Variable jet properties in GRB110721A: Time resolved observations of the jet photosphere. (arXiv:1305.3611v1 [astro-ph.HE])

May 17th, 2013

{\it Fermi Gamma-ray Space Telescope} observations of GRB110721A have
revealed two emission components from the relativistic jet: emission from the
photosphere, peaking at $\sim 100$ keV and a non-thermal component, which peaks
at $\sim 1000$ keV. We use the photospheric component to calculate the
properties of the relativistic outflow. We find a strong evolution in the flow
properties: the Lorentz factor decreases with time during the bursts from
$\Gamma \sim 1000$ to $\sim 150$ (assuming a redshift $z=2$; the values are
only weakly dependent on unknown efficiency parameters). Such a decrease is
contrary to the expectations from the internal shocks and the isolated magnetar
birth models. Moreover, the position of the flow nozzle measured from the
central engine, $r_0$, increases by more than two orders of magnitude. Assuming
a moderately magnetised outflow we estimate that $r_0$ varies from $10^6$ cm to
$\sim 10^9$ cm during the burst. We suggest that the maximal value reflects the
size of the progenitor core. Finally, we show that these jet properties
naturally explain the observed broken power-law decay of the temperature which
has been reported as a characteristic for GRB pulses.

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On the magnetisation of gamma-ray burst blast waves. (arXiv:1305.3689v1 [astro-ph.HE])

May 17th, 2013

Martin Lemoine (IAP), Zhuo Li (PKU), Xiang-Yu Wang (NJU)

The origin of magnetic fields that permeate the blast waves of gamma-ray
bursts is a long-standing problem. The present paper argues that in four GRBs
revealing extended emission at >100 MeV, with follow-up in the radio, optical
and X-ray domains at later times, this magnetisation can be described as the
partial decay of the microturbulence that is generated in the shock precursor.
Assuming that the extended high energy emission can be interpreted as
synchrotron emission of shock accelerated electrons, we model the
multi-wavelength light curves of GRB 090902B, GRB 090323, GRB 090328 and GRB
110731A, using a simplified then a full synchrotron calculation with power law
decaying microturbulence \epsilon_B \propto t^{\alpha_t} (t denotes the time
since injection through the shock, in the comoving blast frame). We find that
these models point to a consistent value of the decay exponent -0.5 < \alpha_t
< -0.4.

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Photospheric emission throughout GRB 100507 detected by Fermi. (arXiv:1305.3287v1 [astro-ph.HE])

May 16th, 2013

G. Ghirlanda (1), A. Pescalli (2), G. Ghisellini (1) ((1) INAF-Osservatorio Astronomico di Brera, (2) Dipartimento di Fisica G. Occhialini, Universita' Milano Bicocca)

Gamma ray bursts with blackbody spectra are only a few and in most cases this
spectral component is accompanied by a dominating non-thermal one. Only four
bursts detected by Batse have a pure blackbody spectrum throughout their
duration. We present the new case of GRB 100507 detected by the Gamma Burst
Monitor on board the Fermi satellite. GRB 100507 has a blackbody spectrum for
the entire duration (~30 s) of the prompt emission. The blackbody temperature
varies between 25 and 40 keV. The flux varies between 1e-7 and 4e-7 erg/cm2 s.
There is no clear evidence of a correlation between the temperature and the
blackbody flux. If the thermal emission in GRB 100507 is due to the fireballs
becoming transparent, we can estimate the radius R_T and bulk Lorentz factor
Gamma_T corresponding to this transition and the radius R_0 where the fireballs
are created. We compare these parameters with those derived for the other four
bursts with a pure blackbody spectrum. In all but one burst, for fiducial
assumptions on the radiative efficiency and distance of the sources,
R_0~1e9-1e10 cm, i.e. much larger than the gravitational radius of a few solar
mass black hole. Possible solutions of this apparent inconsistency are
tentatively discussed considering the dependence of R_0 on the unknown
parameters. Alternatively, such a large R_0 could be where the fireball, still
opaque, converts most of its kinetic energy into internal energy (due to the
impact with some material left over by the progenitor star) and starts to
re-accelerate.

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The Swift Gamma-Ray Burst redshift distribution: selection biases or rate evolution at high-z?. (arXiv:1305.3366v1 [astro-ph.CO])

May 16th, 2013

We employ realistic constraints on selection effects to model the Gamma-Ray
Burst (GRB) redshift distribution using {\it Swift} triggered redshift samples
acquired from optical afterglows and the TOUGH survey. Models for the Malmquist
bias, redshift desert, and the fraction of afterglows missing because of host
galaxy dust extinction, are used to show how the “true” GRB redshift
distribution is distorted to its presently observed biased distribution. Our
analysis, which accounts for the missing fraction of redshifts in the two data
subsets, shows that a combination of selection effects (both instrumental and
astrophysical) can describe the observed GRB redshift distribution. The
observed distribution supports the case for dust extinction as the dominant
astrophysical selection effect that shapes the redshift distribution.

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Constraints on Lorentz Invariance Violation from Fermi-Large Area Telescope Observations of Gamma-Ray Bursts. (arXiv:1305.3463v1 [astro-ph.HE])

May 16th, 2013

We analyze the MeV/GeV emission from four bright Gamma-Ray Bursts (GRBs)
observed by the Fermi-Large Area Telescope to produce robust, stringent
constraints on a dependence of the speed of light in vacuo on the photon energy
(vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by
some Quantum Gravity (QG) theories. First, we use three different and
complementary techniques to constrain the total degree of dispersion observed
in the data. Additionally, using a maximally conservative set of assumptions on
possible source-intrinsic spectral-evolution effects, we constrain any vacuum
dispersion solely attributed to LIV. We then derive limits on the “QG energy
scale” (the energy scale that LIV-inducing QG effects become important, E_QG)
and the coefficients of the Standard Model Extension. For the subluminal case
(where high energy photons propagate more slowly than lower energy photons) and
without taking into account any source-intrinsic dispersion, our most stringent
limits (at 95% CL) are obtained from GRB090510 and are E_{QG,1}>7.6 times the
Planck energy (E_Pl) and E_{QG,2}>1.3 x 10^11 GeV for linear and quadratic
leading order LIV-induced vacuum dispersion, respectively. These limits improve
the latest constraints by Fermi and H.E.S.S. by a factor of ~2. Our results
disfavor any class of models requiring E_{QG,1} \lesssim E_Pl.

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Radio Constraints on Heavily-Obscured Star-Formation within Dark Gamma-Ray Burst Host Galaxies. (arXiv:1305.2941v1 [astro-ph.CO])

May 15th, 2013

Highly dust-obscured starbursting galaxies (submillimeter galaxies and their
ilk) represent the most extreme sites of star-formation in the distant universe
and contribute significantly to overall cosmic star-formation beyond z>1.5.
Some stars formed in these environments may also explode as GRBs and contribute
to the population of “dark” bursts. Here we present VLA wideband
radio-continuum observations of 15 heavily dust-obscured Swift GRBs to search
for radio synchrotron emission associated with intense star-formation in their
host galaxies. We detect four of these targets, although one detection is
marginal and for two others we cannot yet rule out the contribution of a
long-lived radio afterglow. The final detection is secure, but indicates a
star-formation rate consistent with the UV-inferred value. The remaining 11
targets are not detected. Most galaxies hosting obscured GRBs are therefore not
forming stars at extreme rates, and the amount of optical extinction seen along
a GRB afterglow sightline does not clearly correlate with the likelihood that
the host has a sufficiently high star-formation rate to be radio-detectable.
While some submillimeter galaxies do readily produce GRBs, these GRBs are often
unobscured - suggesting that the outer (unobscured) parts of these galaxies
overproduce GRBs and the inner (obscured) parts underproduce GRBs relative to
their respective contributions to star-formation, hinting at strong chemical or

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Thermal emission in the early X-ray afterglows of GRBs: following the prompt phase to the late times. (arXiv:1305.3165v1 [astro-ph.HE])

May 15th, 2013

Thermal radiation, peaking in soft X-rays, has now been detected in a handful
of GRB afterglows and has to date been interpreted as shock break-out of the
GRB's progenitor star. We present a search for thermal emission in the early
X-ray afterglows of a sample of Swift bursts selected by their brightness in
X-rays at early times. We identify a clear thermal component in eight GRBs and
track the evolution. We show that at least some of the emission must come from
highly relativistic material since two show an apparent super-luminal expansion
of the thermal component. Furthermore we determine very large luminosities and
high temperatures for many of the components-too high to originate in a SN
shock break-out. Instead we suggest that the component may be modelled as late
photospheric emission from the jet, linking it to the apparently thermal
component observed in the prompt emission of some GRBs at gamma-ray and hard
X-ray energies. By comparing the parameters from the prompt emission and the
early afterglow emission we find that the results are compatible with the
interpretation that we are observing the prompt quasi-thermal emission
component in soft X-rays at a later point in its evolution.

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Models and possible progenitors of gamma-ray bursts at the test field of the observations. (arXiv:1305.3194v1 [astro-ph.HE])

May 15th, 2013

B. Gendre (Artemis/IRAP), G. Stratta (OaR/INAF), on behalf of the FIGARO collaboration

During the last 15 years, a standard paradigm has emerged to explain both the
progenitor nature and the observed radiations of gamma-ray bursts. In this work
we show three GRBs for which the standard paradigm could be tested with high
statistics due to their exceptional spectral and temporal coverage. While GRB
1110205 represents a very good example of the standard scenario, GRB 090102 and
GRB 111209A do not fit into the standard paradigm.

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Discovery of an extra hard spectral component in the high-energy afterglow emission of GRB 130427A. (arXiv:1305.3217v1 [astro-ph.HE])

May 15th, 2013

Pak-Hin Thomas Tam (NTHU), Qing-Wen Tang (NJU), Shujing Hou (Xiamen Univ, PMO), Ruo-Yu Liu (NJU, MPIK), Xiang-Yu Wang (NJU)

The extended high-energy gamma-ray (>100 MeV) emission occurred after the
prompt gamma-ray bursts is usually characterized by a single power-law
spectrum, which has been explained as the afterglow synchrotron radiation. The
afterglow inverse-Compton emission has long been predicted to be able to
produce a high-energy component as well, but previous observations have not
revealed such a signature clearly, probably due to the small number of >10 GeV
photons even for the brightest GRBs known so far. In this Letter, we report on
the Fermi Large Area Telescope (LAT) observations of the >100 MeV emission from
the very bright and nearby GRB 130427A. We characterize the time-resolved
spectra of the GeV emission from the GRB onset to the afterglow phase. Based on
detection of about a dozen >10 GeV photons from GRB 130427A, we found a strong
evidence of an extra hard spectral component that exists in the extended
high-energy emission of this GRB. We argue that this hard component may arise
from the afterglow inverse Compton emission.

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Evidence for the Connection between Prompt and X-ray Afterglow emission of Swift-Detected Gamma-Ray Bursts. (arXiv:1305.3236v1 [astro-ph.HE])

May 15th, 2013

Dirk Grupe (PSU), John A. Nousek (PSU), Peter Verres (PSU), Binbin Zhang (PSU), Neil Gehrels (NASA/GSFC)

explosion is retained in the properties of the prompt and afterglow emission.
We report on tight relationships between the prompt and X-ray afterglow
emission of Swift-detected Gamma Ray Bursts found from BAT and XRT data between
2004 December and 2013 March. These relations suggest that the prompt and
afterglow emission are closely linked. In particular, we find very strong
correlations between the BAT 15-150keV T90 and the break times before and after
the plateau phase in the X-ray 0.3-10keV afterglow light curves. We also find a
strong anti-correlation between the photon index of the GRB prompt emission and
the X-ray spectral slope of the afterglow. Further, anti-correlations exist
between the rest frame peak energy in the prompt emission, E_ peak, and the
X-ray afterglow decay slope during the plateau phase and the break times after
the plateau phase. The rest-frame break times before and after the plateau
phase are also anti-correlated with the rest-frame 15-150keV luminosity and
isotropic energy during the prompt emission. A Principal Component Analysis
suggests that GRB properties are primarily driven by the luminosity/energy
release in the 15-150 keV band. Luminosity functions derived at various
redshifts from logN-logS analysis, indicate that the density of bright bursts
is significantly lower in the local Universe compared with the Universe at z~3,
where the density of bright GRBs peaks. Using cluster analysis, we find that
the duration of Swift BAT-detected short-duration GRBs is less than 1s.

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