astro-grb

Pablo Cerdá-Durán, José A. Font, Luis Antón, Ewald Müller

We present a new numerical code which solves the general relativistic
magneto-hydrodynamics (GRMHD) equations coupled to the Einstein equations for
the evolution of a dynamical spacetime. This code has been developed with the
main objective of studying astrophysical scenarios in which both, high magnetic
fields and strong gravitational fields appear, such as the magneto-rotational
collapse of stellar cores, the collapsar model of GRBs, and the evolution of
neutron stars. The code is based on an existing and thoroughly tested purely
hydrodynamics code and on its extension to accommodate weakly magnetized fluids
(passive magnetic field approximation). The numerical code we present here is
based on high-resolution shock-capturing schemes to solve the GRMHD equations
together with the flux constraint transport method to ensure the solenoidal
condition of the magnetic field. Since the astrophysical applications envisaged
do not deviate much from spherical symmetry, the conformal flatness condition
approximation is used for the formulation of the Einstein equations. In
addition, the code can handle several equations of state, from simple
analytical expressions to microphysical tabulated ones. In this paper we
present stringent tests of our new GRMHD numerical code, which show its ability
to handle all aspects appearing in the astrophysical scenarios for which the
code is intended, namely relativistic shocks, highly magnetized fluids, and
equilibrium configurations of magnetized neutron stars. As an application,
magneto-rotational core collapse simulations of a realistic progenitor are
presented, comparing the results with our previous finding in the passive
magnetic field approximation.

http://arxiv.org/abs/0804.4572

Shi Qi, Fa-Yin Wang, Tan Lu

The behavior of the dark energy equation of state (EOS) is crucial in
distinguishing different cosmological models. With a model independent
approach, we constrain the possible evolution of the dark energy EOS. Gamma-ray
bursts (GRBs) of redshifts up to $z>6$ are used, in addition to type Ia
supernovae (SNe Ia). We separate the redshifts into 4 bins and assume a
constant EOS parameter for dark energy in each bin. The EOS parameters are
decorrelated by diagonalizing the covariance matrix. And the evolution of dark
energy is estimated out of the uncorrelated EOS parameters. By including GRB
luminosity data, we significantly reduce the confidence interval of the
uncorrelated EOS parameter whose contribution mostly comes from the redshift
bin of $0.5<z<1.8$. At high redshift where we only have GRBs, the constraints
on the dark energy EOS are still very weak. However, we can see an obvious cut
at about zero in the probability plot of the EOS parameter, from which we can
infer that the ratio of dark energy to matter most probably continues to
decrease beyond redshift 1.8. We carried out analyses with and without
including the latest BAO measurements, which themselves favor a dark energy EOS
of $w<-1$. If they are included, the results show some evidence of an evolving
dark energy EOS. If not included, however, the results are consistent with the
cosmological constant within $1 \sigma$ for redshift $0<z \lesssim 0.5$ and $2
\sigma$ for $0.5 \lesssim z<1.8$.

http://arxiv.org/abs/0803.4304

S. Covino, P. D'Avanzo, A. Klotz, D.A. Perley, L. Amati, S. Campana, G. Chincarini, A. Cucchiara, V. D'Elia, D. Guetta, C. Guidorzi, D.A. Kann, A. Küpcü Yoldaş, K. Misra, G. Olofsson, G. Tagliaferri, L.A. Antonelli, E. Berger, J.S. Bloom, M. Böer, C. Clemens, F. D'Alessio, M. Della Valle, S. di Serego Alighieri, A.V. Filippenko, R.J. Foley, D.B. Fox, D. Fugazza, J. Fynbo, B. Gendre, P. Goldoni, J. Greiner, D. Kocevksi, E. Maiorano, N. Masetti, E. Meurs, M. Modjaz, E. Molinari, A. Moretti, E. Palazzi, S.B. Pandey, S. Piranomonte, D. Poznanski, N. Primak, P. Romano, E. Rossi, R. Roy, J.M. Silverman, L. Stella, G. Stratta, V. Testa, S.D. Vergani, F. Vitali, F. Zerbi,

We present and discuss the results of an extensive observational campaign
devoted to GRB071010A, a long-duration gamma-ray burst detected by the Swift
satellite. This event was followed for almost a month in the
optical/near-infrared (NIR) with various telescopes starting from about 2min
after the high-energy event. Swift-XRT observations started only later at about
0.4d. The light-curve evolution allows us to single out an initial rising phase
with a maximum at about 7min, possibly the afterglow onset in the context of
the standard fireball model, which is then followed by a smooth decay
interrupted by a sharp rebrightening at about 0.6d. The rebrightening was
visible in both the optical/NIR and X-rays and can be interpreted as an episode
of discrete energy injection, although various alternatives are possible. A
steepening of the afterglow light curve is recorded at about 1d. The entire
evolution of the optical/NIR afterglow is consistent with being achromatic.
This could be one of the few identified GRB afterglows with an achromatic break
in the X-ray through the optical/NIR bands. Polarimetry was also obtained at
about 1d, just after the rebrightening and almost coincident with the
steepening. This provided a fairly tight upper limit of 0.9% for the
polarized-flux fraction.

http://arxiv.org/abs/0804.4367

F. E. Bauer (1), V. V. Dwarkadas (2), W. N. Brandt (3), S. Immler (4), S. Smartt (5), N. Bartel (6), M. F. Bietenholz (6 and 7) ((1) Columbia, (2) Chicago, (3) Penn State, (4) GSFC, (5) Queen's Belfast, (6) York, (7) Hartebeesthoek Radio Observatory)

We report on new VLT optical spectroscopic and multi-wavelength archival
observations of SN1996cr, a previously identified ULX known as Circinus Galaxy
X-2. Our optical spectrum confirms SN1996cr as a bona fide type IIn SN, while
archival imaging isolates its explosion date to between 1995-02-28 and
1996-03-16. SN1996cr is one of the closest SNe (~3.8 Mpc) in the last several
decades and in terms of flux ranks among the brightest radio and X-ray SNe ever
detected. The wealth of optical, X-ray, and radio observations that exist for
this source provide relatively detailed constraints on its post-explosion
expansion and progenitor history, including an preliminary angular size
constaint from VLBI. The archival X-ray and radio data imply that the
progenitor of SN1996cr evacuated a large cavity just prior to exploding: the
blast wave likely expanded for ~1-2 yrs before eventually striking the dense
circumstellar material which surrounds SN1996cr. The X-ray and radio emission,
which trace the progenitor mass-loss rate, have respectively risen by a factor
of ~2 and remained roughly constant over the past 7 yr. This behavior is
reminiscent of the late rise of SN1987A, but 1000 times more luminous and much
more rapid to onset. Complex Oxygen line emission in the optical spectrum
further hints at a possible concentric shell or ring-like structure. The
discovery of SN1996cr suggests that a substantial fraction of the closest SNe
observed in the last several decades have occurred in wind-blown bubbles. An
Interplanetary Network position allows us to reject a tentative GRB association
with BATSE 4B960202. [Abridged]

http://arxiv.org/abs/0804.3597

Hasan Yuksel, Matthew D. Kistler, John F. Beacom (Ohio State University), Andrew M. Hopkins (University of Sydney)

While the high-z frontier of star formation rate (SFR) studies has advanced
rapidly, direct measurements beyond z ~ 4 remain difficult, as shown by
significant disagreements among different results. Gamma-ray bursts, owing to
their brightness and association with massive stars, offer hope of clarifying
this situation, provided that the GRB rate can be properly related to the SFR.
The Swift GRB data reveal an increasing evolution in the GRB rate relative to
the SFR at intermediate z; taking this into account, we use the highest-z GRB
data to make a new determination of the SFR at z = 4-7. Our results exceed the
lowest direct SFR measurements, and imply that no steep drop exists in the SFR
up to at least z ~ 6.5. We discuss the implications of our result for cosmic
reionization, the efficiency of the universe in producing stellar-mass black
holes, and “GRB feedback'' in star-forming hosts.

http://arxiv.org/abs/0804.4008

S. Desai (Penn State), E.O. Kahya (Florida), R.P. Woodard (Florida)

We discuss the implications for gravity wave detectors of a class of modified
gravity theories which dispense with the need for dark matter. These models,
which are known as Dark Matter Emulators, have the property that weak
gravitational waves couple to the metric that would follow from general
relativity without dark matter whereas ordinary particles couple to a
combination of the metric and other fields which reproduces the result of
general relativity with dark matter. We show that there is an appreciable
difference in the Shapiro delays of gravitational waves and photons or
neutrinos from the same source, with the gravity waves always arriving first.
We compute the expected time lags for GRB 070201, for SN 1987a, and for Sco-X1.
We estimate the probable error by taking account of the uncertainty in
position, and by using three different dark matter profiles.

http://arxiv.org/abs/0804.3804

Roberto Guida, Maria Grazia Bernardini, Carlo Luciano Bianco, Letizia Caito, Maria Giovanna Dainotti, Remo Ruffini

We show the preliminary results of the application of our “fireshell” model
to GRB060124. This source is very peculiar because it is the first event for
which both the prompt and the afterglow emission were observed simultaneously
by the three Swift instruments: BAT (15-350 keV), XRT (0.2-10 keV) and UVOT
(170-650 nm), due to the presence of a precursor ~ 570 s before the main burst.
We analyze GRB060124 within our “canonical” GRB scenario, identifying the
precursor with the P-GRB and the prompt emission with the afterglow peak
emission. In this way we reproduce correctly the energetics of both these two
components. We reproduce also the observed time delay between the precursor
(P-GRB) and the main burst. The effect of such a time delay in our model will
be discussed.

http://arxiv.org/abs/0804.3983

Shlomo Dado, Arnon Dar, A. De Rujula

The X-ray afterglows (AGs) of Gamma-Ray Bursts (GRBs) and X-Ray Flashes
(XRFs) have, after the fast decline phase of their prompt emission, a temporal
behaviour varying between two extremes. A large fraction of these AGs has a
'canonical' light curve which, after an initial shallow-decay 'plateau' phase,
'breaks smoothly' into a fast power-law decline. Very energetic GRBs,
contrariwise, appear not to have a 'break', their AG declines like a power-law
from the start of the observations. Breaks and 'missing breaks' are intimately
related to the geometry and deceleration of the jets responsible for GRBs. In
the frame of the 'cannonball' (CB) model of GRBs and XRFs, we analyze the cited
extreme behaviours (canonical and pure power-law) and intermediate cases
spanning the observed range of X-ray AG shapes. We show that the entire panoply
of X-ray light-curve shapes –measured with Swift and other satellites– are as
anticipated, on very limpid grounds, by the CB model. We test the expected
correlations between the AG's shape and the peak- and isotropic energies of the
prompt radiation, strengthening a simple conclusion of the analysis of AG
shapes: in energetic GRBs the break is not truly 'missing', it is hidden under
the tail of the prompt emission, or it occurs too early to be recorded. We also
verify that the spectral index of the unabsorbed AGs and the temporal index of
their late power-law decline differ by half a unit, as predicted.

http://arxiv.org/abs/0712.1527

F. E. Bauer (1), V. V. Dwarkadas (2), W. N. Brandt (3), S. Immler (4), S. Smartt (5), N. Bartel (6), M. F. Bietenholz (6 and 7) ((1) Columbia, (2) Chicago, (3) Penn State, (4) GSFC, (5) Queen's Belfast, (6) York, (7) Hartebeesthoek Radio Observatory)

We report on new VLT optical spectroscopic and multi-wavelength archival
observations of SN1996cr, a previously identified ULX known as Circinus Galaxy
X-2. Our optical spectrum confirms SN1996cr as a bona fide type IIn SN, while
archival imaging isolates its explosion date to between 1995-02-28 and
1996-03-16. SN1996cr is one of the closest SNe (~3.8 Mpc) in the last several
decades and in terms of flux ranks among the brightest radio and X-ray SNe ever
detected. The wealth of optical, X-ray, and radio observations that exist for
this source provide relatively detailed constraints on its post-explosion
expansion and progenitor history, including an preliminary angular size
constaint from VLBI. The archival X-ray and radio data imply that the
progenitor of SN1996cr evacuated a large cavity just prior to exploding: the
blast wave likely expanded for ~1-2 yrs before eventually striking the dense
circumstellar material which surrounds SN1996cr. The X-ray and radio emission,
which trace the progenitor mass-loss rate, have respectively risen by a factor
of ~2 and remained roughly constant over the past 7 yr. This behavior is
reminiscent of the late rise of SN1987A, but 1000 times more luminous and much
more rapid to onset. Complex Oxygen line emission in the optical spectrum
further hints at a possible concentric shell or ring-like structure. The
discovery of SN1996cr suggests that a substantial fraction of the closest SNe
observed in the last several decades have occurred in wind-blown bubbles.
Finally, cross-correlation with the BATSE GRB catalog yields a ~3sigma match
with 4B 960202, marking perhaps the fourth GRB to be marginally associated with
a type II SN. If correct, it would be the least luminous GRB detected to date.
[Abridged]

http://arxiv.org/abs/0804.3597

Nan Liang, Wei Ke Xiao, Yuan Liu, Shuang Nan Zhang

An important issue related to the application of Gamma-Ray Bursts (GRBs) to
cosmology is that the calibration of GRB luminosity/energy relations depends on
the cosmological model due to the lack of sufficient low-redshift GRB sample.
In this paper, we present a new method to calibrate GRB relations in a
cosmology independent way. Since objects at the same redshift should have the
same luminosity distance and the distance moduli of type Ia Supernovae (SNe Ia)
obtained directly from observations are completely cosmology independent, we
obtain the distance modulus of a GRB at a given redshift by interpolating from
the Hubble diagram of SNe Ia. Then we calibrate seven GRB relations without
assuming a particular cosmological model and construct the GRB Hubble diagram
to constrain cosmological parameters. From the 42 GRBs at $1.4<z\le6.6$, we
obtain $\Omega_{\rm M}=0.25_{-0.05}^{+0.04}$,
$\Omega_{\Lambda}=0.75_{-0.04}^{+0.05}$ for the flat $\Lambda$CDM model, and
for the dark energy model with a constant equation of state
$w_0=-1.05_{-0.40}^{+0.27}$, which is consistent with the concordance model in
1-$\sigma$ confidence region.

http://arxiv.org/abs/0802.4262