astro-grb

Bo Milvang-Jensen, Johan P. U. Fynbo, Daniele Malesani, Jens Hjorth, Páll Jakobsson, Palle Møller

We report the results of a spectroscopic search for Lyman-alpha emission from
gamma-ray burst host galaxies. Based on the well-defined TOUGH sample of 69
X-ray selected Swift GRBs, we have targeted the hosts of a subsample of 20 GRBs
known from afterglow spectroscopy to be in the redshift range 1.8-4.5. We
detect Lya emission from 7 out of the 20 hosts, with the typical limiting
3sigma line flux being 8E-18 erg/cm2/s, corresponding to a Lya luminosity of
6E41 erg/s at z=3. The Lya luminosities for the 7 hosts in which we detect Lya
emission are in the range (0.6-2.3)E42 erg/s corresponding to star-formation
rates of 0.6-2.1 Msun/yr (not corrected for extinction). The rest-frame Lya
equivalent widths (EWs) for the 7 hosts are in the range 9-40A. For 6 of the 13
hosts for which Lya is not detected we place fairly strong 3sigma upper limits
on the EW (<20A), while for others the EW is either unconstrained or has a less
constraining upper limit. We find that the distribution of Lya EWs is
inconsistent with being drawn from the Lya EW distribution of bright Lyman
break galaxies at the 98.3% level, in the sense that the TOUGH hosts on average
have larger EWs than bright LBGs. We can exclude an early indication, based on
a smaller, heterogeneous sample of pre-Swift GRB hosts, that all GRB hosts are
Lya emitters. We find that the TOUGH hosts on average have lower EWs than the
pre-Swift GRB hosts, but the two samples are only inconsistent at the 92%
level. The velocity centroid of the Lya line is redshifted by 200-700 km/s with
respect to the systemic velocity, similar to what is seen for LBGs, possibly
indicating star-formation driven outflows from the host galaxies. There seems
to be a trend between the Lya EW and the optical to X-ray spectral index of the
afterglow (beta_OX), hinting that dust plays a role in the observed strength
and even presence of Lya emission. [ABRIDGED]

http://arxiv.org/abs/1205.3779

Arnon Dar

The upper limit on the flux of ultra high energy neutrinos from gamma-ray
bursts (GRBs) that was reported recently by the IceCube collaboration
contradicts predictions based on the Fireball model of GRBs, but does not
exclude GRBs as a main source of ultra-high energy cosmic rays.

http://arxiv.org/abs/1205.3479

P. Jakobsson, J. Hjorth, D. Malesani, R. Chapman, J. P. U. Fynbo, N. R. Tanvir, B. Milvang-Jensen, P. M. Vreeswijk, G. Letawe, R. L. C. Starling

We present 10 new gamma-ray burst (GRB) redshifts and another five redshift
limits based on host galaxy spectroscopy obtained as part of a large program
conducted at the Very Large Telescope (VLT). The redshifts span the range 0.345
< z < 2.54. Three of our measurements revise incorrect values from the
literature. The homogeneous host sample researched here consists of 69 hosts
that originally had a redshift completeness of 55% (with 38 out of 69 hosts
having redshifts considered secure). Our project, including VLT/X-shooter
observations reported elsewhere, increases this fraction to 77% (53/69), making
the survey the most comprehensive in terms of redshift completeness of any
sample to the full Swift depth, analyzed to date. We present the cumulative
redshift distribution and derive a conservative, yet small, associated
uncertainty. We constrain the fraction of Swift GRBs at high redshift to a
maximum of 14% (5%) for z > 6 (z > 7). The mean redshift of the host sample is
assessed to be <z> > 2.2, with the 10 new redshifts reducing it significantly.
Using this more complete sample, we confirm previous findings that the GRB rate
at high redshift (z > 3) appears to be in excess of predictions based on
assumptions that it should follow conventional determinations of the star
formation history of the universe, combined with an estimate of its likely
metallicity dependence. This suggests that either star formation at high
redshifts has been significantly underestimated, for example due to a dominant
contribution from faint, undetected galaxies, or that GRB production is
enhanced in the conditions of early star formation, beyond that usually
ascribed to lower metallicity.

http://arxiv.org/abs/1205.3490

Remo Ruffini, Gregory Vereshchagin

Electron-positron plasma is believed to play imporant role both in the early
Universe and in sources of Gamma-Ray Bursts (GRBs). We focus on analogy and
difference between physical conditions of electron-positron plasma in the early
Universe and in sources of GRBs. We discuss a) dynamical differences, namely
thermal acceleration of the outflow in GRB sources vs cosmological
deceleration; b) nuclear composition differences as synthesis of light elements
in the early Universe and possible destruction of heavy elements in GRB plasma;
c) different physical conditions during last scattering of photons by
electrons. Only during the acceleration phase of the optically thick
electron-positron plasma comoving observer may find it similar to the early
Universe. This similarity breaks down during the coasting phase. Reprocessing
of nuclear abundances may likely take place in GRB sources. Heavy nuclear
elements are then destroyed, resulting mainly in protons with small admixture
of helium. Unlike the primordial plasma which recombines to form neutral
hydrogen, and emits the Cosmic Microwave Background Radiation, GRB plasma does
not cool down enough to recombine.

http://arxiv.org/abs/1205.3512

Zhe Chang, Hai-Nan Lin, Yunguo Jiang

Recent observations by Fermi-LAT showed that there are delayed arrival of GeV
photons relative to the onset of MeV photons in some GRBs. In order to avoid a
large optical depth, minimum values of Lorentz factor have been estimated to be
higher than 1000 in some brightest bursts. In this paper, we present a detailed
calculation of the time delay between the MeV and GeV photons in the framework
of the magnetic-dominated jet model. We find that the time delay strongly
depends on the terminal bulk Lorentz factor of the jet. Inspired by this fact,
we use this model to calculate the Lorentz factors of four Fermi bursts. The
results show that the Lorentz factors are much lower than that obtained from
“single-zone” scenario. The short GRB 090510 has a minimal Lorentz factor 385,
while the three long GRBs have almost the same Lorentz factors, with an average
value near 260. Another interesting feature is that, for long GRBs, GeV photons
are emitted after the Lorentz factor saturates. For short GRBs, however, MeV
and GeV photons are emitted at the same phase, i.e, either at the expansion
phase or at the coast phase.

http://arxiv.org/abs/1205.3572

Zachariah B. Etienne, Yuk Tung Liu, Vasileios Paschalidis, Stuart L. Shapiro

As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at
the end of a BH-NS binary inspiral, its magnetic fields will be stretched and
amplified. If sufficiently strong, these magnetic fields may impact the
gravitational waveforms, merger evolution and mass of the remnant disk.
Formation of highly-collimated magnetic field lines in the disk+spinning BH
remnant may launch relativistic jets, providing the engine for a short-hard
GRB. We analyze this scenario through fully general relativistic,
magnetohydrodynamic (GRMHD) BHNS simulations from inspiral through merger and
disk formation. Different initial magnetic field configurations and strengths
are chosen for the NS interior for both nonspinning and moderately spinning
(a/M=0.75) BHs aligned with the orbital angular momentum. Only strong interior
(Bmax~10^17 G) initial magnetic fields in the NS significantly influence merger
dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning
and spinning BH cases, respectively. However, detecting the imprint of even a
strong magnetic field may be challenging for Advanced LIGO. Though there is no
evidence of mass outflows or magnetic field collimation during the preliminary
simulations we have performed, higher resolution, coupled with longer disk
evolutions and different initial magnetic field configurations, may be required
to definitively assess the possibility of BHNS binaries as short-hard GRB
progenitors.

http://arxiv.org/abs/1112.0568

Hao-Ning He, Bin-Bin Zhang, Xiang-Yu Wang, Zhuo Li, Peter Mészáros

Recently, Fermi-LAT detected GeV emission during the X-ray flaring activity
in GRB 100728A. We study various scenarios for its origin. The hard spectrum of
the GeV emission favors the external inverse-Compton origin in which X-ray
flare photons are up-scattered by relativistic electrons in the external
forward shock. This external IC scenario, with anisotropic scattering effect
taken into account, can reproduce the temporal and spectral properties of the
GeV emission in GRB 100728A.

http://arxiv.org/abs/1112.2253

T. Uehara, K. Toma, K. S. Kawabata, S. Chiyonobu, Y. Fukazawa, Y. Ikejiri, T. Inoue, R. Itoh, T. Komatsu, H. Miyamoto, T. Mizuno, O. Nagae, H. Nakaya, T. Ohsugi, K. Sakimoto, M. Sasada, H. Tanaka, M. Uemura, M. Yamanaka, T. Yamashita, R. Yamazaki, M. Yoshida

We report that the optical polarization in the afterglow of GRB 091208B is
measured at t = 149 - 706 s after the burst trigger, and the polarization
degree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a
power-law decay with index -0.75 +/- 0.02, which is interpreted as the forward
shock synchrotron emission, and thus this is the first detection of the
early-time optical polarization in the forward shock (rather than that in the
reverse shock reported by Steele et al. (2009). This detection disfavors the
afterglow model in which the magnetic fields in the emission region are random
on the plasma skin depth scales, such as amplified by the plasma instabilities,
e.g., Weibel instability. We suggest that the fields are amplified by the
magnetohydrodynamic instabilities, which would be tested by future observations
of the temporal changes of the polarization degrees and angles for other
bursts.

http://arxiv.org/abs/1205.2149

Arnon Dar

The upper limit on the flux of ultra high energy neutrinos from gamma-ray
bursts (GRBs) that was reported recently by the IceCube collaboration
contradicts predictions based on the Fireball model of GRBs, but does not
exclude GRBs as a main source of ultra-high energy cosmic rays.

http://arxiv.org/abs/1205.3479

Jens Hjorth, Daniele Malesani, Páll Jakobsson, Andreas O. Jaunsen, Johan P. U. Fynbo, Javier Gorosabel, Thomas Krühler, Andrew J. Levan, Michał J. Michałowski, Bo Milvang-Jensen, Palle Møller, Steve Schulze, Nial R. Tanvir, Darach Watson

Long-duration gamma-ray bursts (GRBs) are powerful tracers of star-forming
galaxies at a very wide range of redshifts. We have defined a homogeneous
subsample of 69 Swift GRB-selected galaxies. Special attention has been devoted
to making the sample optically unbiased through simple and well-defined
selection criteria based on the high-energy properties of the bursts and their
positions on the sky. Thanks to our extensive follow-up observations, this
sample has now achieved a comparatively high degree of redshift completeness,
and thus provides a legacy sample, useful for statistical studies of GRBs and
their host galaxies. In this paper we present the survey design and summarize
the results of our observing program conducted at the ESO Very Large Telescope
aimed at obtaining the most basic properties of galaxies in this sample,
including a catalog of R and K magnitudes and redshifts. We detect the host
galaxies for 80 % of the GRBs in the sample, although only 42 % Ks-band
detections, which confirms that GRB-selected host galaxies are generally blue.
The sample is not uniformly blue, however, with two extremely red objects
detected. Moreover, galaxies hosting GRBs with no optical afterglows, whose
identification therefore relies on X-ray localisations, are significantly
brighter and redder than those with an optical afterglow. Our spectroscopic
campaign has resulted in 77 % now having redshift measurements, with a median
redshift of 2.14 +- 0.18. TOUGH alone consists of 17 detected z > 2 Swift GRB
host galaxies suitable for individual and statistical studies. Seven hosts have
detections of the Ly-alpha emission line and we can exclude an early indication
that Ly-alpha emission is ubiquitous among GRB hosts, but confirm that Ly-alpha
is stronger in GRB-selected galaxies than in flux-limited samples of Lyman
break galaxies.

http://arxiv.org/abs/1205.3162