astro-grb

Nozomu Tominaga

Jet-induced supernovae (SNe) have been suggested to occur in gamma-ray bursts
(GRBs) and highly-energetic SNe (hypernovae). I investigate hydrodynamical and
nucleosynthetic properties of the jet-induced explosion of a population III
$40\Msun$ star with a two-dimensional special relativistic hydrodynamical code.
The abundance distribution after the explosion and the angular dependence of
the yield are obtained for the models with high and low energy deposition rates
$\Ed=120\times10^{51}$\ergs and $1.5\times10^{51}$\ergs. I also find that the
peculiar abundance pattern of a Si-deficient metal-poor star HE 1424–0241 can
be reproduced by the angle-delimited yield for $\theta=30^\circ-35^\circ$ of
the model with the energy deposition rate of $\Ed=120\times10^{51}$\ergs. The
ejection of Fe-peak products and the fallback of unprocessed materials can
account for the abundance patterns of the extremely metal-poor (EMP) stars. I
compare the yield of the jet-induced explosion with that of the spherical
explosion and confirm the ejection and fallback in the jet-induced explosion
model is almost equivalent to the “mixing-fallback'' in spherical explosions.
In contrast to the spherical models, however, the high-entropy environment
realized in the jet-induced explosion enhances [(Sc, Ti, V, Cr, Co, Zn)/Fe].
The enhancements of [Sc/Fe] and [Ti/Fe] improve agreements with the abundance
pattern of the EMP stars.

http://arxiv.org/abs/0711.4815

LIGO Scientific Collaboration, K. Hurley

We analyzed the available LIGO data coincident with GRB 070201, a short
duration hard spectrum gamma-ray burst whose electromagnetically determined sky
position is coincident with the spiral arms of the Andromeda galaxy (M31).
Possible progenitors of such short hard GRBs include mergers of neutron stars
or a neutron star and black hole, or soft gamma-ray repeater (SGR) flares.
These events can be accompanied by gravitational-wave emission. No plausible
gravitational wave candidates were found within a 180 s long window around the
time of GRB 070201. This result implies that a compact binary progenitor of GRB
070201, with masses in the range 1 M_sun < m_1 < 3 M_sun and 1 M_sun < m_2 < 40
M_sun, located in M31 is excluded at >99% confidence. Indeed, if GRB 070201
were caused by a binary neutron star merger, we find that D < 3.5 Mpc is
excluded, assuming random inclination, at 90% confidence. The result also
implies that an unmodeled gravitational wave burst from GRB 070201 most
probably emitted less than 4.4 x 10^(-4) M_sun c^2 (7.9 x 10^(50) ergs) in any
100 ms long period within the signal region if the source was in M31 and
radiated isotropically at the same frequency as LIGO's peak sensitivity (f ~
150 Hz). This upper limit does not exclude current models of SGRs at the M31
distance.

http://arxiv.org/abs/0711.1163

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
excellent chances, not only to detect OAs, but also to put strong constraints
on the jet models.

http://arxiv.org/abs/0711.4096

L. Shao (1 and 2), Z. G. Dai (1), N. Mirabal (3) ((1)Nanjing University, China, (2)University of Colorado, USA, (3)Columbia University, USA)

We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs).
We find that the echo emission can provide an alternative way of understanding
X-ray shallow decays and jet breaks. In particular, a shallow decay followed by
a “normal” decay and a further rapid decay of X-ray afterglows can be together
explained as being due to the echo from prompt X-ray emission scattered by dust
grains in a massive wind bubble around a GRB progenitor. We also introduce an
extra temporal break in the X-ray echo emission. By fitting the afterglow light
curves, we can measure the locations of the massive wind bubbles, which will
bring us closer to finding the mass loss rate, wind velocity, and the age of
the progenitors prior to the GRB explosions.

http://arxiv.org/abs/0711.3800

R. O'Shaughnessy (1), V. Kalogera (1), K. Belczynski (2) ((1) Northwestern University, (2) New Mexico State University)

To test whether the short GRB rates, redshift distribution and host galaxies
are consistent with current theoretical predictions, we use avery large
database of population synthesis calculations to examine BH-NS and NS-NS merger
rates in the universe, factoring in (i) the star formation history of the
universe, (ii) a heterogeneous population of star-forming galaxies, including
spirals and ellipticals, and (iii) a simple flux-limited selection model for
short GRB detection. When we require our models reproduce the known short GRB
rates and redshift measurements (and, for NS-NS, the merger rates extrapolated
from binary pulsars in the Galaxy), a small fraction of models reproduce all
observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most
commonly models produce mergers preferentially in spiral galaxies if short GRBs
arise from NS-NS mergers alone. Model universes where present-day binary
mergers occur preferentially in elliptical galaxies necessarily include a
significant fraction of binaries with long delay times between birth and merger
(often $O(10{\rm Gyr})$). Though long delays occur, almost all of our models
predict that a higher proportion of short GRBs should occur at moderate to high
redshift (e.g., $z>1$) than has presently been observed, in agreement with
recent observations which suggest a selection bias towards successful follow-up
of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have
the right combination of masses and spins to make GRBs, then at best only a
small fraction of BH-NS models could be consistent with all {\em current}
available data. (Abridged)

http://arxiv.org/abs/0706.4139

A. Moretti (1), R. Margutti (1,2), F. Pasotti (1,2), A.P. Beardmore (3), S. Campana (1), G. Chincarini (2,1), S. Covino (1), O. Godet (3), C. Guidorzi (2,1) J.P. Osborne (3), P. Romano (2,1), G. Tagliaferri (1) ((1) INAF-OAB; (2) U. Bicocca; (3) U. Leicester)

We aim to investigate the ability of simple spectral models to describe the
GRB early afterglow emission. We performed a time resolved spectral analysis of
a bright GRB sample detected by the Swift Burst Alert Telescope and promptly
observed by the Swift X-ray Telescope,with spectroscopically measured redshift
in the period April 2005 — January 2007. The sample consists of 22 GRBs and a
total of 214 spectra. We restricted our analysis to the softest spectra
sub–sample which consists of 13 spectra with photon index > 3. In this sample
we found that four spectra, belonging to GRB060502A, GRB060729, GRB060904B,
GRB061110A prompt–afterglow transition phase, cannot be modeled neither by a
single power law nor by the Band model. Instead we find that the data present
high energy (> 3 keV, in the observer frame) excesses with respect to these
models. We estimated the joint statistical significance of these excesses at
the level of 4.3 sigma. In all four cases, the deviations can be modeled well
by adding either a second power law or a blackbody component to the usual
synchrotron power law spectrum. The additional power law would be explained by
the emerging of the afterglow, while the blackbody could be interpreted as the
photospheric emission from X-ray flares or as the shock breakout emission. In
one case these models leave a 2.2 sigma excess which can be fit by a Gaussian
line at the energy the highly ionized Nickel recombination. Although the data
do not allow an unequivocal interpretation, the importance of this analysis
consists in the fact that we show that a simple power law model or a Band model
are insufficient to describe the X-ray spectra of a small homogeneous sample of
GRBs at the end of their prompt phase.

http://arxiv.org/abs/0711.3739

R.L.C. Starling (1), P.T. O'Brien, R. Willingale, K.L. Page, J.P. Osborne, M. De Pasquale, Y.E. Nakagawa, N.P.M. Kuin, K. Onda, J.P. Norris, T.N. Ukwatta, N. Kodaka, D.N. Burrows, J.A. Kennea, M.J. Page, M. Perri, C.B. Markwardt ((1) University of Leicester, UK)

The origins of Gamma-ray Burst prompt emission are currently not well
understood and in this context long, well-observed events are particularly
important to study. We present the case of GRB 070616, analysing the
exceptionally long-duration multipeaked prompt emission, and later afterglow,
captured by all the instruments on-board Swift and by Suzaku WAM. The high
energy light curve remained generally flat for several hundred seconds before
going into a steep decline. Spectral evolution from hard to soft is clearly
taking place throughout the prompt emission, beginning at 285 s after the
trigger and extending to 1200 s. We track the movement of the spectral peak
energy, whilst observing a softening of the low energy spectral slope. The
steep decline in flux may be caused by a combination of this strong spectral
evolution and the curvature effect. We investigate origins for the spectral
evolution, ruling out a superposition of two power laws and considering instead
an additional component dominant during the late prompt emission. We also
discuss origins for the early optical emission and the physics of the
afterglow. The case of GRB 070616 clearly demonstrates that both broadband
coverage and good time resolution are crucial to pin down the origins of the
complex prompt emission in GRBs.

http://arxiv.org/abs/0711.3753

S. Schanne, B. Cordier, D. Gotz, A. Gros, P. Kestener, H. Le Provost, B. L'Huillier, M. Mur

Gamma-ray bursts (GRB) sign energetic explosions in the Universe, occurring
at cosmological distances. Multi-wavelength observations of GRB allow to study
their properties and to use them as cosmological tools. In 2012 the space borne
gamma-ray telescope ECLAIRs is expected to provide accurate GRB localizations
on the sky in near real-time, necessary for ground-based follow-up
observations. Led by CEA Saclay, France, the project is currently in its
technical design phase. ECLAIRs is optimized to detect highly red-shifted GRB
thanks to a 4 keV low energy threshold. A coded mask telescope with a 1024 cm^2
detection plane of 80×80 CdTe pixels permanently observes a 2 sr sky field. The
on-board trigger detects GRB using count-rate increase monitors on multiple
time-scales and cyclic images. It computes sky images in the 4-50 keV energy
range by de-convolving detector plane images with the mask pattern and
localizes newly detected sources with <10 arcmin accuracy. While individual GRB
photons are available hours later, GRB alerts are transmitted over a VHF
network within seconds to ground, in particular to robotic follow-up
telescopes, which refine GRB localizations to the level needed by large
spectroscopic telescopes. This paper describes the ECLAIRs concept, with
emphasis on the GRB triggering scheme.

http://arxiv.org/abs/0711.3754

John Middleditch

Early measurements of SN 1987A can be interpreted in light of a beam/jet
(BJ), with a collimation factor >10,000, which impacted polar ejecta (PE) to
produce the “Mystery Spot” (MS), ~24 lt-d away. Other details of SN 1987A
suggest that it came from a merger of 2 stellar cores of a common envelope (CE)
binary, i.e. a “double degenerate” (DD) SN. Even having to penetrate the CE,
the BJ may have caused a long-soft (l)GRB upon hitting the PE, thus DD can
produce lGRBs. Because DD must be the dominant merger/SN mechanism in
elliptical galaxies (EGs), where only short, hard GRBs (sGRBs) have been
observed, DD without CE or PE must also produce sGRBs, and thus NS-NS mergers
may not make GRBs as we know them, and/or be as common as previously thought.
Millisecond pulsars (MSPs) in the non-core-collapsed globular clusters are also
99% DD-formed from WD-WD merger, consistent with their 2.10 ms minimum spin
period, the 2.14 ms signal seen from SN 1987A, and sGRBs offset from the
centers of EGs. The details of Ia's suggest that these are also DD, and the
total thermonuclear disruption paradigm is now in serious doubt as well, a
cause for concern in Ia Cosmology, because Ia's will appear to be Ic's when
viewed from their DD merger poles, given sufficient matter above that lost to
core-collapse. As a DD SN, 1987A appears to be the Rosetta Stone for 99% of
SNe, GRBs and MSPs, including all recent nearby SNe except SN 1986J, and the
more distant SN 2006gy. There is no need to invent exotica, such as
“collapsars,” to account for GRBs.

http://arxiv.org/abs/0708.2263

Charles D. Dermer (NRL)

The discoveries of the GZK cutoff with the HiRes and Auger Observatories and
the discovery by Auger of clustering of >~60 EeV ultra-high energy cosmic rays
(UHECRs) towards nearby <~75 Mpc) AGNs along the supergalactic plane
establishes the astrophysical origin of the UHECRs. The likely sources of the
UHECRs are gamma-ray bursts and radio-loud AGNs because: (1) they are
extragalactic; (2) they are sufficiently powerful; (3) acceleration to
ultra-high energies can be achieved in their relativistic ejecta; (4) anomalous
X-ray and $\gamma$-ray features can be explained by nonthermal hadron
acceleration in relativistic blast waves; and (5) sources reside within the GZK
radius. Two arguments for acceleration to UHE are presented, and limits on
UHECR ion acceleration are set. UHECR ions are shown to be able to survive
without photodisintegrating while passing through the AGN scattered radiation
field, even if launched deep in the broad line region. UHECR injection
throughout cosmic time fits the measured energy spectrum of UHECRs, at least
for protons. Local UHECR proton and ion interaction and energy-loss mean free
paths are calculated using an empirical fit to the extragalactic background
light (EBL) at IR and optical energies. Minimum intergalactic magnetic (IGM)
fields ~1e-11 G are derived from clustering assuming specific source origins,
e.g., Cen A, nearby AGNs, or GRBs for the super-GZK CRs seen with Auger.
Besides distinct cosmic-ray induced gamma-ray signatures that should be
observed with the Gamma ray Large Area Space Telescope (GLAST), source and GZK
neutrino detections and the arrival distribution of UHECR in direction and time
can finally decide the sources of cosmic rays at the highest energies.

http://arxiv.org/abs/0711.2804