astro-grb

Z. Bagoly, L. Borgonovo, I. Horvath, A. Meszaros, L.G. Balazs

A sample of 197 long BATSE GRBs is studied statistically. In the sample 11
variables, describing for any burst the time behavior of the spectra and other
quantities, are collected. The application of the factor analysis on this
sample shows that five factors describe the sample satisfactorily. Both the
pseudo-redshifts coming from the variability and the Amati-relation in its
original form are disfavored.

http://arxiv.org/abs/0902.4610

L. K. Pollack (1), H.-W. Chen (2), J. X. Prochaska (1,3), J. S. Bloom (4) ((1) UC Santa Cruz, (2) KICP/UChicago, (3) UCO/Lick, (4) UC Berkeley)

We present results from an imaging and spectroscopic study of four strong
MgII absorbers of W(2796) >~ 1 Ang revealed by the afterglow of GRB060418 at
z_GRB=1.491. These absorbers, at z=0.603,0.656,1.107 and z_GRB, exhibit large
ion abundances that suggest neutral gas column densities characteristic of
damped Lya systems. The imaging data include optical images obtained using LRIS
on the Keck I telescope and using ACS on board HST, and near-infrared H-band
images obtained using PANIC on the Magellan Baade Telescope and K'-band images
obtained using NIRC2 with LGSAO on the Keck II telescope. These images reveal
six distinct objects at <~ 3.5'' of the afterglow's position, two of which
exhibit well-resolved mature disk morphology, one shows red colors, and three
are blue compact sources. Follow-up spectroscopic observations using LRIS
confirm that one of the disk galaxies coincides with the MgII absorber at
z=0.656. The observed broad-band spectral energy distributions of the second
disk galaxy and the red source indicate that they are associated with the
absorbers at z=0.603 and z=1.107, respectively. These results show that strong
MgII absorbers identified in GRB afterglow spectra are associated with typical
galaxies of luminosity ~ (0.1-1) L* at impact parameter <~ 10 h^-1 kpc. The
close angular separation would preclude easy detections toward a bright quasar.
Finally, we associate the remaining three blue compact sources with the GRB
host galaxy, noting that they are likely star-forming knots located at
projected distances 2-12 h^-1 kpc from the afterglow. At the afterglow's
position, we derive a 2-sigma upper limit to the underlying SFR intensity of
0.0074 M_sun yr^-1 kpc^-2.

http://arxiv.org/abs/0902.3661

Michał J. Michałowski, Jens Hjorth, Daniele Malesani, Tadeusz Michałowski, José María Castro Cerón, Robert F. Reinfrank, Michael A. Garrett, Johan P. U. Fynbo, Darach J. Watson, Uffe G. Jørgensen

We present an analysis of the spectral energy distribution (SED) of the
galaxy ESO 184-G82, the host of the closest known long gamma-ray burst (GRB)
980425 and its associated supernova (SN) 1998bw. We use our observations
obtained at the Australia Telescope Compact Array (the third >3 sigma radio
detection of a GRB host) as well as archival infrared and ultraviolet (UV)
observations to estimate its star formation state. We find that ESO 184-G82 has
a UV star formation rate (SFR) and stellar mass consistent with the population
of cosmological GRB hosts and of local dwarf galaxies. However, it has a higher
specific SFR (per unit stellar mass) than luminous spiral galaxies. The mass of
ESO 184-G82 is dominated by an older stellar population in contrast to the
majority of GRB hosts. The Wolf-Rayet region ~800 pc from the SN site
experienced a starburst episode during which the majority of its stellar
population was built up. Unlike that of the entire galaxy, its SED is similar
to those of cosmological submillimeter/radio-bright GRB hosts with hot dust
content. These findings add to the picture that in general, the environments of
GRBs on 1-3 kpc scales are associated with high specific SFR and hot dust.

http://arxiv.org/abs/0809.0508

En-Wei Liang, Hou-Jun Lv, Bin-Bin Zhang, Bing Zhang

By analyzing the Swift/XRT lightcurves detected before 2008 October, we find
17 cases that decay as a single power law (SPL) from tens of seconds to ~10^5
seconds post the GRB trigger. They are apparently different from the canonical
ones that are characterized by a shallow-to-normal decay transition. We compare
the distributions of the observables of the prompt gamma-rays and X-rays for
the two groups of GRBs, but no statistical difference is found. Interestingly,
the SPL XRT lightcurves in the burst frame merge into a conflux. The normal
decay segment for the canonical lightcurves has the similar feature. These
results likely suggest that both the prompt gamma-rays and the X-rays for the
two groups of GRBs may share the similar physical origins, and the apparent
difference between the groups of XRT lightcurves may not be intrinsic. If the
shallow decay is due to energy injection into the fireball, this suggests that
the total energy budget after injection for both types of GRBs is similar. More
intriguingly, the picture is consistent with the scenario that the apparent
shallow-to-normal behavior is an artifact because of the improper choice of the
zero time point (T_0), as suggested by Yamazaki. We shift T_0 of the canonical
lightcurves to an epoch prior to the GRB triggers to make the lightcurves close
to a SPL with the temporal indices similar to those in the normal decay phase.
It is found that the T_0-shifted lightcurves trace the lightcurves of the SPL
type well. This result likely suggests that the X-rays might be a long-lasting
emission component that is independent of the prompt gamma-rays. The GRBs with
the SPL lightcurves may be those whose T_0's of X-ray decay are comparable to
the trigger times. We discuss the prior X-ray emission from GRBs for both an
external shock origin and an origin from a long-lasting central engine.

http://arxiv.org/abs/0902.3504

Jeffrey E. McClintock, Ronald A. Remillard

In astronomy, the problem of black holes is arguably second in importance
only to the problem of cosmology. A current frontier in black hole research is
the measurement of spin. During the past three years, the spins of several
stellar-mass black holes in X-ray binaries have been measured via two
techniques: fitting the X-ray continuum spectrum and modeling the profile of
the Fe K line. This fledgling enterprise motivates the following decadal goals:
(1) Firmly establish the continuum-fitting and Fe K methods; obtain precise
values of spin for 10-20 black holes, several using both methods; (2) use the
derived masses and spins to test models of jets, GRBs, supernovae, black hole
formation, black hole binary evolution, etc.; (3) serve the IXO mission by
securing the Fe K methodology, which is currently the only means to measure the
spins of supermassive black holes in AGN; (4) identify the correct model of
high-frequency QPOs, thereby opening a third channel for measuring spin; (5)
pursue X-ray polarimetry as a means of securing the continuum-fitting and Fe K
methods, and also as a possible fourth avenue to spin; and (6) develop and test
MHD models of thin disks in strong gravity. Achieving these goals requires the
establishment of an RXTE follow-on mission dedicated to the study of bright and
transient compact objects, as well as strong support for theoretical work on 3D
MHD simulations of accretion flows in the Kerr metric of a spinning black hole.

http://arxiv.org/abs/0902.3488

D. A. Williams, A. D. Falcone, M. G. Baring, J. Buckley, V. Connaughton, P. Coppi, C. Dermer, S. Digel, B. Dingus, C. Fryer, N. Gehrels, J. Granot, D. Horan, J. I. Katz, P. Meszaros, J. Norris, P. Saz Parkinson, A. Pe'er, S. Razzaque, G. Sinnis, X. Y. Wang, T. C. Weekes, B. Zhang

Gamma-ray bursts (GRBs) have been an enigma since their discovery forty years
ago. However, considerable progress unraveling their mysteries has been made in
recent years. Developments in observations, theory, and instrumentation have
prepared the way so that the next decade can be the one in which we finally
answer the question, “What are gamma-ray bursts?” This question encompasses not
only what the progenitors are that produce the GRBs, but also how the enormous
luminosity of the GRBs, concentrated in gamma rays, is achieved. Observations
across the electromagnetic spectrum, from both the ground and space, will be
required to fully tackle this important question. This white paper, mostly
distilled from a recent study commissioned by the Division of Astrophysics of
the American Physical Society, focuses on what very high energy (~100 GeV and
above) gamma-ray observations can contribute. Very high energy gamma rays probe
the most extreme high energy particle populations in the burst environment,
testing models of lepton and proton acceleration in GRBs and constraining the
bulk Lorentz factor and opacity of the outflow. Sensitivity improvements of
more than an order of magnitude in the very high energy gamma-ray band can be
achieved early in the next decade, in order to contribute to this science.

http://arxiv.org/abs/0902.3466

Patrizia Ferrero, Sylvio Klose, David Alexander Kann, Sandra Savaglio, Steve Schulze, Eliana Palazzi, Elisabetta Maiorano, Petra Böhm, Dirk Grupe, Samantha R. Oates, Sebastián F. Sánchez, Lorenzo Amati, Jochen Greiner, Jens Hjorth, Daniele Malesani, Scott D. Barthelmy, Javier Gorosabel, Nicola Masetti, Martin M. Roth

The long and relatively faint gamma-ray burst GRB 060605 detected by
\emph{Swift}/BAT lasted about 20 sec. Its afterglow could be observed with
\emph{Swift}/XRT for nearly 1 day, while \emph{Swift}/UVOT could detect the
afterglow during the first 6 hours after the event. Here, we report on integral
field spectroscopy of its afterglow performed with PMAS/PPak mounted at the
Calar Alto 3.5 m telescope. In addition, we report on a detailed analysis of
XRT and UVOT data and on the results of deep late-time VLT observations that
reveal the GRB host galaxy. We find that the burst occurred at a redshift of
$z$=3.773, possibly associated with a faint, $R_C=26.4 \pm 0.3$ host. Based on
the optical and X-ray data, we deduce information on the SED of the afterglow,
the position of the cooling frequency in the SED, the nature of the circumburst
environment, its collimation factor, and its energetics. We find that the GRB
fireball was expanding into a constant-density medium and that the explosion
was collimated with a narrow half-opening angle of about 2.4 degrees. The
initial Lorentz factor of the fireball was about 250; however, its
beaming-corrected energy release in the gamma-ray band was comparably low. The
optical, X-ray afterglow, on the other hand, was rather luminous. Finally, we
find that the data are consistent within the error bars with an achromatic
evolution of the afterglow during the suspected jet break time at about 0.27
days after the burst.

http://arxiv.org/abs/0804.2457

Zhuo Li (PKU)

The gamma-rays from gamma-ray bursts (GRBs) are believed to be produced by
internal shocks driven by small timescale, ~1 ms, variation in the GRB
outflows, and a pair-production spectral cutoff is generally expected around
the GeV range. However, the observed optical flashes accompanying GRBs suggest
that the delayed residual collisions due to large timescale variation continue
to accelerate electrons. We show here that the inverse-Compton (IC) scattering
of the prompt gamma-rays by these residual internal shock electrons leads to a
high energy emission beyond the previously thought spectral cutoff, in
agreement with the previous detections of GeV photons by EGRET in several GRBs
in conjunction with MeV emission. We expect a spectral break due to the
transition from the primary to residual internal shock emission at the
previously thought spectral cutoff, and expect systematic time delays of high
energy photons relative to MeV emission, the discovery of which would provide
stringent constraint on the outflow properties, but requires large enough
collection of high energy photons by, e.g., Fermi and AGILE satellites.

The recent Fermi-detected bright GRB 080916c unambiguously shows the shifting
of the prompt emission toward later times as the photon energy increases, as
predicted in the residual collision model. The detected prompt, highest energy
photon (about 70 GeV in the rest frame of the GRB) may not be produced by
primary internal shocks but by IC emission in residual collisions. With the
method involving time delays of high energy emission, the bulk Lorentz factor
of GRB 080916c is determined to be \Gamma ~ 300.

http://arxiv.org/abs/0810.2932

X. Dai (1,2), K. Z. Stanek (1), P. M. Garnavich (3) ((1)Ohio State Univ. (2) Univ. of Michigan (3) Univ. of Notre Dame)

Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB
afterglows to systematically probe the late time behaviors of afterglows
including jet breaks, flares, and supernova bumps. In particular, the LBT
observations have typical flux limits of 25-26 mag in the Sloan r' band, which
allows us to extend the temporal baseline for measuring jet breaks by another
decade in time scale. We detected four jet breaks (including a “textbook” jet
break in GRB070125) and a fifth candidate, all of which are not detectable
without deep, late time optical observations. In the other four cases, we do
not detect the jet breaks either because of contamination from the host galaxy
light, the presence of a supernova bump, or the intrinsic faintness of the
optical afterglow. This suggests that the basic picture that GRBs are
collimated is still valid and that the apparent lack of Swift jet breaks is due
to poorly sampled afterglow light curves, particularly at late times. Besides
the jet breaks, we also detected late time flares, which could attribute to
late central engine activities, and two supernova bumps.

http://arxiv.org/abs/0902.3285

D.Fargion, D. D'Armiento

After a decade of Fireball reign there is a hope for thin collimated Jet to
solve the Supernova-GRB mystery

http://arxiv.org/abs/0902.3293