astro-grb

Wolfgang Bietenholz

The free photon dispersion relation is a reference quantity for high
precision tests of Lorentz Invariance. We first outline effective approaches to
a conceivable Lorentz Invariance Violation (LIV). Next we address
phenomenological tests based on the propagation of cosmic rays, in particular
in Gamma Ray Bursts (GRBs). As a specific concept, which could imply LIV, we
then focus on field theory in a non-commutative (NC) space, and we present
non-perturbative results for the dispersion relation of the NC photon.

http://arxiv.org/abs/0812.3564

D. Fargion, D. D'Armiento, P. Oliva, F. Manniti

Last and nearest GRB-XRF 080109 has been an exceptional lesson on GRB nature.
After a decade (since 25 April 08) we know that Supernovae may often contain a
Jet. Its persistent activity may shine on axis as a GRBs. Such a persistent,
thin beamed gamma jet may be powered by either a BH (Black Holes) or Pulsars.
Late stages of these jets may loose the SN traces and appear as a short GRB or
a long orphan GRB (depending on jet angular velocity and view angle). XRF are
peripherical viewing of the jets. These precessing and spinning gamma jet are
originated by Inverse Compton and-or Synchrotron Radiation at pulsars or
micro-quasars sources, by ultra-relativistic electrons. These Jets are most
powerful at Supernova birth, blazing, once on axis, to us and flashing GRB
detector. The trembling of the thin jet explains naturally the observed erratic
multi-explosive structure of different GRBs. The jets are precessing (by binary
companion or inner disk asymmetry) and decaying by power on time scales of few
hours, but they keep staying inside the observer cone view only a few seconds
duration times (GRB); the jet is thinner in gamma and wider in X band. This
explain the wider and longer X GRB afterglow duration and the rare presence of
X-ray precursors.

http://arxiv.org/abs/0901.4524

N.P.M. Kuin, W.B. Landsman, M.J. Page, P. Schady, M. Still, A.A. Breeveld, M. De Pasquale, P.J. Brown, M.Carter, C. James, P.A. Curran, A. Cucciara, C. Gronwall, S.T. Holland, E. A. Hoversten, S. Hunsberger, T. Kennedy, S. Koch, H. Lamoureux, F.E. Marshall, S.R. Oates, A. Parsons, D. Palmer, P. Roming, P.J. Smith

We present the earliest ever ultraviolet spectrum of a gamma-ray burst (GRB)
as observed with the Swift-UVOT. The GRB 081203A spectrum was observed for 50
seconds with the UV grism starting 251 seconds after the Swift-BAT trigger when
the GRB was of u ~13.4 mag and still rising to its peak optical brightness. The
UV grism spectrum shows a damped Ly-alpha line, Ly-beta, and the Lyman
continuum break at a redshift z = 2.05 +/- 0.01. A model fit to the Lyman
absorption implies log N(HI) = 22.0 +/- 0.2 cm-2, which is typical for GRB host
galaxies with damped Ly-alpha absorbers. This observation of GRB 081203A
demonstrates that for GRBs brighter than v ~14 mag and with 0.5 < z < 3.5 the
UVOT will be able to provide redshifts, and probe for damped Ly-alpha absorbers
within 4-6 minutes from the time of the Swift-BAT trigger.

http://arxiv.org/abs/0812.2943

A. Tramacere (1 and 2) P. Giommi (3), M. Perri (3), F. Verrecchia (3), G. Tosti (4 and 5) ((1) CIFS - Torino, Italy, (2) SLAC- CA, USA, (3) ASI Science Data Center - Italy (4) Dipartimento di Fisica - Perugia, Italy (5) INFN Perugia - Perugia, Italy)

We present results from a deep spectral analysis of all the Swift
observations of Mrk 421 from April 2006 to July 2006, when it reached its
largest X-ray flux recorded until 2006. The peak flux was about 85 milli-Crab
in the 2.0-10.0 keV band, with the peak energy (Ep) of the spectral energy
distribution (SED) laying often at energies larger than 10 keV. We performed
spectral analysis of the Swift observations investigating the trends of the
spectral parameters in terms of acceleration and energetic features
phenomenologically linked to the SSC model parameters, predicting their effects
in the gamma-ray band, in particular the spectral shape expected in the Fermi
Gamma-ray Space Telescope-LAT band. We confirm that the X-ray spectrum is well
described by a log-parabolic distribution close to Ep, with the peak flux of
the SED (Sp) being correlated with Ep, and Ep anti-correlated with the
curvature parameter (b). During the most energetic flares the UV-to-soft-X-ray
spectral shape requires an electron distribution spectral index s about 2.3.
Present analysis shows that the UV-to-X-ray emission from Mrk 421 is likely to
be originated by a population of electrons that is actually curved, with a low
energy power-law tail. The observed spectral curvature is consistent both with
stochastic acceleration or energy dependent acceleration probability
mechanisms, whereas the power-law slope form XRT-UVOT data is very close to
that inferred from the GRBs X-ray afterglow and in agreement with the universal
first-order relativistic shock acceleration models. This scenario hints that
the magnetic turbulence may play a twofold role: spatial diffusion relevant to
the first order process and momentum diffusion relevant to the second order
process.

http://arxiv.org/abs/0901.4124

Shlomo Dado, Arnon Dar

During the past ten years, the predictions of the cannonball (CB) model of
gamma ray bursts (GRBs) were repeatedly confronted with the mounting data from
space- and ground-based observations of GRBs and their afterglows (AGs). The
two underlying radiation mechanisms of the model, inverse Compton scattering
(ICS) and synchrotron radiation (SR), provided an accurate description of the
prompt and afterglow emission in all of the many well-sampled GRBs that were
studied. Simple as they are, these two mechanisms and the burst environment
were shown to generate the observed rich structure of the GRB light-curves at
all observed frequencies and times.

http://arxiv.org/abs/0901.4260

John Ellis, N.E. Mavromatos, D.V. Nanopoulos

We have used a stringy model of quantum space-time foam to suggest that the
vacuum may exhibit a non-trivial refractive index depending linearly on
gamma-ray energy: eta -1 ~ E_gamma/M_QG1, where M_QG1 is some mass scale
typical of quantum gravity that may be ~ 10^18 GeV: see Phys. Lett. B 665, 412
(2008) and references therein. The MAGIC, HESS and Fermi gamma-ray telescopes
have recently probed the possible existence of such an energy-dependent vacuum
refractive index. All find indications of time-lags for higher-energy photons,
but cannot exclude the possibility that they are due to intrinsic delays at the
sources. However, the MAGIC and HESS observations of time-lags in emissions
from AGNs Mkn 501 and PKS 2155-304 are compatible with each other and a
refractive index depending linearly on the gamma-ray energy, with M_QG1 ~ 10^18
GeV. We combine their results to estimate the time-lag Delta t to be expected
for the highest-energy photon from GRB 080916c measured by the Fermi telescope,
which has an energy ~ 13.2 GeV, assuming the redshift z = 4.2 \pm 0.3 measured
by GROND. In the case of a refractive index depending linearly on the gamma-ray
energy we predict Delta t = 25 \pm 11 s. This is compatible with the time-lag
Delta t <= 16.5 s reported by the Fermi Collaboration, whereas the time-lag
would be negligible in the case of a refractive index depending quadratically
on the gamma-ray energy. We suggest a strategy for future observations that
could distinguish between a quantum-gravitational effect and other
interpretations of the time-lags observed by the MAGIC, HESS and Fermi
gamma-ray telescopes.

http://arxiv.org/abs/0901.4052

N. Bucciantini (1), E. Quataert (1), B.D. Metzger (1), T.A. Thompson (2), J. Arons (1), L. Del Zanna (3) ((1) U.C. Berkeley, (2) Ohio State Univ., (3) Univ. di Firenze)

We use ideal axisymmetric relativistic magnetohydrodynamic simulations to
calculate the spindown of a newly formed millisecond, B ~ 10^{15} G, magnetar
and its interaction with the surrounding stellar envelope during a
core-collapse supernova (SN) explosion. The mass, angular momentum, and
rotational energy lost by the neutron star are determined self-consistently
given the thermal properties of the cooling neutron star's atmosphere and the
wind's interaction with the surrounding star. The magnetar drives a
relativistic magnetized wind into a cavity created by the outgoing SN shock.
For high spindown powers (~ 10^{51}-10^{52} ergs/s), the magnetar wind is
super-fast at almost all latitudes, while for lower spindown powers (~ 10^{50}
erg/s), the wind is sub-fast but still super-Alfvenic. In all cases, the rates
at which the neutron star loses mass, angular momentum, and energy are very
similar to the corresponding free wind values (<~ 30% differences), in spite of
the causal contact between the neutron star and the stellar envelope. In
addition, in all cases that we consider, the magnetar drives a collimated
(~5-10 deg.) relativistic jet out along the rotation axis of the star. Nearly
all of the spindown power of the neutron star escapes via this polar jet,
rather than being transferred to the more spherical SN explosion. The
properties of this relativistic jet and its expected late-time evolution in the
magnetar model are broadly consistent with observations of long duration
gamma-ray bursts (GRBs) and their associated broad-lined Type Ic SN.

http://arxiv.org/abs/0901.3801

P. D'Avanzo, D. Malesani, S. Covino, S. Piranomonte, A. Grazian, D. Fugazza, R. Margutti, V. D'Elia, L. A. Antonelli, S. Campana, G. Chincarini, M. Della Valle, F. Fiore, P. Goldoni, J. Mao, R. Perna, R. Salvaterra, L. Stella, G. Stratta, G. Tagliaferri

Short GRBs are commonly thought to originate from the merging of double
compact object binaries but direct evidence for this scenario is still missing.
Optical observations of short GRBs allow us to measure redshifts, firmly
identify host galaxies, characterize their properties, and accurately localize
GRBs within them. Multiwavelength observations of GRB afterglows provide useful
information on the emission mechanisms at work. These are all key issues that
allow one to discriminate among different models of these elusive events. We
carried out photometric observations of the short/hard GRB 051227, GRB 061006,
and GRB 071227 with the ESO-VLT starting from several hours after the explosion
down to the host galaxy level several days later. For GRB 061006 and GRB 071227
we also obtained spectroscopic observations of the host galaxy. We compared the
results obtained from our optical observations with the available X-ray data of
these bursts. For all the three above bursts, we discovered optical afterglows
and firmly identified their host galaxies. About half a day after the burst,
the optical afterglows of GRB 051227 and GRB 061006 present a decay
significatly steeper than in the X-rays. In the case of GRB 051227, the optical
decay is so steep that it likely indicates different emission mechanisms in the
two wavelengths ranges. The three hosts are blue, star forming galaxies at
moderate redshifts and with metallicities comparable to the Solar one. The
projected offsets of the optical afterglows from their host galaxies centers
span a wide range, but all afterglows lie within the light of their hosts and
present evidence for local absorption in their X-ray spectra. We discuss our
findings in light of the current models of short GRB progenitors.

http://arxiv.org/abs/0901.4038

John Ellis, N.E. Mavromatos, D.V. Nanopoulos

We have used a stringy model of quantum space-time foam to suggest that the
vacuum may exhibit a non-trivial refractive index depending linearly on
gamma-ray energy: eta -1 ~ E_gamma/M_QG1, where M_QG1 is some mass scale
typical of quantum gravity that may be ~ 10^18 GeV: see Phys. Lett. B 665, 412
(2008) and references therein. The MAGIC, HESS and Fermi gamma-ray telescopes
have recently probed the possible existence of such an energy-dependent vacuum
refractive index. All find indications of time-lags for higher-energy photons,
but cannot exclude the possibility that they are due to intrinsic delays at the
sources. However, the MAGIC and HESS observations of time-lags in emissions
from AGNs Mkn 501 and PKS 2155-304 are compatible with each other and a
refractive index depending linearly on the gamma-ray energy, with M_QG1 ~ 10^18
GeV. We combine their results to estimate the time-lag Delta t to be expected
for the highest-energy photon from GRB 080916c measured by the Fermi telescope,
which has an energy ~ 13.2 GeV, assuming the redshift z = 4.2 \pm 0.3 measured
by GROND. In the case of a refractive index depending linearly on the gamma-ray
energy we predict Delta t = 25 \pm 11 s. This is compatible with the time-lag
Delta t <= 16.5 s reported by the Fermi Collaboration, whereas the time-lag
would be negligible in the case of a refractive index depending quadratically
on the gamma-ray energy. We suggest a strategy for future observations that
could distinguish between a quantum-gravitational effect and other
interpretations of the time-lags observed by the MAGIC, HESS and Fermi
gamma-ray telescopes.

http://arxiv.org/abs/0901.4052

Keiichi Maeda, Nozomu Tominaga

Theoretically expected natures of a supernova driven by a wind/jet are
discussed. Approximate analytical formulations are derived to clarify basic
physical processes involved in the wind/jet-driven explosions, and it is shown
that the explosion properties are characterized by the energy injection rate
(Edot_iso) and the mass injection rate (Mdot_iso). To explain observations of
SN 1998bw associated with Gamma-Ray Burst (GRB) 980425, the following
conditions are required: Edot_iso Mdot_iso > ~ 10^{51} erg M_sun s^{-2} and
Edot_iso > ~ 2 x 10^{52} erg s^{-1} (if the wind Lorentz factor Gamma_w ~ 1) or
Edot_iso > ~ 7 x 10^{52} erg s^{-1} (if Gamma_w >> 1). In SN 1998bw, 56Ni (~
0.4M_sun) is probably produced in the shocked stellar mantle, not in the wind.
The expected natures of SNe, e.g., ejected 56Ni masses and ejecta masses, vary
depending on Edot_iso and Mdot_iso. The sequence of the SN properties from high
Edot_iso and Mdot_iso to low Edot_iso and Mdot_iso is the following: SN
1998bw-like — intermediate case — low mass ejecta (< ~ 1M_sun$) where 56Ni is
from the wind — whole collapse. This diversity may explain the diversity of
supernovae associated with GRBs. Our result can be used to constrain natures of
the wind/jet, which is linked to the central engine of GRBs, by studying
properties of the associated supernovae.

http://arxiv.org/abs/0901.0410