astro-grb

Daniel A. Perley, A. N. Morgan, A. Updike, F. Yuan, C. W. Akerlof, A. A. Miller, J. S. Bloom, S. B. Cenko, W. Li, A. V. Filippenko, J. X. Prochaska, D. A. Kann, N. R. Butler, P. Christian, D. H. Hartmann, P. Milne, E. S. Rykoff, W. Rujopakarn, J. C. Wheeler, G. G. Williams

We present early-time optical through infrared photometry of the bright
gamma-ray burst GRB 080607, starting only 6 s following the initial trigger in
the rest frame. Complemented by our previously published spectroscopy, this
high-quality photometric dataset allows us to solve for the extinction
properties of the redshift 3.036 sightline, giving perhaps the most detailed
information on the ultraviolet continuum absorption properties of any sightline
outside our Local Group to date. The extinction properties are not adequately
modeled by any ordinary extinction template (including the average Milky Way,
Large Magellanic Cloud, and Small Magellanic Cloud curves), partially because
the 2175-Angstrom feature (while present) is weaker by about a factor of two
than when seen under similar circumstances locally. However, the spectral
energy distribution is exquisitely fitted by the more general Fitzpatrick &
Massa (1990) parameterization of Local-Group extinction, putting it in the same
family as some peculiar Milky Way extinction curves. After correcting for this
(considerable, A_V = 3.3 +/- 0.4 mag) extinction, GRB 080607 is revealed to
have been among the most optically luminous events ever observed, comparable to
the naked-eye burst GRB 080319B. Its early peak time (t_rest < 6 s) indicates a
high initial Lorentz factor (Gamma > 600), while the extreme luminosity may be
explained in part by a large circumburst density. Only because of its early
high luminosity could the afterglow of GRB 080607 be studied in such detail in
spite of the large attenuation and great distance, making this burst an
excellent prototype for the understanding of other highly obscured
extragalactic objects, and of the class of “dark” GRBs in particular.

http://arxiv.org/abs/1009.0004

R. Margutti, G. Bernardini, R. Barniol Duran, C. Guidorzi, R. F. Shen, G. Chincarini

Gamma-ray burst X-ray flares are believed to mark the late time activity of
the central engine. We compute the temporal evolution of the average flare
luminosity $< L >$ in the common rest frame energy band of 44 GRBs taken from
the large \emph{Swift} 5-years data base. Our work highlights the importance of
a proper consideration of the threshold of detection of flares against the
contemporaneous continuous X-ray emission. In the time interval $30
\rm{s}<t<1000\,\rm{s}$ we find $< L >\propto t^{-2.7\pm 0.1}$; this implies
that the flare isotropic energy scaling is $E_{\rm{iso,flare}}\propto
t^{-1.7}$. The decay of the continuum underlying the flare emission closely
tracks the average flare luminosity evolution, with a typical flare to
steep-decay luminosity ratio which is $L_{\rm{flare}}/L_{\rm{steep}}=4.7$: this
suggests that flares and continuum emission are deeply related to one another.
We infer on the progenitor properties considering different models. According
to the hyper-accreting black hole scenario, the average flare luminosity
scaling can be obtained in the case of rapid accretion ($t_{\rm{acc}}\ll t$) or
when the last $\sim 0.5 M_{\sun}$ of the original $14 M_{\sun}$ progenitor star
are accreted. Alternatively, the steep $\propto t^{-2.7}$ behaviour could be
triggered by a rapid outward expansion of an accretion shock in the material
feeding a convective disk. If instead we assume the engine to be a rapidly
spinning magnetar, then its rotational energy can be extracted to power a jet
whose luminosity is likely to be between the monopole ($L\propto e^{-2t}$) and
dipole ($L\propto t^{-2}$) cases. In both scenarios we suggest the variability,
which is the main signature of the flaring activity, to be established as a
consequence of different kinds of instabilities.

http://arxiv.org/abs/1009.0172

Junichi Aoi, Kohta Murase, Keitaro Takahashi, Kunihito Ioka, Shigehiro Nagataki

We revisit the high-energy spectral cutoff originating from the
electron-positron pair creation in the prompt phase of gamma-ray bursts (GRBs)
with numerical and analytical calculations. We show that the conventional
exponential and/or broken power law cutoff should be drastically modified to a
shallower broken power-law in practical observations that integrate emissions
from different internal shocks. Since the steepening is tiny for observations,
this “smearing” effect can generally reduce the previous estimates of the
Lorentz factor of the GRB outflows. We apply our formulation to GRB 080916C,
recently detected by the Large Area Telescope detector on the Fermi satellite,
and find that the minimum Lorentz factor can be ~600 (or even smaller values),
which is below but consistent with the previous result of ~900. Observing the
steepening energy (so-called “pair-break energy”) is crucial to diagnose the
Lorentz factor and/or the emission site in the future observations, especially
current and future Cherenkov telescopes such as MAGIC, VERITAS, and CTA.

http://arxiv.org/abs/0904.4878

J. M. Castro Cerón (1,2), M. J. Michałowski (1,3), J. Hjorth (1), D. Malesani (1), J. Gorosabel (4), D. Watson (1), J. P. U. Fynbo (1), M. Morales Calderón (5); ((1) Dark Cosmology Centre (NBI) Copenhagen; (2) Herschel Science Centre (ESA/ESAC) Madrid; (3) Institute for Astronomy (ROE) Edinburgh; (4) Instituto de Astrofísica de Andalucía (CSIC) Granada; (5) Spitzer Science Centre (Caltech) Pasadena)

We analyse Spitzer images of 30 long-duration gamma-ray burst (GRB) host
galaxies. We estimate their total stellar masses (M*) based on the rest-frame
K-band luminosities (L_Krest) and constrain their star formation rates (SFRs,
not corrected for dust extinction) based on the rest-frame UV continua.
Further, we compute a mean M*/L_Krest = 0.45 Msun/Lsun. We find that the hosts
are low M*, star-forming systems. The median M* in our sample (<M*> = 10^9.7
Msun) is lower than that of “field” galaxies (e.g., Gemini Deep Deep Survey).
The range spanned by M* is 10^7 Msun < M* < 10^11 Msun, while the range spanned
by the dust-uncorrected UV SFR is 10^-2 Msun yr^-1 < SFR < 10 Msun yr^-1. There
is no evidence for intrinsic evolution in the distribution of M* with redshift.
We show that extinction by dust must be present in at least 25% of the GRB
hosts in our sample and suggest that this is a way to reconcile our finding of
a relatively lower UV-based, specific SFR (PHI = SFR/M*) with previous claims
that GRBs have some of the highest PHI values. We also examine the effect that
the inability to resolve the star-forming regions in the hosts has on PHI.

http://arxiv.org/abs/0803.2235

The Fermi LAT, the GBM Collaboration: A. A. Abdo, et al.

The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the “baseline” model of Stecker et al.
(2006) can be ruled out with high confidence.

http://arxiv.org/abs/1005.0996

Z. Mao, Yun-Wei Yu, Z. G. Dai, C. M. Pi, X. P. Zheng

Context: Swift observations suggest that the X-ray afterglow emission of some
gamma-ray bursts (GRB) may have internal origins, and the conventional external
shock (ES) cannot be the exclusive source of the afterglow emission. Aims: If
the central compact objects of some GRBs are millisecond magentars, the
magnetar winds could play an important role in the (internal) X-ray afterglow
emission, which is our focus here. Methods: The dynamics and the synchrotron
radiation of the termination shock (TS) of the magmnetar winds, as well as the
simultaneous GRB ES, are investigated by considering the magnetization of the
winds. Results: As a result of the competition between the emission of the wind
TS and the GRB ES, two basic types of X-ray afterglows are predicted, i.e., the
TS-dominated and the ES-dominated types. Moreover, our results also show that
both of the two types of afterglows have a shallow-decay phase and a
normal-decay one, as observed by the \textit{Swift} satellite. This indicates
that some observed X-ray afterglows could be (internally) produced by the
magnetar winds, but not necessarily GRB ESs.

http://arxiv.org/abs/1008.4899

Men-Quan Liu

The steady equilibrium conditions for a mixed gas of neutrons, protons,
electrons, positrons and radiation field (abbreviated as $npe^{\pm}$ gas)
with/without external neutrino flux are investigated, and a general chemical
potential equilibrium equation $\mu_n=\mu_p+C\mu_e$ is obtained to describe the
steady equilibrium at high temperatures ($T>10^9$K). An analytic fitting
formula of coefficient $C$ is presented for the sake of simplicity as the
neutrino and antineutrino are transparent. It is a simple method to estimate
the electron fraction for the steady equilibrium $npe^{\pm}$ gas that using the
corresponding equilibrium condition. As an example, we apply this method to the
GRB accretion disk and approve the composition in the inner region is
approximate equilibrium as the accretion rate is low. For the case with
external neutrino flux, we calculate the initial electron fraction of
neutrino-driven wind from proto-neutron star model M15-l1-r1. The results show
that the improved equilibrium condition makes the electron fraction decrease
significantly than the case $\mu_n=\mu_p+\mu_e$ when the time is less than 5
seconds post bounce, which may be useful for the r-process nucleosynthesis

http://arxiv.org/abs/1008.5025

Todd A. Thompson, Brian D. Metzger, Niccolò Bucciantini

The millisecond proto-magnetar model for the central engine of long-duration
gamma-ray bursts is briefly reviewed. Limitations and uncertainties in the
model are highlighted. A short discussion of the maximum energy, maximum
duration, radiative efficiency, jet formation mechanism, late-time energy
injection, and (non-)association with supernovae of millisecond
magnetar-powered GRBs is provided.

http://arxiv.org/abs/1008.5138

Ivan Zalamea, Andrei M. Beloborodov (Columbia University)

Hyper-accretion discs around black holes emit copious neutrinos and
anti-neutrinos. A fraction of the emitted neutrinos convert to
electron-positron plasma above the disc through the annihilation reaction
$\nu\bar\nu\to e^+e^-$. This process may drive relativistic jets associated
with GRB explosions. We calculate the efficiency of energy deposition by
neutrinos. Our calculation is fully relativistic and based on a
geodesic-tracing method. We find that the efficiency of neutrino heating is a
well-defined function of (i) accretion rate and (ii) spin of the black hole. It
is practically independent of the details of neutrino transport in the opaque
zone of the disc. The results help identify accretion discs whose neutrino
emission can power GRBs.

http://arxiv.org/abs/1003.0710

Davide Lazzati (NCSU), Chris H. Blackwell (NCSU), Bran J. Morsony (UW Madison), Mitch C. Begelman (UC Boulder)

We present a numerical simulation of a gamma-ray burst jet from a
long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine
is active for 6000 s with a luminosity that decays in time as a power-law with
index -5/3. Even though there is no short time-scale variability in the engine
luminosity, we find that the jet's kinetic luminosity outside the progenitor
star is characterized by fluctuations with relatively short time scale. We
analyze the temporal characteristics of those fluctuations and we find that
they are consistent with the properties of observed flares in the X-ray
afterglows. The peak to continuum flux ratio of the flares in the simulation is
consistent with some, but not all, the observed flares. We propose that
propagation instabilities, rather than variability in the engine luminosity,
may be responsible for the X-ray flares with moderate contrast. Strong flares
such as the one detected in GRB 050502B, instead, cannot be reproduced in this
model and require strong variability in the engine activity.

http://arxiv.org/abs/1008.4364