astro-grb

F. Y. Wang (NJU), Z. G. Dai (NJU)

The high-redshift star formation rate (SFR) is difficult to measure directly
even by modern approaches. Long-duration gamma-ray bursts (GRBs) can be
detected to the edge of the visible universe because of their high
luminorsities. The collapsar model of long gamma-ray bursts indicates that they
may trace the star formation history. So long gamma-ray bursts may be a useful
tool of measuring the high-redshift SFR. Observations show that long gamma-ray
bursts prefer to form in a low-metallicity environment. We study the
high-redshift SFR up to z~8.3 considering the Swift GRBs tracing the star
formation history and the cosmic metallicity evolution in different background
cosmological models including $\Lambda$CDM, quintessence, quintessence with a
time-varying equation of state, and brane-world model. We use latest Swift GRBs
including two highest-$z$ GRBs, GRB 080913 at $z=6.7$ and GRB 090423 at
$z=8.3$. We find that the SFR at $z>4$ shows a steep decay with a slope of
$\sim -5.0$ in $\Lambda$CDM. In the other three models, the high-redshift SFR
is slightly different from $\Lambda$CDM model, and also shows a steep decay.

http://arxiv.org/abs/0912.5140

F. Y. Wang (NJU), Z. G. Dai (NJU), Shi. Qi (PMO)

In this paper we investigate the deceleration, jerk and snap parameters to
distinguish between the dark energy and modified gravity models by using high
redshift gamma-ray bursts (GRBs) and supernovae (SNe). We first derive the
expressions of deceleration, jerk and snap parameters in dark energy and
modified gravity models. In order to constrain the cosmographic parameters, we
calibrate the GRB luminosity relations without assuming any cosmological models
using SNe Ia. Then we constrain the models (including dark energy and modified
gravity models) parameters using type Ia supernovae and gamma-ray bursts.
Finally we calculate the cosmographic parameters. GRBs can extend the redshift
- distance relation up to high redshifts, because they can be detected to high
redshifts.We find that the statefinder pair (r,s) could not be used to
distinguish between some dark energy and modified gravity models, but these
models could be differentiated by the snap parameter. Using the
model-independent constraints on cosmographic parameters, we conclude that the
\LambdaCDM model is consistent with the current data.

http://arxiv.org/abs/0912.5141

J. Deng, W. Zheng, M. Zhai, L. Xin, Y. Qiu, A. Stefanescu, A. Pozanenko, M. Ibrahimov, A. Volnova

We present optical photometry of the GRB 060912A afterglow obtained with
ground-based telescopes, from about 100 sec after the GRB trigger till about
0.3 day later, supplemented with the Swift optical afterglow data released in
its official website. The optical light curve (LC) displays a smooth single
power-law decay throughout the observed epochs, with a power-law index of about
-1 and no significant color evolution. This is in contrast to the X-ray LC
which has a plateau phase between two normal power-law decays of a respective
index of about -1 and -1.2. It is shown by our combined X-ray and optical data
analysis that this asynchronous behavior is difficult to be reconciled with the
standard afterglow theory and energy injection hypothesis. We also construct an
optical-to-X-ray spectral energy distribution at about 700 sec after the GRB
trigger. It displays a significant flux depression in the B-band, reminding us
of the possibility of a host-galaxy (at z=0.937) 2175-A dust absorption similar
to the one that characterizes the Milky Way extinction law. Such an
identification, although being tentative, may be confirmed by our detailed
analysis using both template extinction laws and the afterglow theory. So far
the feature is reported in very few GRB afterglows. Most seem to have a host
galaxy either unusually bright for a GRB, just like this one, or of an early
type, supporting the general suggestion of an anti-correlation between the
feature and star-forming activities.

http://arxiv.org/abs/0912.5435

D. Viganò (1,2), S. Mereghetti (2) ((1) Università degli Studi di Milano, Italy, (2) INAF-IASF Milano, Italy)

The Anti Coincidence Shield (ACS) of the INTEGRAL SPI instrument provides an
excellent sensitivity for the detection of Gamma Ray Bursts (GRBs) above ~
75keV, but no directional and energy information is available. We studied the
ACS response by using GRBs with known localizations and good spectral
information derived by other satellites. We derived a count rate to flux
conversion factor for different energy ranges and studied its dependence on the
GRB direction and spectral hardness. For a typical GRB spectrum, we found that
1 ACS count corresponds on average to ~ 1E-10 erg/cm^2 in the 75keV-1MeV range,
for directions orthogonal to the satellite pointing axis. This is broadly
consistent with the ACS effective area derived from the Monte Carlo
simulations, but there is some indication that the latter slightly
overestimates the ACS sensitivity, especially for directions close to the
instrument axis.

http://arxiv.org/abs/0912.5329

Shu-Fu Qin, En-Wei Liang, Rui-Jing Lu, Jian-Yan Wei, Shuang-Nan Zhang

Since the launch of Swift satellite, detections of high-z (z>4 and up to
about 8.3 currently) long gamma-ray bursts (LGRBs) have been rapidly growing
up, even approaching the very early Universe. The observed high-z LGRB rate
shows significant excess over that estimated from the star formation history.
We investigate what may be responsible for this high productivity of GRBs at
high-z through Monte Carlo simulations based on current Swift LGRB sample.
Elaborated effective Swif/BAT trigger probability and redshift detection
probability for LGRBs are estimated with current Swift/BAT sample and
CGRO/BATSE LGRB sample. We compare our simulations to the Swift observations
via log N-\log P and L-z distributions. In the case that LGRB rate is purely
proportional to the star formation rate (SFR), our simulations poorly reproduce
the LGRB rate at z>4, although the simulated \log N-\log P distribution is in
good agreement with the observed one. Assuming that the excess of high-z GRB
rate is due to the cosmic metallicity evolution or unknown LGRB rate increase
parameterized as (1+z)^{\delta}, we find that although the two scenarios can
make better consistency between our simulations and observations, they cannot
simultaneously reproduce the observations alone. Incorporation of the two
scenarios gives great agreement between our simulations and observations,
indicating that both GRB rate evolution and cosmic metallicity evolution would
result in the observed high-z GRB rate excess. With increasing detections of
GRBs at z>4 (~ 15% of GRBs in current Swift LGRB sample based on our
simulations), a window for very early Universe is opening up by Swift and
up-coming SVOM missions.

http://arxiv.org/abs/0912.5382

F. Ryde, M. Axelsson, B.B. Zhang, S. McGlynn, A. Pe'er, C. Lundman, S. Larsson, M. Battelino, B. Zhang, E. Bissaldi, J. Bregeon, M.S. Briggs, J. Chiang, F. de Palma, S. Guiriec, J. Larsson, F. Longo, S. McBreen, N. Omodei, V. Petrosian, R. Preece, A.J. van der Horst

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B,
lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and
the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300
GeV is covered. Here we show that the prompt burst spectrum is consistent with
emission from the jet photosphere combined with non-thermal emission described
by a single power-law with photon index -1.9. The photosphere gives rise to a
strong quasi-blackbody spectrum which is somewhat broader than a single Planck
function and has a characteristic temperature of ~290keV. We derive the
photospheric radius Rph = (1.1 \pm 0.3) x 10^12 Y^{1/4} cm and the bulk Lorentz
factor of the flow, which is found to vary by a factor of two and has a maximal
value of Gamma = 750 Y^{1/4}. Here Y is the ratio between the total fireball
energy and the energy emitted in the gamma-rays. We find that during the first
quarter of the prompt phase the photospheric emission dominates, which explains
the delayed onset of the observed flux in the LAT compared to the GBM. We model
the photospheric emission with a multi-color blackbody and its shape indicates
that the photospheric radius increases at higher latitudes. We interpret the
broad band emission as synchrotron emission at R ~ 4×10^15 cm. Our analysis
emphasize the importance of having high temporal resolution when performing
spectral analysis on GRBs, since there is strong spectral evolution.

http://arxiv.org/abs/0911.2025

En-Wei Liang (GXU, Unlv), Shuang-xi Yi (GXU), Jin Zhang (GXTU), Hou-Jun LV (GXU), Bin-Bin Zhang (UNLV), Bing Zhang (UNLV)

The onset of GRB afterglow is characterized by a smooth bump in the early
afterglow lightcurve. We make an extensive search for such a feature. Twenty
optically selected GRBs and 12 X-ray selected GRBs are found, among which 17
optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements.
We fit the lightcurves with a smooth broken power-law and measure the temporal
characteristic timescales of the bumps at FWHM. Strong mutual correlations
among these timescales are found, and a dimmer and broader bump tends to peak
at a later peak time. The ratio of rising to decaying timescales is almost
universal among bursts, but the ratio of the rising time to the peak time
varies from 0.3~1. The E_iso is tightly correlated with the peak luminosity and
the peak time of the bump in the burst frame. Assuming that the bumps signal
the deceleration of the GRB fireballs in a constant density medium, we
calculate the initial Lorentz factor (Gamma_0) and the deceleration radius
(R_dec) of the GRBs in the optical-selected sample. It is found that Gamma_0
are typically a few hundreds, and the typical deceleration radius is
R_dec~10^{17} cm. More intriguingly, a tight correlation between the Gamma_0
and E_iso is found, namely Gamma_0 ~ 195 E_iso, 52}^{0.27} (satisfied for both
the optical and X-ray z-known samples). It is helpful to understand GRB
physics, and may serve as an indicator of Gamma_0. We find that the early
bright X-rays are usually dominated by a different component from the external
shock emission, but occasionally (for one case) an achromatic deceleration
feature is observed. Components in X-rays would contribute to the diversity of
the observed X-ray lightcurves (abridge).

http://arxiv.org/abs/0912.4800

Evgeny Gorbovskoy, Kirill Ivanov, Vladimir Lipunov, Victor Kornilov, Alexander Belinski, Nikolaj Shatskij, Dmitry Kuvshinov, Nataly Tyurina, Pavel Balanutsa, Vadim Chazov, Artem Kuznetsov, Petr Kortunov, Andrey Tlatov, Alexander Parkhomenko, Vadim Krushinsky, Ivan Zalozhnyh, Alexander Popov, Taisia Kopytova, Sergey Yazev, Alexander Krylov

At present time Robotic observatory making is of current importance. Having a
large field of view and being able to point at anywhere, Robotic astronomical
systems are indispensable when they looking for transients like grb, supernovae
explosions, novae etc, as it's impossible in these cases to foresee what you
should point you telescope at and when. In work are described prompt GRB
observations received on wide-field chambers MASTER-VWF, and also methods of
the images analysis and transients classifications applied in real-time data
processing in this experiment. For 7 months of operation 6 synchronous
observations of gamma-ray burst had been made by MASTER VWF in Kislovodsk and
Irkutsk. In all cases a high upper limits have been received (see tabl \ref
{tab_grbwf} and fig. \ref {allgrb}).

http://arxiv.org/abs/0907.1118

R.J. Bouwens (UC Santa Cruz), G.D. Illingworth (UC Santa Cruz), I. Labbe (Carnegie Observatories), P.A. Oesch (ETH Zurich), M. Carollo (ETH Zurich), M. Trenti (U Colorado), P.G. van Dokkum (Yale), M. Franx (Leiden), M. Stiavelli (STScI), V. Gonzalez (UC Santa Cruz), D. Magee (UC Santa Cruz)

The first galaxies likely formed a few hundred million years after the Big
Bang. Until recently, it has not been possible to detect galaxies earlier than
~750 million years after the Big Bang. The new HST WFC3/IR camera changed this
when the deepest-ever, near-IR image of the universe was obtained with the
HUDF09 program. Here we use this image to identify three redshift z~10 galaxy
candidates in the heart of the reionization epoch when the universe was just
500 million years old. These would be the highest redshift galaxies yet
detected, higher than the recent detection of a GRB at z~8.2. The HUDF09 data
previously revealed galaxies at z~7 and z~8. Galaxy stellar population models
predict substantial star formation at z>9-10. Verification by direct
observation of the existence of galaxies at z~10 is the next step. Our
conservative search and extensive testing for contamination and spurious images
suggests that we can set reliable constraints based upon our 3 z~10 candidates,
unlike a recent claim of 20 z~10 sources which appear to be spurious. The
detection of galaxies at z>8 is further enhanced by our detailed analysis of 2
other faint sources likely at z~8.4 and z~8.7. Our z~10 sample suggests that
the luminosity function and star formation rate density evolution found at
lower redshifts continues to z~10, and pushes back the timescale for early
galaxy buildup to z>10, increasing the likely role of galaxies in providing the
UV flux needed to reionize the universe. The true nature of these galaxies, at
just 4% of the age of the universe, will remain hidden until JWST is launched.

http://arxiv.org/abs/0912.4263

K. Hurley, M. Briggs, V. Connaughton, C. Meegan, T. Cline, I. Mitrofanov, D. Golovin, M. L. Litvak, A. B. Sanin, W. Boynton, C. Fellows, K. Harshman, R. Starr, S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, D. M. Smith, C. Wigger, A. Rau, A. von Kienlin, K. Yamaoka, M. Ohno, Y. Fukazawa, T. Takahashi, M. Tashiro, Y. Terada, T. Murakami, K. Makishima, S. Barthelmy, J. Cummings, N. Gehrels, H. Krimm, J. Goldsten, E. Del Monte, M. Feroci, M. Marisaldi

We are integrating the Fermi Gamma-Ray Burst Monitor (GBM) into the
Interplanetary Network (IPN) of Gamma-Ray Burst (GRB) detectors. With the GBM,
the IPN will comprise 9 experiments. This will 1) assist the Fermi team in
understanding and reducing their systematic localization uncertainties, 2)
reduce the sizes of the GBM and Large Area Telescope (LAT) error circles by 1
to 4 orders of magnitude, 3) facilitate the identification of GRB sources with
objects found by ground- and space-based observatories at other wavelengths,
from the radio to very high energy gamma-rays, 4) reduce the uncertainties in
associating some LAT detections of high energy photons with GBM bursts, and 5)
facilitate searches for non-electromagnetic GRB counterparts, particularly
neutrinos and gravitational radiation. We present examples and demonstrate the
synergy between Fermi and the IPN. This is a Fermi Cycle 2 Guest Investigator
project.

http://arxiv.org/abs/0912.4294