq 1$ in
both cases. Furthermore, the constant of proportionality is nearly identical
for long and short bursts. Therefore, for a given fluence, the afterglows of
short GRBs are not significantly dimmer than those of long GRBs in the optical
and the X-ray bands. This result is difficult to explain in the framework of
the standard scenario, since it requires that either (1) the average number
density of the surrounding medium of short bursts is comparable to, or even
larger than the number density of long bursts; or (2) short bursts explode into
a density profile, $n(r) \propto r^{-2}$. We therefore find it likely that
either basic assumptions on the properties of the circumburst environment of
short GRBs, and thus the merger origin theory of short bursts, are incorrect,
or else the standard models of GRB emission must be fully re-examined.

http://arxiv.org/abs/0806.3607

Roberto Guida, Maria Grazia Bernardini, Carlo Luciano Bianco, Letizia Caito, Maria Giovanna Dainotti, Remo Ruffini

(Shortened) CONTEXT: […] AIMS: Motivated by the relation proposed by Amati
and collaborators, we look within the “fireshell'' model for a relation
between the peak energy E_p of the \nu F_\nu total time-integrated spectrum of
the afterglow and the total energy of the afterglow E_{aft}, which in our model
encompasses and extends the prompt emission. METODS: […] Within the fireshell
model […] We can then build two sets of “gedanken'' GRBs varying the total
energy of the electron-positron plasma E^{e^\pm}_{tot} and keeping the same
baryon loading B of GRB050315. The first set assumes for the effective CBM
density the one obtained in the fit of GRB050315. The second set assumes
instead a constant CBM density equal to the average value of the GRB050315
prompt phase. RESULTS: For the first set of “gedanken'' GRBs we find a
relation E_p\propto (E_{aft})^a, with a = 0.45 \pm 0.01, whose slope strictly
agrees with the Amati one. Such a relation, in the limit B \to 10^{-2},
coincides with the Amati one. Instead, in the second set of “gedanken'' GRBs
no correlation is found. CONCLUSIONS: Our analysis excludes the Proper-GRB
(P-GRB) from the prompt emission, extends all the way to the latest afterglow
phases and is independent on the assumed cosmological model, since all
“gedanken'' GRBs are at the same redshift. The Amati relation, on the other
hand, includes also the P-GRB, focuses on the prompt emission only, and is
therefore influenced by the instrumental threshold which fixes the end of the
prompt emission, and depends on the assumed cosmology. This may well explain
the intrinsic scatter observed in the Amati relation.

http://arxiv.org/abs/0806.3705

E. Moreno Mendez, G.E. Brown, C.-H. Lee, F.M. Walter

Recent measurements of the Kerr parameters a* for two black-hole binaries in
our Galaxy (Shafee et al. 2006), GRO J1655-40 and 4U 1543-47 of a*=0.65-0.75
and a*=0.75-0.85, respectively, fitted well the predictions of Lee et al.
(2002), of a*= 0.8. In this report we also note that Lee et al. (2002)
predicted a* >0.5 for 80% of the Soft X-ray Transient Sources. The maximum
available energy in the Blandford-Znajek formalism for a* > 0.5 gives E > 3 x
10^{53} ergs, orders of magnitude larger than the energy needed for the GRB and
hypernova explosion. We interpret the Soft X-ray Transients to be relics of
GRBs and Hypernovae, but most of them were subluminous ones which could use
only a small part of the available rotational energy.

http://arxiv.org/abs/astro-ph/0612461

Y.-P. Qin

The curvature effect is explored in the case of extremely short intrinsic
emission. Assuming a $\delta$ function emission we get formulas that get rid of
the impacts from the intrinsic emission duration, which are applicable to any
forms of continuum. The formulas predict that the same form of spectrum could
be observed at different times, with the peak energy of the spectrum shifting
from higher energy bands to lower bands following $E_{peak}\propto t^{-1}$.
When the emission is early enough the light curve in the form $f_{\nu
}(t)t^{2}$ will possess exactly the intrinsic spectral form, for which the
temporal power law index and the spectral power law index will be related by
$\alpha =2+\beta $. The analysis shows that there do exist a temporal steep
decay phase and a spectral softening which occur simultaneously, and both are
caused by the shifting of the Band function spectrum. According to the
analysis, we predict that the softening due to the curvature effect will appear
at different frequencies; it occurs earlier for higher frequencies and later
for lower frequencies; the maximum spectral index time follows the
$t_{b,max}\propto \nu^{-1}$ law. We also find: the softening duration would be
linearly correlated with the maximum spectral index time; the observed
$\beta_{min}$ and $\beta_{max}$ are determined by the low and high energy
indexes of the observed Band function spectrum. We propose to check the
curvature effect with the $\log f_{\nu}(t)t^{3}$ vs. $log t$ curve which would
be in agreement with the $\log \nu f_{\nu}$ vs. $log \nu$ curve. Applying this
to GRB 060614 shows that the peak energy of its observed spectrum is expected
to pass through the observation band at $\sim $175 s.

http://arxiv.org/abs/0806.3339

Kentaro Nagamine (UNLV, IPMU), Bing Zhang (UNLV), Lars Hernquist (Harvard)

We study the incidence rate of damped Lya systems associated with the host
galaxies of gamma-ray bursts (GRB-HOST-DLAs) as functions of neutral hydrogen
column density (N_HI) and projected star formation rate (SFR) using
cosmological SPH simulations. Assuming that the occurrence of GRBs is
correlated with the local SFR, we find that the median N_HI of GRB-HOST-DLAs
progressively shifts to lower N_HI values with increasing redshift, and the
incidence rate of GRB-HOST-DLAs with log N_HI > 21.0 decreases rapidly at z>=6.
Our results suggest that the likelihood of observing the signature of IGM
attenuation in GRB afterglows increases towards higher redshift, because it
will not be blocked by the red damping wing of DLAs in the GRB host galaxies.
This enhances the prospects of using high-redshift GRBs to probe the
reionization history of the Universe. The overall incidence rate of
GRB-HOST-DLAs decreases monotonically with increasing redshift, whereas that of
QSO-DLAs increases up to z=6. A measurement of the difference between the two
incidence rates would enable an estimation of the value of \eta_grb, which is
the mass fraction of stars that become GRBs for a given amount of star
formation.

http://arxiv.org/abs/0806.2899