astro-grb

Zhi-Bin Zhang, Chul-Sung Choi

We report the systematic analysis of the durations for Swift gamma-ray bursts
(GRBs) and compare the results with those of pre-Swift data. For 95 GRBs with
known redshift, we show that the observed durations have two lognormal
distributions that are clearly divided at $T_{90}\simeq2$ s. This is consistent
with the earlier BATSE results. The intrinsic durations also show a bimodal
distribution but shift systematically toward the smaller value and the
distribution exhibits a narrower width compared with the observed one. We find
that the intrinsic distributions of long GRBs between Swift and pre-Swift are
significantly different particularly in the width and the median value. In
addition, the Swift data exhibit a wider duration dynamic range. Our present
study also confirms the spectra of short GRBs are in general harder than the
long GRBs.

http://arxiv.org/abs/0708.4049

Kohta Murase, Katsuaki Asano, Shigehiro Nagataki

Regenerated high-energy emissions from gamma-ray bursts (GRBs) are studied in
detail. If the intrinsic primary spectrum extends to the TeV range, these very
high-energy photons are absorbed by the cosmic infrared background (CIB).
Created high-energy electron-positron pairs up-scatter mainly cosmic microwave
background (CMB) photons, and secondary photons are generated in the GeV-TeV
range. These secondary delayed photons may be observed in the near future,
which are useful for a consistency check of the intrinsic primary spectrum. In
this paper, we focus on effects of the CIB on delayed secondary emissions. In
particular, we show that not only up-scattered CMB photons but also
up-scattered CIB ones are important, especially for low redshift bursts. They
also give us additional information on the CIB, whose photon density is not
definitely determined so far.

http://arxiv.org/abs/astro-ph/0703759

M. J. Michałowski, J. Hjorth, J. M. Castro Cerón, D. Watson (Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Denmark)

We present detailed fits of the spectral energy distributions (SEDs) of four
submillimeter (submm) galaxies selected by the presence of a gamma-ray burst
(GRB) event (GRBs 980703, 000210, 000418 and 010222). These faint ~3 mJy submm
emitters at redshift ~1 are characterized by an unusual combination of long-
and short-wavelength properties, namely enhanced submm and/or radio emission
combined with optical faintness and blue colors. We exclude an active galactic
nucleus as the source of long-wavelength emission. From the SED fits we
conclude that the four galaxies are young (ages <2 Gyr), highly starforming
(star formation rates ~150 MSun/yr), low-mass (stellar masses ~10^10 MSun) and
dusty (dust masses ~3×10^8 MSun). Their high dust temperatures (Td>45 K)
indicate that GRB host galaxies are hotter, younger, and less massive
counterparts to submm-selected galaxies detected so far. Future facilities like
Herschel, JCMT/SCUBA-2 and ALMA will test this hypothesis enabling measurement
of dust temperatures of fainter GRB-selected galaxies.

http://arxiv.org/abs/0708.3850

Daisuke Yonetoku, Sachiko Tanabe, Toshio Murakami, Naomi Emura, Yuka Aoyama, Takashi Kidamura, Hironobu Kodaira, Yoshiki Kodama, Ryota Kozaka, Takuro Nashimoto, Shinya Okuno, Satoshi Yokota, Satoru Yoshinari, Keiichi Abe, Kaori Onda, Makoto S. Tashiro, Yuji Urata, Yujin E. Nakagawa, Satoshi Sugita, Kazutaka Yamaoka, Atsumasa Yoshida, Takuto Ishimura, Nobuyuki Kawai, Takashi Shimokawabe, Kenzo Kinugasa, Takayoshi Kohmura, Kaori Kubota, Kei Sugiyasu, Yoshihiro Ueda, Kensuke Masui, Kazuhiro Nakazawa, Tadayuki Takahashi, Shouta Maeno, Eri Sonoda, Makoto Yamauchi, Makoto Kuwahara, Toru Tamagawa, Daisuke Matsuura, Motoko Suzuki, Scott Barthelmy, Neil Gehrels, John Nousek

We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku
satellites. We found rapid spectral softening during both the prompt tail phase
and the decline phase of an X-ray flare in the BAT and XRT data. The observed
spectra were fit by power-law photon indices which rapidly changed from $\Gamma
= 1.51^{+0.04}_{-0.03}$ to $\Gamma = 5.30^{+0.69}_{-0.59}$ within a few hundred
seconds in the prompt tail. This is one of the steepest X-ray spectra ever
observed, making it quite difficult to explain by simple electron acceleration
and synchrotron radiation. Then, we applied an alternative spectral fitting
using a broken power-law with exponential cutoff (BPEC) model. It is valid to
consider the situation that the cutoff energy is equivalent to the synchrotron
frequency of the maximum energy electrons in their energy distribution. Since
the spectral cutoff appears in the soft X-ray band, we conclude the electron
acceleration has been inefficient in the internal shocks of GRB 060904A. These
cutoff spectra suddenly disappeared at the transition time from the prompt tail
phase to the shallow decay one. After that, typical afterglow spectra with the
photon indices of 2.0 are continuously and preciously monitored by both XRT and
Suzaku/XIS up to 1 day since the burst trigger time. We could successfully
trace the temporal history of two characteristic break energies (peak energy
and cutoff energy) and they show the time dependence of $\propto t^{-3} \sim
t^{-4}$ while the following afterglow spectra are quite stable. This fact
indicates that the emitting material of prompt tail is due to completely
different dynamics from the shallow decay component. Therefore we conclude the
emission sites of two distinct phenomena obviously differ from each other.

http://arxiv.org/abs/0708.3968

Christina C. Thoene, Klaas Wiersema, Cedric Ledoux, Rhaana L. C. Starling, Johan P. U. Fynbo, Peter A. Curran, Javier Gorosabel, Alexander J. van der Horst, Lisa J. Kewley, Andrew J. Levan, Alvaro LLorente, Evert Rol, Nial R. Tanvir, Antonio de Ugarte Postigo, Paul M. Vreeswijk, Ralph A. M. J. Wijers

Context. The spectra of afterglows can provide us with detailed information
on the line-of-sight towards high redshift gamma-ray bursts (GRBs). This allows
us to use GRB afterglows as sensitive probes of interstellar matter in their
host galaxies, and the circumstellar material around the progenitor star. Aims.
In this paper we present early WHT/ISIS optical spectroscopy of the afterglow
of the gamma-ray burst GRB 060206 at z = 4.048, detecting a range of metal
absorption lines and their fine-structure transitions. Additional information
is provided by properties derived from the afterglow lightcurve and from deep
imaging of the host galaxy. Methods. The resolution and wavelength range of the
spectra and the bright afterglow facilitate a detailed study of the circumburst
and host galaxy environment through fitting of the absorption line systems.
Their column densities allow us to derive properties for the different detected
velocity components. We also use the deep imaging to detect the host galaxy and
probe the nature of the intervening system that is seen in absorption in the
afterglow spectra. Results. We detect several discrete velocity systems in
several absorption lines, best explained by shells within and/or around the
host created by starburst winds. From the strength of the fine-structure line
in the different systems, we conclude that the redmost component is the one
closest to the burst and most likely excited by indirect UV pumping, whereas
the other components originate from collisional excitation. The host is
detected in deep imaging with r

http://arxiv.org/abs/0708.3448

M. Nysewander, D. E. Reichart, J. A. Crain, A. Foster, J. Haislip, K. Ivarsen, A. Lacluyze, A. Trotter

PROMPT (Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes)
observed the early-time optical afterglow of GRB 060607A and obtained a densely
sampled multiwavelength light curve that begins only tens of seconds after the
GRB. Located at Cerro Tololo Inter-American Observatory in Chile, PROMPT is
designed to observe the afterglows of gamma-ray bursts using multiple automated
0.4-m telescopes that image simultaneously in many filters when the afterglow
is bright and may be highly variable. The data span the interval from 44
seconds after the GRB trigger to 3.3 hours in the Bgri filters. We observe an
initial peak in the light curve at approximately three minutes, followed by
rebrightenings peaking around 40 minutes and again at 66 minutes. Although our
data overlap with the early Swift gamma-ray and x-ray light curves, we do not
see a correlation between the optical and high-energy flares. We measure the
early (t < 3 minutes) spectral index and find that it does not agree with the
assumption that the initial peak is the result of the passage of nu_m. We model
the variations in the light curves and find that the most likely cause of the
rebrightening episodes is a refreshment of the forward shock preceded by a
rapidly fading reverse shock component, although other explanations are
plausible.

http://arxiv.org/abs/0708.3444

G. Stratta, P. D'Avanzo, S. Piranomonte, S. Cutini, B. Preger, M. Perri, M.L. Conciatore, S. Covino, L. Stella, D. Guetta, F.E. Marshall, S. T. Holland, M. Stamatikos, C. Guidorzi, V. Mangano, L. A. Antonelli, D. Burrows, S. Campana, M. Capalbi, G. Chincarini, G. Cusumano, V. D'Elia, P.A. Evans, F. Fiore, D. Fugazza, P. Giommi, J.P. Osborne, V. La Parola, T. Mineo, A. Moretti, K.L. Page, P. Romano, G. Tagliaferri

Our knowledge of the intrinsic properties of short duration Gamma-Ray Bursts
has relied, so far, only upon a few cases for which the estimate of the
distance and an extended, multiwavelength monitoring of the afterglow have been
obtained. We carried out multiwavelength observations of the short GRB 061201
aimed at estimating its distance and studying its properties. We performed a
spectral and timing analysis of the prompt and afterglow emission and discuss
the results in the context of the standard fireball model. A clear temporal
break was observed in the X-ray light curve about 40 minutes after the burst
trigger. We find that the spectral and timing behaviour of the X-ray afterglow
is consistent with a jet origin of the observed break, although the optical
data can not definitively confirm this and other scenarios are possible. No
underlying host galaxy down to R~26 mag was found after fading of the optical
afterglow. Thus, no secure redshift could be measured for this burst. The
nearest galaxy is at z=0.111 and shows evidence of star formation activity. We
discuss the association of GRB 061201 with this galaxy and with the ACO S 995
galaxy cluster, from which the source is at an angular distance of 17'' and
8.5', respectively. We also test the association with a possible undetected,
positionally consistent galaxy at z~1. In all these cases, in the jet
interpretation, we find a jet opening angle of 1-2 degrees.

http://arxiv.org/abs/0708.3553

Brian D. Metzger, Todd A. Thompson, Eliot Quataert

We calculate the structure and neutron content of neutrino-heated MHD winds
driven from the surface of newly-formed magnetars (“proto-magnetars'') and
from the midplane of hyper-accreting disks, two of the possible central engines
for gamma-ray bursts (GRBs) and hyper-energetic supernovae (SNe). Both the
surface of proto-magnetars and the midplane of neutrino-cooled accretion flows
(NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p
>> 1). If this substantial free neutron excess is preserved to large radii in
ultra-relativistic outflows, several important observational consequences may
result. Weak interaction processes, however, can drive n/p to ~1 in the
nondegenerate regions that obtain just above the surfaces of NDAFs and
proto-magnetars. Our calculations show that mildly relativistic neutron-rich
outflows from NDAFs are possible in the presence of a strong poloidal magnetic
field. However, we find that neutron-rich winds possess a minimum mass-loss
rate that likely precludes simultaneously neutron-rich and ultra-relativistic
(Lorentz factor > 100) NDAF winds accompanying a substantial accretion power.
In contrast, proto-magnetars are capable of producing neutron-rich
long-duration GRB outflows ~10-30 seconds following core bounce for
sub-millisecond rotation periods; such outflows would, however, accompany only
extremely energetic events, in which the GRB + SN energy budget exceeds ~ 4e52
ergs. Neutron-rich highly relativistic outflows may also be produced during
some short-duration GRBs by geometrically thick accretion disks formed from
compact object mergers. The implications for r-process nucleosynthesis, optical
transients due to non-relativistic neutron-rich winds, and Nickel production in
proto-magnetar and NDAF winds are also briefly discussed.

http://arxiv.org/abs/0708.3395

P. Ferrero, S.F. Sanchez, D.A. Kann, S. Klose, J.Greiner, J. Gorosabel, D.H. Hartmann, A.A. Henden, P. Møller, E. Palazzi, A. Rau, B. Stecklum, A.J. Castro-Tirado, J.P.U. Fynbo, J. Hjorth, P. Jakobsson, C. Kouveliotou, N. Masetti, E. Pian, N. Tanvir, R.A.M.J. Wijers

We report early follow-up observations of the error box of the short burst
050813 using the telescopes at Calar Alto and at Observatorio Sierra Nevada
(OSN), followed by deep VLT/FORS2 I-band observations obtained under very good
seeing conditions 5.7 and 11.7 days after the event. Neither a fading
afterglow, nor a rising SN component was found, so the potential GRB host
galaxy has not been identified based on a comparison of the two VLT images
taken at different epoches. We discuss if any of the galaxies present in the
original 10 arcsec XRT error circle could be the host. In any case, the optical
afterglow of GRB 050813 was of very low luminosity. We conclude that all these
properties are consistent with the binary compact merger hypothesis for the
progenitor of GRB 050813.

http://arxiv.org/abs/astro-ph/0610255

H. Bartko, for the MAGIC collaboration

The MAGIC telescope with its 17m diameter mirror is today the largest
operating single-dish Imaging Air Cherenkov Telescope (IACT). It is located on
the Canary Island La Palma, at an altitude of 2200m above sea level, as part of
the Roque de los Muchachos European Northern Observatory. The MAGIC telescope
detects celestial very high energy gamma-radiation in the energy band between
about 50 GeV and 10 TeV. Since the autumn of 2004 MAGIC has been taking data
routinely, observing various objects, like supernova remnants (SNRs), gamma-ray
binaries, Pulsars, Active Galactic Nuclei (AGN) and Gamma-ray Bursts (GRB). We
briefly describe the observational strategy, the procedure implemented for the
data analysis, and discuss the results of observations of Galactic Sources.

http://arxiv.org/abs/0708.2934