astro-grb

Emily M. Levesque, Edo Berger, Lisa J. Kewley, Megan M. Bagley

We present the first observations from a large-scale survey of nearby (z < 1)
long-duration gamma-ray burst (LGRB) host galaxies, which consist of eight
rest-frame optical spectra obtained at Keck and Magellan. Along with two host
galaxy observations from the literature, we use optical emission line
diagnostics to determine metallicities, ionization parameters, young stellar
population ages, and star formation rates. We compare the LGRB host
environments to a variety of local and intermediate-redshift galaxy
populations, as well as the newest grid of stellar population synthesis and
photoionization models generated with the Starburst99/Mappings codes. With
these comparisons we investigate whether the GRB host galaxies are consistent
with the properties of the general galaxy population, and therefore whether
they may be used as reliable tracers of star formation. We find that LGRB host
galaxies generally have low-metallicity ISM environments out to z ~ 1. The ISM
properties of our GRB hosts, including metallicity, ionization parameter, and
young stellar population age, are significantly different from the general
galaxy population, host galaxies of nearby broad-lined Type Ic supernovae, and
nearby metal-poor galaxies.

http://arxiv.org/abs/0907.4988

K. Hurley, A. Rowlinson, E. Bellm, D. Perley, I. G. Mitrofanov, D. V. Golovin, A. S. Kozyrev, M. L. Litvak, A. B. Sanin, W. Boynton, C. Fellows, K. Harshmann, M. Ohno, K. Yamaoka, Y. E. Nakagawa, D. M. Smith, T. Cline, N.R. Tanvir, P.T. O'Brien, K. Wiersema, E. Rol, A. Levan, J. Rhoads, A. Fruchter, D. Bersier, J.J. Kavelaars, N. Gehrels, H. Krimm, D. M. Palmer, R. C. Duncan, C. Wigger, W. Hajdas, J.-L. Atteia, G. Ricker, R. Vanderspek, A. Rau, A. von Kienlin

GRB 051103 is considered to be a candidate soft gamma repeater (SGR)
extragalactic giant magnetar flare by virtue of its proximity on the sky to
M81/M82, as well as its time history, localization, and energy spectrum. We
have derived a refined interplanetary network localization for this burst which
reduces the size of the error box by over a factor of two. We examine its time
history for evidence of a periodic component, which would be one signature of
an SGR giant flare, and conclude that this component is neither detected nor
detectable under reasonable assumptions. We analyze the time-resolved energy
spectra of this event with improved time- and energy resolution, and conclude
that although the spectrum is very hard, its temporal evolution at late times
cannot be determined, which further complicates the giant flare association. We
also present new optical observations reaching limiting magnitudes of R > 24.5,
about 4 magnitudes deeper than previously reported. In tandem with
serendipitous observations of M81 taken immediately before and one month after
the burst, these place strong constraints on any rapidly variable sources in
the region of the refined error ellipse proximate to M81. We do not find any
convincing afterglow candidates from either background galaxies or sources in
M81, although within the refined error region we do locate two UV bright star
forming regions which may host SGRs. A supernova remnant (SNR) within the error
ellipse could provide further support for an SGR giant flare association, but
we were unable to identify any SNR within the error ellipse. These data still
do not allow strong constraints on the nature of the GRB 051103 progenitor, and
suggest that candidate extragalactic SGR giant flares will be difficult,
although not impossible, to confirm.

http://arxiv.org/abs/0907.2462

Xiang-Yu Wang (NJU), Hao-Ning He (NJU), Zhuo Li (PKU), Xue-Feng Wu (PSU, PMO), Zi-Gao Dai (NJU)

Extended high-energy(>100MeV) gamma-ray emission that lasts much longer than
the prompt sub-MeV emission has been detected from quite a few gamma-ray bursts
(GRBs) by Fermi Large Area Telescope (LAT) recently. A plausible scenario is
that this emission is the afterglow synchrotron emission produced by electrons
accelerated in the forward shocks. In this scenario, the electrons that produce
synchrotron high-energy emission also undergo inverse-Compton (IC) loss and the
IC scattering with the synchrotron photons should be in the Klein-Nishina
regime. Here we study effects of the Klein-Nishina scattering on the
high-energy synchrotron afterglow emission. We find that, at early times the
Klein-Nishina suppression effect on those electrons that produce the
high-energy emission is usually strong and therefore their inverse-Compton loss
is small with a Compton parameter Y < a few for a wide range of parameter
space. This leads to a relatively bright synchrotron afterglow at high energies
that can be detected by Fermi LAT. As the Klein-Nishina suppression effect
weakens with time, the inverse-Compton loss increases and could dominate over
the synchrotron loss in some parameter space. This will lead to a faster
temporal decay of the high-energy synchrotron emission than what is predicted
by the standard synchrotron model, which may explain the observed rapid decay
of the early high-energy gamma-ray emission in GRB090510 and GRB090902B.

http://arxiv.org/abs/0911.4189

Hao-Ning He (NJU), Xiang-Yu Wang (NJU), Yun-Wei Yu (NJU, HNU), Peter Meszaros (PSU)

The observations of gamma-ray bursts (GRBs) such as 980425, 031203 and
060218, with luminosities much lower than those of other classic bursts, lead
to the definition of a new class of GRBs — low-luminosity GRBs. The nature of
the outflow responsible for them is not clear yet. Two scenarios have been
suggested: one is the conventional relativistic outflow with initial Lorentz
factor of order of $\Gamma_0\ga 10$ and the other is a trans-relativistic
outflow with $\Gamma_0\simeq 1-2$. Here we compare the high energy gamma-ray
afterglow emission from these two different models, taking into account both
synchrotron self inverse-Compton scattering (SSC) and the external
inverse-Compton scattering due to photons from the cooling supernova or
hypernova envelope (SNIC). We find that the conventional relativistic outflow
model predicts a relatively high gamma-ray flux from SSC at early times ($<10^4
{\rm s}$ for typical parameters) with a rapidly decaying light curve, while in
the trans-relativistic outflow model, one would expect a much flatter light
curve of high-energy gamma-ray emission at early times, which could be
dominated by both the SSC emission and SNIC emission, depending on the
properties of the underlying supernova and the shock parameter $\epsilon_e$ and
$\epsilon_B$. The Fermi Gamma-ray Space Telescope should be able to distinguish
between the two models in the future.

http://arxiv.org/abs/0911.4217

Alessandra Corsi, Dafne Guetta, Luigi Piro

We investigate the origin of the prompt and delayed emission observed in the
short GRB 090510. We first attempt to explain the soft-to-hard spectral
evolution associated to the delayed onset of a GeV tail, in the hypothesis that
the prompt burst and the high energy tail both originate from a single process,
namely synchrotron emission from internal shocks. Considerations on the
compactness of the source imply that the high energy tail should be produced in
a lately emitted shell, characterized by a Lorentz factor greater than the one
generating the prompt burst. However, in this hypothesis, the evolution of the
synchrotron peak frequency does not agree with the observed soft-to-hard one,
unless a substantial change in the micro-physics of the shocks developing in
the two shells does happen. Given the difficulties of a single mechanism
hypothesis, we test two alternative double-component scenarios. In the first,
the prompt burst is explained as synchrotron radiation from internal shocks,
while the high energy emission (up to about 1 s since the trigger) as internal
shock synchrotron-self-Compton. In the second scenario, in view of its long
duration (\sim 100 s), the high energy tail is decoupled from the prompt burst
and has an external shock origin. In this case, we show that a reasonable
choice of parameters does indeed exist to accommodate the optical-to-GeV data,
provided the Lorentz factor of the shocked shell is sufficiently high. We
finally attempt to model the chromatic break observed around \sim 10^3 s under
the hypothesis of a structured jet model, finding that this might be a viable
explanation, which lowers the high value of the burst energy derived assuming
isotropy (\sim 1e53 ergs), down to \sim 1e49 ergs, more compatible with the
energetics from a binary merger progenitor.

http://arxiv.org/abs/0911.4453

Paul A Crowther (Sheffield), Joanne L Bibby (Sheffield), James P Furness (Sheffield), J Simon Clark (Open University)

We highlight how the downward revision in the distance to the star cluster
associated with SGR 1806-20 by Bibby et al. reconciles the apparent low
contamination of BATSE short GRBs by intense flares from extragalactic
magnetars without recourse to modifying the frequency of one such flare per 30
years per Milky Way galaxy. We also discuss the variety in progenitor initial
masses of magnetars based upon cluster ages, ranging from ~50 Msun for SGR
1806-20 and 1E 1647-455 in Westerlund 1 to ~15 Msun for SGR 1900+14 according
to Davies et al. and presumably also 1E 1841-045 if it originated from one of
the massive RSG clusters #2 or #3.

http://arxiv.org/abs/0908.2773

Krzysztof Belczynski, Daniel E. Holz, Chris L. Fryer, Edo Berger, Dieter H. Hartmann, Brian O'Shea

GRB 080913 and GRB 090423 are the most distant gamma-ray bursts (GRBs) known
to-date, with spectroscopically determined redshifts of z=6.7 and z=8.1,
respectively. The detection of bursts at this early epoch of the Universe
significantly constrains the nature of GRBs and their progenitors. We perform
population synthesis studies of the formation and evolution of early stars, and
calculate the resulting formation rates of short and long-duration GRBs at high
redshift. The peak of the GRB rate from Population II stars occurs at z=7 for a
model with efficient/fast mixing of metals, while it is found at z=3 for an
inefficient/slow metallicity evolution model. We show that in the redshift
range 6<z<10 essentially all GRBs originate from Population II stars,
regardless of metallicity evolution model. These stars (having small, but
non-zero metallicity) are the most likely progenitors for both long GRBs
(collapsars) and short GRBs (NS-NS or BH-NS mergers) at this epoch. Although
the predicted intrinsic rates of long and short GRBs are similar at these high
redshifts, observational selection effects lead to higher (factor of 10)
observed rates for long GRBs. We conclude that the two recently observed high-z
GRB events are most likely long GRBs originating from Population II collapsars.

http://arxiv.org/abs/0812.2470

Shlomo Dado, Arnon Dar

Emission of high energy (HE) photons above 100 MeV that is delayed and lasts
much longer than the prompt MeV emission has been detected from several long
duration gamma ray bursts (LGRBs) and short hard bursts (SHBs) by the Compton,
Fermi and AGILE gamma ray observatories. In this paper we show that the main
observed properties of this HE emission are those predicted by the cannonball
(CB) model of GRBs: In the CB model all the observed radiations in a GRB are
produced by the interaction of a highly relativistic jet of plasmoids (CBs)
with the environment. The prompt X-ray and MeV $\gamma$-ray pulses are produced
by inverse Compton scattering (ICS) of glory photons -photons scattered/emitted
into a cavity created by the wind/ejecta blown from the progenitor star or a
companion star long before the GRB- by the thermal electrons in the CBs. A
simultaneous optical and high energy emission begins shortly after each MeV
pulse when the CB collides with the wind/ejecta, and continues during the
deceleration of the CB in the interstellar medium. The optical emission is
dominated by synchrotron radiation (SR) from the swept-in and knocked-on
electrons which are Fermi accelerated to high energies by the turbulent
magnetic fields in the CBs, while ICS of these SR photons dominates the
emission of HE photons. The lightcurves of the optical and HE emissions have
approximately the same temporal behaviour but have different power-law spectra.
The emission of very high energy (VHE) photons above 100 TeV is dominated by
the decay of $\pi^0$'s produced in hadronic collisions of Fermi accelerated
protons in the CBs. The CB model explains well all the observed radiations,
including the high energy radiation from both LGRBs and SHBs as demonstrated
here for GRB 090902B and SHB 090510.

http://arxiv.org/abs/0910.0687

Davide Lazzati (NCSU), Brian J. Morsony (UW Madison), Mitchell C. Begelman (JILA)

We present 2D high resolution hydrodynamic simulations of the relativistic
outflows of long-duration gamma-ray burst progenitors. We analyze the
properties of the outflows at wide off-axis angles, produced by the expansion
of the hot cocoon that surrounds the jet inside the progenitor star. We find
that the cocoon emission at wide angles has properties that are strikingly
similar to the properties of the subclass of short-duration gamma-ray bursts
with persistent X-ray emission. We compute the predicted duration distribution,
redshift distribution, and afterglow brightness and we find that they are all
in agreement with the observed properties of short GRBs with persistent
emission. We suggest that late afterglow observations can be used as a crucial
test to verify this model.

http://arxiv.org/abs/0911.3313

S. B. Cenko, N. R. Butler, E. O. Ofek, D. A. Perley, A. N. Morgan, D. A. Frail, J. Gorosabel, J. S. Bloom, A. J. Castro-Tirado, J. Cepa, P. C. Chandra, A. de Ugarte Postigo, A. V. Filippenko, C. R. Klein, S. R. Kulkarni, A. A. Miller, P. E. Nugent, D. L. Starr

We present broadband (gamma-ray, X-ray, near-infrared, optical, and radio)
observations of the gamma-ray burst (GRB) 090709A and its afterglow in an
effort to ascertain the origin of this high-energy transient. Previous analyses
suggested that GRB 090709A exhibited quasi-periodic oscillations with a period
of 8.06 s, a trait unknown in long-duration GRBs but typical of flares from
soft gamma-ray repeaters. When properly accounting for the underlying shape of
the power-density spectrum of GRB 090709A, we find no conclusive (> 3 sigma)
evidence for the reported periodicity. In conjunction with the location of the
transient (far from the Galactic plane and absent any nearby host galaxy in the
local universe) and the evidence for extinction in excess of the Galactic
value, we consider a magnetar origin relatively unlikely. A long-duration GRB,
however, can account for the majority of the observed properties of this
source. GRB 090709A is distinguished from other long-duration GRBs primarily by
the large amount of obscuration from its host galaxy (A_K,obs >~ 2 mag).

http://arxiv.org/abs/0911.3150