astro-grb

S. Gallerani, A. Ferrara, X. Fan, Choudhury T. Roy, R. Salvaterra

We provide measurements of the neutral hydrogen fraction xHI at z~6, by
comparing semi-analytical models of the Lyalpha forest with observations of
high-z quasars and Gamma Ray Bursts absorption spectra. We analyze the
transmitted flux in a sample of 17 QSOs spectra at 5.74<zem<6.42 studying
separately the narrow transmission windows (peaks) and the wide dark portions
(gaps) in the observed absorption spectra. By comparing the statistics of these
spectral features with our models, we conclude that xHI evolves smoothly from
10^{-4.4} at z=5.3 to 10^{-4.2} at z=5.6, with a robust upper limit xHI<0.36 at
z=6.3. We show the results of the first-ever detected transverse proximity
effect in the HI Lyalpha forest, produced by the HII region of the faint quasar
RD J1148+5253 at z=5.70 intervening along the LOS of SDSS J1148+5251 at z=6.42.
Moreover, we propose a novel method to study cosmic reionization using
absorption line spectra of high-redshift GRBs afterglows. We show that the time
evolution and the statistics of gaps in the observed spectra represent
exquisite tools to discriminate among different reionization models. By
applying our methods to GRB 050904 detected at z=6.29, we show that the
observation of this burst provides strong indications of a highly ionized
intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction
xHI=7.0\pm 4.0\times 10^{-4} along that line of sight.

http://arxiv.org/abs/0801.4674

Francisco Virgili, Enwei Liang, Bing Zhang

The intriguing observations of {Swift}/BAT GRB 060218 and CGRO/BATSE burst
980425, both with much lower luminosity and redshift compared to other observed
bursts, lead naturally to the question whether these low-luminosity (LL) bursts
constitute a separate population from high-luminosity (HL) bursts. Utilizing
Monte Carlo simulations we compare various single-component luminosity function
(LF)models (single power law or broken power law) with the two-component
luminosity function model proposed by Liang et al. Using various criteria, we
demonstrate that the single-component LF models have great difficulty in
simultaneously reproducing both the high local LL-GRB rate and the oberved
distributions of redshift, luminosity, and $\log N-\log P$ for HL-GRBs. We
argue that the two-component LF model is necessary, and we use the observed
BATSE and Swift $\log N-\log P$ distributions to add constrains to the LL and
HL-LF parameters. The LL-LF can be modeled by a smoothed, broken power law with
a break at around $10^{47}$ erg s$^{-1}$, dropping steeply above this
luminosity. The local rate of LL-GRBs is $\sim$ 100 Gpc$^{-3}$ yr$^{-1}$ at the
break luminosity, much larger than that of HL-GRBs. The recently discovered
peculiar X-ray transient XRF 080109/SN 2008D strengthens this conclusion, and
requires that the LL-population LF extends further down in luminosity with a
probably even higher local rate at lower luminosities.

http://arxiv.org/abs/0801.4751

J. L. Racusin (1), E.-W. Liang (2,3), D. N. Burrows (1), D. C. Morris (1), B. B. Zhang (2), B. Zhang (2) ((1) Pennsylvania State University, (2) University of Nevada, Las Vegas, (3) Guangxi University)

We present a systematic survey of the temporal and spectral properties of all
GRB X-ray afterglows observed by Swift-XRT between January 2005 and July 2007.
We have constructed a catalog of all light curves and spectra and investigate
the physical origin of each afterglow segment in the framework of the forward
shock models by comparing the data with the closure relations. We search for
possible jet-like breaks in the lightcurves and try to explain some of the
“missing” X-ray jet breaks in the lightcurves.

http://arxiv.org/abs/0801.4749

Andre Maeder, Georges Meynet, Sylvia Ekstrom, Raphael Hirschi, Cyril Georgy

Some useful developments in the model physics are briefly presented, followed
by model results on chemical enrichments and WR stars. We discuss the expected
rotation velocities of WR stars. We emphasize that the (C+O)/He ratio is a
better chemical indicator of evolution for WC stars than the C/He ratios. With
or without rotation, at a given luminosity the (C+O)/He ratios should be higher
in regions of lower metallicity Z. Also, for a given (C+O)/He ratio the WC
stars in lower Z regions have higher luminosities. The WO stars, which are
likely the progenitors of supernovae SNIc and of some GRBs, should
preferentially be found in regions of low Z and be the descendants of very high
initial masses. Finally, we emphasize the physical reasons why massive rotating
low Z stars may also experience heavy mass loss

http://arxiv.org/abs/0801.4712

Philip Chang (1), Anatoly Spitkovsky (2), Jonathan Arons (1) ((1) Univ. of California, Berkeley (2) Princeton University)

We study the long term evolution of magnetic fields generated by an initially
unmagnetized collisionless relativistic $e^+e^-$ shock. Our 2D particle-in-cell
numerical simulations show that downstream of such a Weibel-mediated shock,
particle distributions are approximately isotropic, relativistic Maxwellians,
and the magnetic turbulence is highly intermittent spatially, nonpropagating,
and decaying. Using linear kinetic theory, we find a simple analytic form for
these damping rates. Our theory predicts that overall magnetic energy decays
like $(\omega_p t)^{-q}$ with $q \sim 1$, which compares favorably with
simulations, but predicts overly rapid damping of short wavelength modes.
Magnetic trapping of particles within the magnetic structures may be the origin
of this discrepancy. We conclude that initially unmagnetized relativistic
shocks in electron-positron plasmas are unable to form persistent downstream
magnetic fields. These results put interesting constraints on synchrotron
models for the prompt and afterglow emission from GRBs.

http://arxiv.org/abs/0801.4583

Jason Harris, Dennis Zaritsky

We present a detailed reconstruction of the star-formation history of the
Constellation III region in the Large Magellanic Cloud, to constrain the
formation mechanism of this enigmatic feature. Star formation in Constellation
III seems to have taken place during two distinct epochs: there is the 8-15 Myr
epoch that had previously been recognized, but we also see strong evidence for
a separate “burst” of star formation 25-30 Myr ago. The “super-supernova” or
GRB blast wave model for the formation of Constellation III is difficult to
reconcile with such an extended, two-epoch star formation history, because the
shock wave should have induced star formation throughout the structure
simultaneously, and any unconsumed gas would quickly be dissipated, leaving
nothing from which to form a subsequent burst of activity. We propose a “truly
stochastic” self-propagating star formation model, distinct from the canonical
model in which star formation proceeds in a radially-directed wave from the
center of Constellation III to its perimeter. As others have noted, and we now
confirm, the bulk age gradients demanded by such a model are simply not present
in Constellation III. In our scenario, the prestellar gas is somehow pushed
into these large-scale arc structures, without simultaneously triggering
immediate and violent star formation throughout the structure. Rather, star
formation proceeds in the arc according to the local physical conditions of the
gas. Self-propagating star formation is certainly possible, but in a truly
stochastic manner, without a directed, large scale pattern.

http://arxiv.org/abs/0712.2077

P. A. Evans, A.P. Beardmore, M.R. Goad, J.P. Osborne (U. Leicester), D.N. Burrows (Penn State University), N. Gehrels (NASA/GSFC)

Since GRBs fade rapidly, it is important to publish accurate, precise
positions at early times. For Swift-detected bursts, the best promptly
available position is most commonly the X-ray Telescope (XRT) position. We
present two processes, developed by the Swift team at Leicester, which are now
routinely used to improve the precision and accuracy of the XRT positions
reported by the Swift team. Both methods, which are fully automated, make use
of a PSF-fitting approach which accounts for the bad columns on the CCD. The
first method yields positions with 90% error radii <4.4″ 90% of the time,
within 10–20 minutes of the trigger. The second method astrometrically
corrects the position using UVOT field stars and the known mapping between the
XRT and UVOT detectors, yielding enhanced positions with 90% error radii of
<2.8″ 90% of the time, usually ~2 hours after the trigger.

http://arxiv.org/abs/0801.4462

K.-I. Nishikawa (NSSTC/Uah), Y. Mizuno (NASA/MSFC/NSSTC), G. J. Fishiman (NASA/MSFC), P. Hardee (UA)

Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray
bursts (GRBs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations using injected relativistic electron-ion
(electro-positron) jets show that acceleration occurs within the downstream
jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
other two-streaming instability, and the Weibel instability) created in the
shocks are responsible for particle (electron, positron, and ion) acceleration.
The simulation results show that the Weibel instability is responsible for
generating and amplifying highly nonuniform, small-scale magnetic fields. These
magnetic fields contribute to the electron's transverse deflection behind the
jet head. The “jitter'' radiation from deflected electrons has different
properties than synchrotron radiation which assumes a uniform magnetic field.
This jitter radiation may be important to understanding the complex time
evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and
supernova remnants.

http://arxiv.org/abs/0801.4390

Dong Xu, Yuan-Chuan Zou, Yi-Zhong Fan

We analyze the hitherto available space-based X-ray data as well as
ground-based optical data of the X-ray transient 080109/SN2008D. From the data
we suggest that (i) The initial transient (\lesssim 800 sec) is attributed to
the reverse shock emission of a mildly relativistic (\Gamma \sim a few) outflow
stalled by the dense stellar wind. (ii) The subsequent X-ray afterglow
(\lesssim 2\times 10^4 sec) can be ascribed to the forward shock emission of
the outflow, with a kinetic energy \sim 10^{46} erg, when sweeping up the
stellar wind medium. (iii) The late X-ray flattening (\gtrsim 2\times 10^4$
sec) is powered by the fastest non-decelerated component of SN2008D's ejecta.
(iv) The local event rate of X-ray transient has a lower limit of \sim
1.6\times 10^4 yr^{-1} Gpc^{-3}, indicating a vast majority of X-ray transients
have a wide opening angle of \gtrsim 100 degree. The off-axis viewing model is
less likely. (v) Transient 080109/SN2008D may lead to a continuum from GRB-SN
to under-luminous GRB-/XRF-SN to X-ray transient-SN and to ordinary Ibc SN (if
not every Ibc SN has a relativistic jet), as shown in Figure 2 of this Letter.

http://arxiv.org/abs/0801.4325

Pijushpani Bhattacharjee, Sovan Chakraborty, Srirupa Das Gupta, Kamales Kar (Saha Inst., Kolkata, India)

We derive upper limits on the ratio $f_{GRB/CCSN}(z) \equiv
R_{GRB}(z)/R_{CCSN}(z) \equiv f_{GRB/CCSN}(0)(1+z)^\alpha$, the ratio of the
rate, $R_{GRB}$, of long-duration Gamma Ray Bursts (GRBs) to the rate,
$R_{CCSN}$, of core-collapse supernovae (CCSNe) in the Universe ($z$ being the
cosmological redshift and $\alpha\geq 0$), by using the upper limit on the
diffuse TeV–PeV neutrino background given by the AMANDA-II experiment in the
South Pole, under the assumption that GRBs are sources of TeV–PeV neutrinos
produced from decay of charged pions produced in $p\gamma$ interaction of
protons accelerated to ultrahigh energies at internal shocks within GRB jets.
For the assumed “concordance model'' of cosmic star formation rate, $R_{SF}$,
with $R_{CCSN}(z) \propto R_{SF}(z)$, our conservative upper limits are
$f_{GRB/CCSN}(0)\leq 5.0\times10^{-3}$ for $\alpha=0$, and $f_{GRB/CCSN}(0)\leq
1.1\times10^{-3}$ for $\alpha=2$, for example. These limits are already
comparable to (and, for $\alpha\geq 1$ already more restrictive than) the
current upper limit on this ratio inferred from other astronomical
considerations, thus providing a useful independent probe of and constraint on
the CCSN-GRB connection. Non-detection of a diffuse TeV–PeV neutrino
background by the up-coming IceCube detector in the South pole after three
years of operation, for example, will bring down the upper limit on
$f_{GRB/CCSN}(0)$ to below few $\times10^{-5}$ level, while a detection will
confirm the hypothesis of proton acceleration to ultrahigh energies in GRBs and
will potentially also yield the true rate of occurrence of these events in the
Universe.

http://arxiv.org/abs/0710.5922