astro-grb

B. D. Metzger, A. L. Piro, E. Quataert

We present time-dependent models of the accretion disks created during
compact object mergers, focusing on the energy available from accretion at late
times and the composition of the disk and its outflows. We calculate the
dynamics near the outer edge of the disk, which contains the majority of the
mass and sets the accretion rate onto the central black hole. This allows us to
follow the evolution over much longer timescales than current hydrodynamic
simulations. At late times the disk becomes advective and its properties
asymptote to self-similar solutions with accretion rate dM/dt ~ t^(-4/3)
(neglecting outflows). This late-time accretion can in principle provide
sufficient energy to power the late-time activity observed from some
short-duration gamma-ray bursts (GRBs). However, because outflows during the
advective phase unbind the majority of the remaining mass, it is difficult for
the remnant disk alone to produce significant accretion power well beyond the
onset of the advective phase. Unless the viscosity is quite low (alpha ~1e-3),
this occurs before the start of observed flaring at ~ 30 s; continued mass
inflow thus appears required to explain the late-time activity from short GRBs.
We show that the composition of the disk freezes-out relatively neutron-rich
(electron fraction Ye ~ 0.3). Roughly 1e-2 Msun of this neutron-rich material
is ejected by winds at late times. During earlier, neutrino-cooled phases of
accretion, neutrino irradiation of the disk produces a wind with Ye ~ 0.5,
which synthesizes at most ~ 1e-3 Msun of Ni56. We highlight what conditions are
favorable for Ni56 production and predict, in the best cases, optical and
infrared transients peaking ~ 0.5-2 days after the burst, with fluxes a factor
of ~ 10 below the current observational limits.

http://arxiv.org/abs/0805.4415

R. Salvaterra, C. Guidorzi, S. Campana, G. Chincarini, G. Tagliaferri

We compute the luminosity function (LF) and the formation rate of long gamma
ray bursts (GRBs) by fitting the observed differential peak flux distribution
obtained by the BATSE satellite in two different scenarios: i) the GRB
luminosity evolves with redshift and ii) GRBs form preferentially in
low-metallicity environments. In both cases, model predictions are consistent
with the Swift number counts and with the number of detections at z>2.5 and
z>3.5. To discriminate between the two evolutionary scenarios, we compare the
model results with the number of luminous bursts (i.e. with isotropic peak
luminosity in excess of 10^53 erg s^-1) detected by Swift in its first three
years of mission. Our sample conservatively contains only bursts with good
redshift determination and measured peak energy. We find that models in which
GRBs are characterised by a constant LF are ruled out, underproducing the
number of luminous GRBs. Only for a metallicity threshold for GRB formation as
low as 1/10 \Zsun, the model is marginally consistent with the observed number
of bright GRBs. Using these new constraints, we derive robust upper limits on
the bright-end of the GRB LF, showing that this can not be steeper than ~2.5.
In conclusion, we find that available Swift observations (both the number of
high-z GRBs and of bright GRBs) point to a scenario in which GRBs were more
luminous in the past as we see them today.

http://arxiv.org/abs/0805.4104

Kohta Murase, Kunihito Ioka, Shigehiro Nagataki, Takashi Nakamura

Gamma-ray bursts (GRBs) are one of the candidates of ultra-high-energy
(around 10^18.5 eV) cosmic-ray (UHECR) sources. We investigate high-energy
cosmic-ray acceleration including heavy nuclei in GRBs by using Geant 4, and
discuss its various implications, taking both of high-luminosity (HL) and
low-luminosity (LL) GRBs into account. This is because LL GRBs may also make a
significant contribution to the observed UHECR flux if they form a distinct
population. We show that not only protons but also heavier nuclei can be
accelerated up to ultra-high energies in the internal, (external) reverse and
forward shock models. We also show that the condition for ultra-high-energy
heavy nuclei such as iron to survive is almost the same as that for about TeV
gamma-rays to escape from the source and for high-energy neutrinos not to be
much produced. The multi-messenger astronomy by neutrino and GeV-TeV gamma-ray
telescopes such as IceCube and KM3Net, GLAST and MAGIC will be important to see
whether GRBs can be accelerators of ultra-high-energy heavy nuclei. We also
demonstrate expected spectra of high-energy neutrinos and gamma rays, and
discuss their detectabilities. In addition, we discuss implictaions of the
GRB-UHECR hypothesis. We point out, since the number densities of HL-GRBs and
LL-GRBs are quite different, its detemination by UHECR observations is also
important.

http://arxiv.org/abs/0801.2861

S. Gallerani, R. Salvaterra, A. Ferrara, T. Roy Choudhury

We propose to study cosmic reionization using absorption line spectra of
high-redshift Gamma Ray Burst (GRB) afterglows. We show that the statistics of
the dark portions (gaps) in GRB absorption spectra represent exquisite tools to
discriminate among different reionization models. We then compute the
probability to find the largest gap in a given width range [Wmax, Wmax + dW] at
a flux threshold Fth for burst afterglows at redshifts 6.3 < z < 6.7. We show
that different reionization scenarios populate the (Wmax, Fth) plane in a very
different way, allowing to distinguish among different reionization histories.
We provide here useful plots that allow a very simple and direct comparison
between observations and model results. Finally, we apply our methods to GRB
050904 detected at z = 6.29. We show that the observation of this burst
strongly favors reionization models which predict a highly ionized
intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction
xHI = 6.4 \pm 0.3 \times 10^-5 along the line of sight towards GRB 050904.

http://arxiv.org/abs/0710.1303

V.V.Sokolov

If the threshold for $e^{-}e^{+}$ pair production depends on an angle between
photon momenta, and if the $\gamma$-rays are collimated right in gamma-ray
burst (GRB) source then another solution of the compactness problem is
possible. The list of basic assumptions of the scenario describing the GRB with
energy release $< 10^{49}$ erg is adduced: the matter is about an alternative
to the ultrarelativistic fireball if all long-duration GRBs are physically
connected with core-collapse supernovae (SNe). The questions about radiation
pressure and how the jet arises on account of even a small radiation field
asymmetry in a compact GRB source of size $\lesssim 10^8$ cm, and observational
consequences of the compact model of GRBs are considered.

http://arxiv.org/abs/0805.3262

S. McGlynn (1,2), S. Foley (1), S. McBreen (3), L. Hanlon (1), R. O'Connor (1), A. Martin Carrillo (1), B. McBreen (1) ((1) University College Dublin, (2) Royal Institute of Technology (KTH), Stockholm (3) MPE Garching)

INTEGRAL has observed 47 long-duration GRBs (T_90 > 2s) and 1 short-duration
GRB (T_90 < 2s) in five years of observation since October 2002. This work
presents the properties of the prompt emission of GRB 070707, which is the
first short hard GRB observed by INTEGRAL. The spectral and temporal properties
of GRB 070707 were determined using the two sensitive coded-mask gamma-ray
instruments on board INTEGRAL, IBIS and SPI. The T_90 duration was 0.8s, and
the spectrum of the prompt emission was obtained by joint deconvolution of IBIS
and SPI data to yield a best fit power-law with photon index alpha = -1.19
+0.14 -0.13, which is consistent with the characteristics of short-hard
gamma-ray bursts. The peak flux over 1 second was 1.79 photons/cm^2/s and the
fluence over the same interval was 2.07 x 10^-7 erg/cm^2 in the energy range
20-200keV. The spectral lag measured between 25-50keV and 100-300keV is 20 +/-
5ms, consistent with the small or negligible lags measured for short bursts.
The spectral and temporal properties of GRB 070707 are comparable to those of
the short hard bursts detected by other gamma-ray satellites, including BATSE
and Swift. We estimate a lower limit on the Lorentz factor Gamma >~ 25 for GRB
070707, assuming the typical redshift for short GRBs of z=0.35. This limit is
consistent with previous estimates for short GRBs and is smaller than the lower
limits of Gamma >~ 100 calculated for long GRBs. If GRB 070707 is a member of
the recently postulated class of short GRBs at z ~ 1, the lower limit on Gamma
increases to Gamma >~ 35.

http://arxiv.org/abs/0805.2880

A. Galli, L. Piro (INAF/Isaf-Roma)

The detection of the 100 GeV-TeV emission by a Gamma-Ray Burst (GRB) will
provide an unprecedented opportunity to enlighten the nature of the central
engine and the interaction between the relativistic flow and the environment of
the burst progenitor. In this paper we show that there are exciting prospects
of detecting by MAGIC high energy (HE) emission from the burst during the
“early” X-ray flaring activity and, later, during the “normal” afterglow phase.
We also identify the best observational strategy (trigger conditions and time
period of observation). We determine the expected HE emission from the flaring
and afterglow phases of GRBs in the context of the External Shock scenario and
compare them with the MAGIC threshold. We find that a X-ray flare with the
average properties of the class, can be detected in the 100 GeV range by MAGIC
provided that z<1. The requested observational window with MAGIC should then
start from 10-20 sec after the burst and cover about 1000-2000 sec.
Furthermore, we demonstrate that there are solid prospects to detect the late
afterglow emission in the same energy range for most of the bursts with z<1 if
the density of the external medium is n > a few cm^{-3}. In this case the MAGIC
observation shall extend to about 10-20 ksec. We provide recipes to tailor this
prediction to the observational properties of each burst, in particular the
fluence in the prompt emission and the redshift, thus allowing an almost “real
time” decision procedure to decide whether to continue the follow up
observation of a burst at late times.

http://arxiv.org/abs/0805.2884

T. Krühler, A. Küpcü Yoldaş, J. Greiner, C. Clemens, S. McBreen, N. Primak, S. Savaglio, A. Yoldaş, G. P. Szokoly

We present optical and near-infrared photometry of the afterglow of the long
Gamma-Ray Burst GRB 070802 at redshift 2.45 obtained with the ESO/MPI 2.2 m
telescope equipped with the multi-channel imager GROND. Follow-up observations
in g'r'i'z' and JHK_S bands started ~17 min and extended up to 28 h post burst.
We find an increase in brightness of the afterglow at early times, which can be
explained by the superposition of reverse and forward shock (FS) emission or
the onset of the afterglow FS. Additionally, we detect a strong broad-band
absorption feature in the i' band, which we interpret as extinction from the
redshifted 2175 A bump in the GRB host galaxy. This is one of the first and
clearest detections of the 2175 A feature at high redshift. It is strong
evidence for a carbon rich environment, indicating that Milky Way or Large
Magellanic Cloud like dust was already formed in substantial amounts in a
galaxy at z=2.45.

http://arxiv.org/abs/0805.2824

D. A. Perley, W. Li, R. Chornock, J. X. Prochaska, N. R. Butler, P. Chandra, L. K. Pollack, J. S. Bloom, A. V. Filippenko, C. Akerlof, M. W. Auger, S. B. Cenko, H.-W. Chen, C. D. Fassnacht, D. Fox, D. Frail, E. M. Johansson, D. Le Mignant, M. Modjaz, M. A. Skinner, G. H. Smith, H. Swan, M. A. van Dam, F. Yuan

The optical afterglow of long-duration gamma-ray burst (GRB) 071003 is among
the brightest yet to be detected from any GRB, with R ~ 12 mag in Katzman
Automatic Imaging Telescope observations starting 42 s after the GRB trigger,
including filtered detections during prompt emission. However, our high
signal-to-noise ratio afterglow spectrum displays only extremely weak
absorption lines at what we argue is the host redshift of z=1.60435 - in
contrast to the three other, much stronger Mg II absorption systems observed at
lower redshifts. Together with Keck Adaptive Optics observations which fail to
reveal a host galaxy coincident with the burst position, our observations
suggest a halo progenitor and offer a cautionary tale about the use of Mg II
for GRB redshift determination. We present early through late-time observations
spanning the electromagnetic spectrum, constrain the connection between the
prompt emission and early variations in the light curve (we observe no
correlation), and discuss possible origins for an unusual, marked rebrightening
that occurs a few hours after the burst: likely either a late-time refreshed
shock or a wide-angle secondary jet. Analysis of the late-time afterglow is
most consistent with a wind environment, suggesting a massive star progenitor.
Together with GRB 070125, this may indicate that a small but significant
portion of star formation in the early universe occurred far outside what we
consider a normal galactic disk.

http://arxiv.org/abs/0805.2394

Hylke B. J. Koers

Neutrinos produced in gamma-ray bursters (GRBers) may provide a unique probe
for the physics of these extreme astrophysical systems. Here we discuss
neutrino production in inelastic neutron-proton collisions within the
relativistic outflows associated with GRBers. We consider both the widely used
fireball model and a recently proposed magneto-hydrodynamic (MHD) model for the
GRB outflow.

http://arxiv.org/abs/0805.2514