astro-grb

A. Galli (1,2,3), D. Guetta (4) ((1) IASF-Roma/INAF, (2) Univerisita' di Roma "La Sapienza", (3) INFN-Trieste, (4) OAR/INAF)

In this paper we study synchrotron and synchrotron self Compton (SSC)
emission from internal shocks (IS) during the prompt and X-ray flare phases of
Gamma-Ray Bursts (GRBs). The aim is to test the IS model for the flare emission
and for whether GRBs can be GeV sources. We determine the parameters for which
the IS model can account for the observed prompt and X-ray flares emission, and
study the detectability of the high energy SSC emission by the AGILE and GLAST
satellites. We find that the detectability of the SSC emission during the
prompt phase of GRBs improves for higher values of the fireball Lorentz factor
and of the temporal variability. If IS is the mechanism responsible of the
flare emission, and the Lorentz factor of the shells producing the flare is of
the order of 100, the flare light curves are expected to present some
substructures with temporal variability of 10-100 ms which are much smaller
than the average duration of flares, and similar to those observed during the
prompt phase of GRBs. If one assumes lower Lorentz factors, such as 10-25, then
a larger temporal variaibility of 40 s can also account for the observed flare
properties. However in this case we predict that X-ray flares do not have a
counterpart at very high energies (MeV-GeV). An investigation on the
substructures of the X-ray flares light curves, and simultaneous X-ray and high
energy observations, will allow us to corroborate the hypothesis that late IS
are responsible of the X-ray flares.

http://arxiv.org/abs/0709.4568

Xiang-Yu Wang, Soebur Razzaque, Peter Meszaros

The chemical composition of the ultra-high-energy (UHE) cosmic rays serves as
an important clue for their origin. Recent measurements of the elongation rates
by the Pierre Auger Observatory hint at the possible presence of heavy or
intermediate mass nuclei in the UHE cosmic rays. Gamma-ray bursts (GRBs) and
hypernovae have been suggested as possible sources of the UHE cosmic rays. Here
we derive the constraints on the physical conditions under which UHE heavy
nuclei, if they are accelerated in these sources, can survive in their intense
photon fields. We find that in the GRB external shock and in the hypernova
scenarios, UHE nuclei can easily survive photo-disintegration. In the GRB
internal shock scenario, UHE nuclei can also survive, provided the dissipation
radius and/or the bulk Lorentz factor of the relativistic outflow are
relatively large, or if the low-energy self-absorption break in the photon
spectrum of the prompt emission occurs above several KeV. In internal shocks
and in the other scenarios, intermediate-mass UHE nuclei have a higher
probability of survival against photo-disintegration than UHE heavy nuclei such
as Fe.

http://arxiv.org/abs/0711.2065

Yi-Zhong Fan, Tsvi Piran, Ramesh Narayan, Da-Ming Wei

We calculate the very high energy (sub-GeV to TeV) inverse Compton emission
of GRB afterglows. We argue that this emission provides a powerful test of the
currently accepted afterglow model. We focus on two processes: synchrotron
self-Compton (SSC) emission within the afterglow blast wave, and external
inverse Compton (EIC) emission which occurs when flare photons (produced by an
internal process) pass through the blast wave. We show that if our current
interpretations of the Swift XRT data are correct, there should be a canonical
high energy afterglow emission light curve. Our predictions can be tested with
high energy observatories such as GLAST, Whipple, H.E.S.S. and MAGIC. Under
favorable conditions we expect afterglow detections in all these detectors.

http://arxiv.org/abs/0704.2063

E. O. Ofek, M. Muno, R. Quimby, S. R. Kulkarni, H. Stiele, W. Pietsch, E. Nakar, A. Gal-Yam, A. Rau, P. B. Cameron, S. B. Cenko, M. M. Kasliwal, D. B. Fox, P. Chandra, A. K. H. Kong, R. Barnard

The gamma-ray burst (GRB) 070201 was a bright short-duration hard-spectrum
GRB detected by the Inter-Planetary Network (IPN). Its error quadrilateral,
which has an area of 0.124 sq. deg, intersects some prominent spiral arms of
the nearby M31 (Andromeda) galaxy. Given the properties of this GRB, along with
the fact that LIGO data argues against a compact binary merger origin in M31,
this GRB is an excellent candidate for an extragalactic Soft Gamma-ray Repeater
(SGR) giant flare, with energy of 1.4×10^45 erg. Analysis of ROTSE-IIIb visible
light observations of M31, taken 10.6 hours after the burst and covering 42% of
the GRB error region, did not reveal any optical transient down to a limiting
magnitude of 17.1. We inspected archival and proprietary XMM-Newton X-ray
observations of the intersection of the GRB error quadrilateral and M31,
obtained about four weeks prior to the outburst, in order to look for periodic
variable X-ray sources. No SGR or Anomalous X-ray Pulsar (AXP) candidates
(periods in range 1 to 20 s) were detected. We discuss the possibility of
detecting extragalactic SGRs/AXPs by identifying their periodic X-ray light
curves. Our simulations suggest that the probability of detecting the periodic
X-ray signal of one of the known Galactic SGRs/AXPs, if placed in M31, is about
10% (50%), using 50 ks (2 Ms) XMM-Newton exposures.

http://arxiv.org/abs/0712.3585

D. Vanbeveren, H. Belkus, J. Van Bever, N. Mennekens

In the present paper we combine an N-body code that simulates the dynamics of
young dense stellar systems with a massive star evolution handler that accounts
in a realistic way for the effects of stellar wind mass loss. We discuss two
topics:

1. The formation and the evolution of very massive stars (with a mass >120
Mo) is followed in detail. These very massive stars are formed in the cluster
core as a consequence of the successive (physical) collison of 10-20 most
massive stars of the cluster (the process is known as runaway merging). The
further evolution is governed by stellar wind mass loss during core hydrogen
burning and during core helium burning (the WR phase of very massive stars).
Our simulations reveal that as a consequence of runaway merging in clusters
with solar and supersolar values, massive black holes can be formed but with a
maximum mass of 70 Mo. In small metallicity clusters however, it cannot be
excluded that the runaway merging process is responsible for pair instability
supernovae or for the formation of intermediate mass black holes with a mass of
several 100 Mo.

2. Massive runaways can be formed via the supernova explosion of one of the
components in a binary (the Blaauw scenario) or via dynamical interaction of a
single star and a binary or between two binaries in a star cluster. We argue
that the most massive runaways (e.g., zeta Pup, ambda Cep, BD+433654) are
probably not formed via the binary scenario, but may be the product of the
collision and merger of 2 or 3 massive stars. They are long period GRB
candidates.

http://arxiv.org/abs/0712.3343

Krzysztof Belczynski, K.Z. Stanek, Chris L. Fryer

We investigate the effect of including a significant “binary twin''
population (binaries with almost equal mass stars, q = M2/M1 > 0.95) for the
production of double compact objects and some resulting consequences, including
LIGO inspiral rate and some properties of short-hard gamma-ray bursts. We
employ very optimistic assumptions on the twin fraction (50%) among all
binaries, and therefore our calculations place an upper limits on the influence
of twins on double compact object populations. We show that for LIGO the effect
of including twins is relatively minor: although the merger rates does indeed
increase when twins are considered, the rate increase is fairly small (1.5).
Also, chirp mass distribution for double compact objects formed with or without
twins are almost indistinguishable. If double compact object are short-hard GRB
progenitors, including twins in population synthesis calculations does not
alter significantly the earlier rate predictions for the event rate. However,
for one channel of binary evolution, introducing twins more than doubles the
rate of “very prompt'' NS-NS mergers (time to merger less than 1 Myr) compared
to models with the “flat'' q distribution. In that case, 70% of all NS-NS
binaries merge within 100 Myr after their formation, indicating a possibility
of a very significant population of “prompt'' short-hard gamma-ray bursts,
associated with star forming galaxies. We also point out that, independent of
assumptions, fraction of such prompt neutron star mergers is always high,
35–70%. We note that recent observations (e.g., Berger et al.) indicate that
fraction of short-hard GRBs found in young hosts is at least 40% and possibly
even 80%.

http://arxiv.org/abs/0712.3309

M. Modjaz (1,2), L. Kewley (3), R. P. Kirshner (1), K. Z. Stanek (4), P. Challis (1), P. M. Garnavich (5), J. E. Greene (6), P. Kelly (7), J. L. Prieto (4) ((1) CfA, (2) UC Berkeley, (3) University of Hawaii, (4) Ohio State, (5) Notre Dame, (6) Princeton, (7) KIPAC/Stanford)

We compare the chemical abundances at the sites of 12 nearby (z <0.14) Type
Ic supernovae (SN Ic) that showed broad lines, but had no observed Gamma-Ray
Burst (GRB), with the chemical abundances in 5 nearby (z < 0.25) galaxies at
the sites of GRB where broad-lined SN Ic were seen after the fireball had
faded. It has previously been noted that GRB hosts are low in luminosity and
low in their metal abundances. If low metallicity is sufficient to force the
evolution of massive stars to end their lives as GRB with an accompanying
broad-lined SN Ic, then we would expect higher metal abundances for the
broad-lined SN Ic that have no detected GRB. This is what we observe, and this
trend is independent of the choice of metallicity calibration we adopt, and the
mode of SN survey that found the broad-lined SN. A unique feature of this
analysis is that we present new spectra of the host galaxies and analyze all
the measurements of both samples in the same set of ways, using the galaxy
emission-line measurements corrected for extinction and stellar absorption, via
independent metallicity diagnostics of Kewley & Dopita (2002), of McGaugh
(1991) and of Pettini & Pagel (2004). In our small sample, the boundary between
galaxies that have GRB accompanying their broad-lined SN Ic and those that have
broad-lined SN Ic without a GRB lies at an oxygen abundance of 12+log(O/H)_KD02
~ 8.5, which corresponds to 0.2-0.6 Z_sun depending on the adopted metallicity
scale and solar abundance value. Even when we limit the comparison to SN Ic
that were found in untargeted supernova surveys, the environment of every
broad-lined SN Ic that had no GRB is more metal rich than the site of any
broad-lined SN Ic where a GRB was detected.

http://arxiv.org/abs/astro-ph/0701246

B. Vukotic, M.M. Cirkovic

We investigate the effects of correlated global regulation mechanisms,
especially Galactic gamma-ray bursts (GRBs), on the temporal distribution of
hypothetical inhabited planets, using simple Monte Carlo numerical experiments.
Starting with recently obtained models of planetary ages in the Galactic
Habitable Zone (GHZ), we obtain that the times required for biological
evolution on habitable planets of the Milky Way are highly correlated. These
results run contrary to the famous anti-SETI anthropic argument of Carter, and
give tentative support to the ongoing and future SETI observation projects.

http://arxiv.org/abs/0712.1508

Elena M. Rossi (JILA), Rosalba Perna (JILA), Frédéric Daigne (IAP)

The paucity of reliable achromatic breaks in Gamma-Ray Burst afterglow light
curves motivates independent measurements of the jet aperture. Orphan afterglow
(OA) searches, 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
nearly isotropic afterglows to GRBs scales with the jet solid angle. We
consider, instead, an almost isotropic outflow with a luminosity that decreases
across the emitting surface. The total GRB energy can be lower than for an
isotropic top-hat jet, and the current lack of positive detections can be more
easily explained. In particular, we adopt the universal structured jet (USJ)
model, that reproduces the observed afterglow phenomenology to the same extent
as the top-hat jet. We compute, within the framework of the USJ, the number and
rate of orphan 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. A comparison with the top-hat
model is also performed. 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 and Pan-Starrs (in the optical band) will have
chances, not only to detect OAs, but also to put strong constraints on the jet
models.

http://arxiv.org/abs/0711.4096

Houri Ziaeepour, Stephen T. Holland, Patricia T. Boyd, Kim L. Page, Samantha Oates, Craig B. Markwardt, Peter Meszaros, Neil Gehrels, Francis E. Marshall, Jay Cummings, Mike Goad

The early optical emission of the moderately high redshift ($z=3.08$) GRB
060607A shows a remarkable broad and strong peak with a rapid rise and a
relatively slow power-law decay. It is not coincident with the strong
early-time flares seen in the X-ray and gamma-ray energy bands. There is weak
evidence for variability superposed on this dominant component in several
optical bands that can be related to flares in high energy bands. While for a
small number of GRBs, well-sampled optical flares have been observed
simultaneously with X-ray and gamma ray pulses, GRB 060607A is one of the few
cases where the early optical emission shows no significant evidence for
correlation with the prompt emission. In this work we first report in detail
the broad band observations of this burst by Swift. Then by applying a simple
model for the dynamics and the synchrotron radiation of a relativistic shock,
we show that the dominant component of the early emissions in optical
wavelengths has the same origin as the tail emission produced after the main
gamma ray activity. The most plausible explanation for the peak in the optical
light curve seems to be the cooling of the prompt after the main collisions,
shifting the characteristic synchrotron frequency to the optical bands. It
seems that the cooling process requires a steepening of the electron energy
distribution and/or a break in this distribution at high energies. The sharp
break in the X-ray light curve at few thousands of seconds after the trigger,
is not observed in the IR/optical/UV bands, and therefore can not be a jet
break. Either the X-ray break is due to a change in the spectrum of the
accelerated electrons or the lack of an optical break is due to the presence of
a related delayed response component (Abbreviated).

http://arxiv.org/abs/0712.3269