astro-grb

J. Grindlay, J. Bloom, P. Coppi, A. Soderberg, J. Hong, H. Moseley, S. Barthelmy, G. Tagliaferri, G. Ghisellini, R. Della Ceca, P. Ubertini

The epochs of origin of the first stars and galaxies, and subsequent growth
of the first supermassive black holes, are among the most fundamental
questions. Observations of the highest redshift Gamma-Ray Bursts (GRBs) will be
the most compelling in situ probe of the history of initial star formation and
consequent epoch of reionization if their prompt and precise detection can be
followed immediately by sensitive near-IR imaging and spectroscopy. Blazars are
the persistent analogs of GRBs and for the same reason (beaming) can be
observed at highest redshifts where they might best trace the high accretion
rate-driven jets and growth of supermassive black holes in galaxies. The
proposed EXIST mission can uniquely probe these questions, and many others,
given its unparalled combination of sensitivity and spatial-spectral-temporal
coverage and resolution. Here we provide a brief summary of the mission design,
key science objectives, mission plan and readiness for EXIST, as proposed to
Astro2010.

http://arxiv.org/abs/1002.4823

M. Patel, S. J. Warren, D. J. Mortlock, J. P. U. Fynbo

Aims: We reanalyse optical spectra of the z=6.7 gamma-ray burst GRB 080913,
adding hitherto unpublished spectra, in order to reassess the measurement of
the neutral fraction of the IGM at high redshifts.

Methods: In the data reduction we take particular care to minimise systematic
errors in the sky subtraction, which are evident in the published spectrum, and
compromise the analysis. The final combined spectrum has a higher S/N than the
previously published spectrum by a factor of 1.3.

Results: We find a single significant absorption line redward of the Ly-alpha
continuum break, which we identify with the SII+SiII blend (rest wavelength of
0.1260 micron), at z=6.733. The sharp spectral break at Ly-alpha implies a
comparatively low total column density of neutral hydrogen along the line of
sight, log [N(HI)/cm^-2] < 20. We model the absorption with a host-galaxy DLA,
surrounded by an ionised region of unknown size r, within the IGM of neutral
fraction, x(HI). Despite knowing the source redshift, and the improved S/N of
the spectrum, when fitting only over wavelengths redward of Ly-alpha, no useful
constraints on x(HI) can be obtained. We consider the possibility of including
the ionised region, blueward of Ly-alpha, in constraining the fit. For the
optimistic assumption that the ionised region is transparent, tau_{GP} << 1, we
find that the region is of small size r < 2 proper Mpc, and we obtain an upper
limit to the neutral fraction of the IGM at z=6.7 of x(HI) < 0.73.

http://arxiv.org/abs/1002.4663

Elisabetta Bissaldi, for the Fermi GBM Collaboration

We present the analysis of two long Gamma-Ray Bursts, GRB 090323 and GRB
090328, which triggered the Fermi Gamma-Ray Burst Monitor (GBM) and generated
an Autonomous Repoint Request to the Fermi Large Area Telescope. The GBM light
curves show multi-peaked structures for both events. Here, we present
time-integrated and time-resolved burst spectra fitted with different models by
the GBM detectors.

http://arxiv.org/abs/1002.4194

Yidong Xu, Andrea Ferrara, Francisco S. Kitaura, Xuelei Chen

We use a model developed by Xu et al. (2010) to compute the 21 cm line
absorption signatures imprinted by star-forming dwarf galaxies (DGs) and
starless minihalos (MHs). The method, based on a statistical comparison of the
equivalent width (W_\nu) distribution and flux correlation function, allows us
to derive a simple selection criteria for candidate DGs at very high (z >= redshift. We find that ~ 18% of the total number of DGs along a line of sight
to a target radio source (GRB or quasar) can be identified by the condition
W_\nu < 0; these objects correspond to the high-mass tail of the DG
distribution at high redshift, and are embedded in large HII regions. The
criterion W_\nu > 0.37 kHz instead selects ~ 11% of MHs. Selected candidate DGs
could later be re-observed in the near-IR by the JWST with high efficiency,
thus providing a direct probe of the most likely reionization sources.

http://arxiv.org/abs/1002.4356

Matthew G. Baring

It is widely accepted that the prompt transient signal in the 10 keV - 10 GeV
band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the
ultra-relativistic expansion. The detailed understanding of the dissipation and
accompanying acceleration at these shocks is a currently topical subject. This
paper explores the relationship between GRB prompt emission spectra and the
electron (or ion) acceleration properties at the relativistic shocks that
pertain to GRB models. The focus is on the array of possible high-energy
power-law indices in accelerated populations, highlighting how spectra above 1
MeV can probe the field obliquity in GRB internal shocks, and the character of
hydromagnetic turbulence in their environs. It is emphasized that diffusive
shock acceleration theory generates no canonical spectrum at relativistic MHD
discontinuities. This diversity is commensurate with the significant range of
spectral indices discerned in prompt burst emission. Such system diagnostics
are now being enhanced by the broadband spectral coverage of bursts by the
Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM)
provides key diagnostics on the lower energy portions of the particle
population, the focus here is on constraints in the non-thermal, power-law
regime of the particle distribution that are provided by the Large Area
Telescope (LAT).

http://arxiv.org/abs/1002.3848

M. Arimoto, N. Kawai, K. Asano, K. Hurley, M. Suzuki, Y. E. Nakagawa, T. Shimokawabe, N. V. Pazmino, R. Sato, M. Matsuoka, A. Yoshida, T. Tamagawa, Y. Shirasaki, S. Sugita, I. Takahashi, J.-L. Atteia, A. Pelangeon, R. Vanderspek, C. Graziani, G. Prigozhin, J. Villasenor, J. G. Jernigan, G. B. Crew, T. Sakamoto, G. R. Ricker, S. E. Woosley, N. Butler, A. Levine, J. P. Doty, T. Q. Donaghy, D. Q. Lamb, E. Fenimore, M. Galassi, M. Boer, J.-P. Dezalay, J.-F. Olive, J. Braga, R. Manchanda, G. Pizzichini

Using a pulse-fit method, we investigate the spectral lags between the
traditional gamma-ray band (50-400 keV) and the X-ray band (6-25 keV) for 8
GRBs with known redshifts (GRB 010921, GRB 020124, GRB 020127, GRB 021211, GRB
030528, GRB 040924, GRB 041006, GRB 050408) detected with the WXM and FREGATE
instruments aboard the HETE-2 satellite. We find several relations for the
individual GRB pulses between the spectral lag and other observables, such as
the luminosity, pulse duration, and peak energy (Epeak). The obtained results
are consistent with those for BATSE, indicating that the BATSE correlations are
still valid at lower energies (6-25 keV). Furthermore, we find that the photon
energy dependence for the spectral lags can reconcile the simple curvature
effect model. We discuss the implication of these results from various points
of view.

http://arxiv.org/abs/1002.3849

Tayyaba Zafar (1), Darach J. Watson (1), Daniele Malesani (1), Paul M. Vreeswijk (1), Johan P.U. Fynbo (1), Jens Hjorth (1), Andrew J. Levan (2), Michał J. Michałowski (1,3) ((1) DARK, (2) Univ. Warwick., (3) Univ. Edinburgh)

Context: GRB afterglows are excellent probes of gas and dust in star-forming
galaxies at all epochs. It has been posited that dust in the early Universe
must be different from dust at lower z. To date two reports directly support
this contention, one of which is based on the spectral shape of GRB 050904 at z
= 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We
address the claimed evidence for unusual (SN-origin) dust in its host galaxy by
simultaneously examining the X-ray and optical/NIR spectrophotometric data.
Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4
days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn
photometric data, the spectroscopic and z'-band photometric data at ~3 days
from the Subaru telescope, as well as the critical UKIRT Z-band photometry at
0.47 days, upon which the claim of dust detection largely relies. Results: We
find no evidence of dust extinction in the SED. We compute flux densities at
lambda_rest = 1250 AA directly from the observed counts at all epochs. In the
earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3
+- 0.2 mag) than previously reported and statistically insignificant (<1.5
sigma). Furthermore we find that the photometry of this band is unstable and
difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that
there is no evidence for dust extinction — the SED at all times can be
reproduced without dust, and at 1.25 days in particular, significant extinction
can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type
extinction curve. We conclude that there is no evidence of any extinction in
the afterglow of GRB 050904 and that the presence of SN-origin dust in the host
of GRB 050904 must be viewed skeptically. [abridged]

http://arxiv.org/abs/1002.4101

Carlo Graziani

Beginning with the 2002 discovery of the “Amati Relation” of GRB spectra,
there has been much interest in the possibility that this and other
correlations of GRB phenomenology might be used to make GRBs into standard
candles. One recurring apparent difficulty with this program has been that some
of the primary observational quantities to be fit as “data” — to wit, the
isotropic-equivalent prompt energy $E_{iso}$ and the collimation-corrected
“total” prompt energy energy $E_{\gamma}$ — depend for their construction on
the very cosmological models that they are supposed to help constrain. This is
the so-called “Circularity Problem” of standard candle GRBs. This paper is
intended to point out that the “Circularity Problem” is not in fact a problem
at all, except to the extent that it amounts to a self-inflicted wound. It
arises essentially because of an inapt choice of data variables –
“source-frame” variables such as $E_{iso}$, which are unnecessarily encumbered
by cosmological considerations. If, instead, the empirical correlations of GRB
phenomenology which are formulated in source-variables are {\it mapped to the
primitive observational variables} (such as fluence) and compared to the
observations in that space, then all taint of “circularity” disappears. I also
describe procedures for encoding high-dimensional empirical correlations (such
as between $E_{iso}$, $E_{pk}$, $t_{jet}$, and $T_{45}$) in a “Gaussian Tube”
model that includes both the correlation and its intrinsic scatter, and how
that source-variable model may easily be mapped to the space of primitive
observables, to be convolved with the measurement errors and fashioned into a
likelihood.

http://arxiv.org/abs/1002.3434

G. Ghisellini, G. Ghirlanda (INAF - Osservatorio Astronomico di Brera)

The GeV emission of Gamma Ray Bursts, first detected by EGRET in an handful
of bursts, is now an established property of roughly the 10% of all bursts,
thanks to the Fermi/LAT observations. GRB 090510, a short burst, is
particularly interesting because the good timing allows to derive a severe
limit to theories of quantum gravity. With the dozen bursts detected in the
0.1-30 GeV band so far, we start to see some common properties: (i) the
duration is often longer than the duration of the softer emission detected by
the Gamma Burst Monitor (GBM) onboard Fermi; (ii) the spectrum is consistent
with F(v)~v^{-1} with no strong spectral evolution; (iii) for the brightest
bursts, the flux detected by the LAT decays as a power law with a typical
slope: t^{-1.5}; iv) the peak energy of the GBM emission exceeds 500 keV (rest
frame). These properties suggest a similar process for the origin of the GeV
flux. We propose that it is afterglow synchrotron emission shortly following
the start of the prompt phase. The steep decay slope suggests that the fireball
emits in the radiative regime, i.e. all dissipated energy is radiated away. The
large peak energy of the GBM flux suggests that electron-positron pairs might
play a crucial role. The rapid onset, but with some delay, of the GeV flux with
respect to the GBM one suggests that the bulk Lorentz factor Gamma of these
bursts is of the order of 1000. Therefore the relatively small fraction of
bursts detected at high energies might correspond to the fraction of bursts
having the largest Gamma. If the emission occurs in the radiative regime we can
start to understand why the observed X-ray and optical afterglow energetics are
much smaller than the energetics emitted during the prompt phase.

http://arxiv.org/abs/1002.3377

Fermi LAT Collaboration, Fermi GBM Collaboration

We report on the detailed analysis of the high-energy extended emission from
the short Gamma-Ray Burst (GRB) 081024B, detected by the Fermi Gamma-ray Space
Telescope. Historically, this represents the first clear detection of temporal
extended emission from a short GRB. The light curve observed by the Fermi
Gamma-ray Burst Monitor lasts approximately 0.8 seconds whereas the emission in
the Fermi Large Area Telescope lasts for about 3 seconds. Evidence of longer
lasting high-energy emission associated with long bursts has been already
reported by previous experiments. Our observations, together with the earlier
reported study of the bright short GRB 090510, indicate similarities in the
high-energy emission of short and long GRBs and open the path to new
interpretations.

http://arxiv.org/abs/1002.3205