astro-grb

B.D. Metzger (Princeton University)

Discovered over forty years ago, Gamma-Ray Bursts (GRBs) remain a forefront
topic in modern astrophysics. Perhaps the most fundamental question associated
with GRBs is the nature of the astrophysical agent (or agents) that ultimately
powers them: the central engine. In this review, I focus on the possible
central engines of long-duration GRBs, and the constraints that present
observations place on these models. Long GRBs are definitively associated with
the deaths of massive stars, but whether the central engine is an accreting
black hole or a rapidly-spinning, highly-magnetized neutron star (a
“proto-magnetar”) remains unsettled. This distinction has been brought into
particular focus by recent MHD simulations of the core-collapse of massive,
rotating “collapsar progenitors,” which suggest that powerful
magneto-centrifugal outflows from the proto-neutron star may stave off black
hole formation entirely. Although both black hole and magnetar GRB models
remain viable, I argue that the magnetar model is more mature in the sense that
it provides quantitative explanations for the durations, energies, Lorentz
factors, and collimation of long GRB outflows. Given these virtues, one
promising strategy to break the present stalemate is to further develop the
magnetar model until inescapable (and falsifiable) predictions emerge. This
course of action signals a renewed challenge to translate time-dependent jet
properties (power, magnetization, and Lorentz factor) into observables
(gamma-ray light curves and spectra).

http://arxiv.org/abs/1001.5046

Z. Paragi (1 and 2), G. B. Taylor (3), C. Kouveliotou (4), J. Granot (5), E. Ramirez-Ruiz (6), M. Bietenholz (7 and 8), A. J. van der Horst (4), Y. Pidopryhora (1), H. J. van Langevelde (1 and 10), M. A. Garrett (9, 10, 11), A. Szomoru (1), M. Argo (12), S. Bourke (1), B. Paczynski (*) ((1) Joint Institute for VLBI in Europe, (2) MTA Research Group for Physical Geodesy and Geodynamics (3) University of New Mexico, Department of Physics and Astronomy (4) Space Science Office, NASA/Marshall Space Flight Center (5) Centre for Astrophysics Research, University of Hertfordshire (6) Department of Physics and Astronomy, University of California (7) Hartebeesthoek Radio Observatory (8) Department of Physics and Astronomy, York University (9) Netherlands Institute for Radio Astronomy (ASTRON) (10) Leiden Observatory, Leiden University (11) Centre for Astrophysics and Supercomputing, Swinburne University of Technology (12) Curtin Institute of Radio Astronomy, Curtin University of Technology; * Deceased.)

The class of type Ic supernovae have drawn increasing attention since 1998
owing to their sparse association (only four so far) with long duration
gamma-ray bursts. Although both phenomena originate from the core collapse of a
massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit
mostly in soft gamma-rays or hard X-rays. Though the GRB central engine
generates ultra-relativistic jets, which beam the early emission into a narrow
cone, no relativistic outflows have hitherto been found in type Ib/c supernovae
explosions, despite theoretical expectations and searches. Here we report radio
(interferometric) observations that reveal a mildly relativistic expansion in a
nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days
apart, we detect expansion of the source and establish a conservative lower
limit for the average apparent expansion velocity of 0.6c. Independently, a
second mildly relativistic supernova has been reported. Contrary to the radio
data, optical observations of SN 2007gr indicate a typical type Ic supernova
with ejecta velocities ~6000 km/s, much lower than in GRB-associated
supernovae. We conclude that in SN 2007gr a small fraction of the ejecta
produced a low-energy mildly relativistic bipolar radio jet, while the bulk of
the ejecta were slower and, as shown by optical spectro-polarimetry, mildly
aspherical.

http://arxiv.org/abs/1001.5060

K. M. Svensson, A. J. Levan, N. R. Tanvir, A. S. Fruchter, L. -G. Strolger

We present a comparative study of the galactic and small scale environments
of gamma-ray bursts (GRB) and core collapse supernovae (CCSN). We use a sample
of 34 GRB hosts at z<1.2, and a comparison sample of 58 supernova hosts located
within the Great Observatories Origins Deep Survey footprint. We fit template
spectra to the available photometric data, which span the range 0.45-24 micron,
and extract absolute magnitudes, stellar masses and star formation rates from
the resulting fits. Our results broadly corroborate previous findings, but
offer significant enhancements in spectral coverage and a factor 2-3 increase
in sample size. Specifically, we find that CCSN occur frequently in massive
spirals (spiral fraction ~50%). In contrast GRBs occur in small, relatively low
mass galaxies with high specific and surface star formation rates, and have a
spiral fraction of only ~10%. A comparison of the rest frame absolute
magnitudes of the GRB and CCSN sample is less conclusive than found in previous
work, suggesting that while GRB hosts are typically both smaller and bluer than
those of CCSN their total blue light luminosities are only slightly lower. We
suggest this is likely due to rapid periods of intensified star formation
activity, as indicated by the high specific star formation rates, which both
create the GRB progenitors and briefly significantly enhance the host galaxy
blue luminosity. Finally, our analysis of local environments of GRBs and CCSN
shows that GRBs are highly concentrated on their host light, and further occur
in regions of higher absolute surface luminosity than CCSN.

http://arxiv.org/abs/1001.5042

Daniel A. Perley, J. S. Bloom, C. R. Klein, S. Covino, T. Minezaki, P. Wozniak, W. T. Vestrand, G. G. Williams, P. Milne, N. R. Butler, A. C. Updike, T. Krühler, P. Afonso, A. Antonelli, L. Cowie, P. Ferrero, J. Greiner, D. H. Hartmann, Y. Kakazu, A. Küpcü Yoldas, A. N. Morgan, P. A. Price, J. X. Prochaska, Y. Yoshii

We present observations and analysis of the broadband afterglow of Swift GRB
071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric
redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour
observations begin at ~30 s after the GRB trigger in the host frame and during
the initial rising phase of the afterglow. We associate the light curve peak at
580 s in the observer frame with the formation of the forward shock, giving an
estimate of the initial Lorentz factor Gamma_0 ~ 200. The red spectral energy
distribution (even in regions not affected by the Lyman-alpha break) provides
secure evidence of a large dust column. However, the inferred extinction curve
shows a prominent flat component between 2000-3000 Angstroms in the rest-frame,
inconsistent with any locally observed template but well-fit by models of dust
formed by supernovae. Time-dependent fits to the extinction profile reveal no
evidence of dust destruction and limit the decrease in the extinction column to
Delta A_3000 < 0.54 mag after t = 50 s in the rest frame. Our observations
provide evidence of a transition in dust properties at z~5, in agreement with
studies of high-z quasars, and suggest that SN-formed dust continues to
dominate the opacity of typical galaxies at this redshift.

http://arxiv.org/abs/0912.2999

A. M. Soderberg, S. Chakraborti, G. Pignata, R. A. Chevalier, P. Chandra, A. Ray, M. H. Wieringa, A. Copete, V. Chaplin, V. Connaughton, S. D. Barthelmy, M. F. Bietenholz, N. Chugai, M. D. Stritzinger, M. Hamuy, C. Fransson, O. Fox, E. M. Levesque, J. E. Grindlay, P. Challis, R. J. Foley, R. P. Kirshner, P. A. Milne, M. A. P. Torres

Long duration gamma-ray bursts (GRBs) mark the explosive death of some
massive stars and are a rare sub-class of Type Ibc supernovae (SNe Ibc). They
are distinguished by the production of an energetic and collimated relativistic
outflow powered by a central engine (an accreting black hole or neutron star).
Observationally, this outflow is manifested in the pulse of gamma-rays and a
long-lived radio afterglow. To date, central engine-driven SNe have been
discovered exclusively through their gamma-ray emission, yet it is expected
that a larger population goes undetected due to limited satellite sensitivity
or beaming of the collimated emission away from our line-of-sight. In this
framework, the recovery of undetected GRBs may be possible through radio
searches for SNe Ibc with relativistic outflows. Here we report the discovery
of luminous radio emission from the seemingly ordinary Type Ibc SN 2009bb,
which requires a substantial relativistic outflow powered by a central engine.
The lack of a coincident GRB makes SN 2009bb the first engine-driven SN
discovered without a detected gamma-ray signal. A comparison with our extensive
radio survey of SNe Ibc reveals that the fraction harboring central engines is
low, ~1 percent, measured independently from, but consistent with, the inferred
rate of nearby GRBs. Our study demonstrates that upcoming optical and radio
surveys will soon rival gamma-ray satellites in pinpointing the nearest
engine-driven SNe. A similar result for a different supernova is reported
independently.

http://arxiv.org/abs/0908.2817

Shlomo Dado, Arnon Dar

The many similarities between the prompt emission pulses in gamma ray bursts
(GRBs) and X-ray flares during the fast decay and afterglow phases of GRBs
suggest a common origin. In the cannonball (CB) model of GRBs, this common
origin is mass accretion episodes of fall-back matter on a newly born compact
object. The prompt emission pulses are produced by a bipolar jet of highly
relativistic plasmoids (CBs) ejected in the early, major episodes of mass
accretion. As the accretion material is consumed, one may expect the engine's
activity to weaken. X-ray flares ending the prompt emission and during the
afterglow phase are produced in such delayed episodes of mass accretion. The
common engine, environment and radiation mechanisms (inverse Compton scattering
and synchrotron radiation) produce their observed similarities. Flares in both
long GRBs and short hard gamma ray bursts (SHBs) can also be produced by
bipolar ejections of CBs following a phase transition in compact objects due to
loss of angular momentum and/or cooling. Optical flares, however, are mostly
produced in collisions of CBs with massive stellar winds/ejecta or with density
bumps along their path. In this paper we show that the master formulae of the
CB model of GRBs and SHBs, which reproduce very well their prompt emission
pulses and their smooth afterglows, seem to reproduce also very well the
lightcurves and spectral evolution of the prominent X-ray and optical flares
that are well sampled.

http://arxiv.org/abs/0908.0650

Dirk Grupe (PSU), David N. Burrows (PSU), Xue-Feng Wu (PSU), Xiang-Yu Wang (Nanjing University), Bing Zhang (University of Nevada), En-Wei Liang (Guangxi University), Gordon Garmire (PSU), John A. Nousek (PSU), Neil Gehrels (NASA/GSFC), George Ricker (MIT), Marshall W. Bautz (MIT)

We report on 5 Chandra observations of the X-ray afterglow of the Gamma-Ray
Burst GRB 060729 performed between 2007 March and 2008 May. In all five
observations the afterglow is clearly detected. The last Chandra pointing was
performed on 2008-May-04, 642 days after the burst - the latest detection of a
GRB X-ray afterglow ever. A reanalysis of the Swift XRT light curve together
with the three detections by Chandra in 2007 reveals a break at about 1.0 Ms
after the burst with a slight steepening of the decay slope from alpha = 1.32
to 1.61. This break coincides with a significant hardening of the X-ray
spectrum, consistent with a cooling break in the wind medium scenario, in which
the cooling frequency of the afterglow crosses the X-ray band. The last two
Chandra observations in 2007 December and 2008 May provide evidence for another
break at about one year after the burst. If interpreted as a jet break, this
late-time break implies a jet half opening angle of about 14 degrees for a wind
medium. Alternatively, this final break may have a spectral origin, in which
case no jet break has been observed and the half-opening angle of the jet of
GRB 060729 must be larger than about 15 degrees for a wind medium. We compare
the X-ray afterglow of GRB 060729 in a wind environment with other bright X-ray
afterglows, in particular GRBs 061121 and 080319B, and discuss why the X-ray
afterglow of GRB 060729 is such an exceptionally long-lasting event.

http://arxiv.org/abs/0903.1258

T. N. Ukwatta, M. Stamatikos, K. S. Dhuga, T. Sakamoto, S. D. Barthelmy, A. Eskandarian, N. Gehrels, L. C. Maximon, J. P. Norris, W. C. Parke

Spectral lag, the time difference between the arrival of high-energy and
low-energy photons, is a common feature in Gamma-ray Bursts (GRBs). Norris et
al. 2000 reported a correlation between the spectral lag and the isotropic peak
luminosity of GRBs based on a limited sample. More recently, a number of
authors have provided further support for this correlation using arbitrary
energy bands of various instruments. In this paper we report on a systematic
extraction of spectral lags based on the largest Swift sample to date of 31
GRBs with measured redshifts. We extracted the spectral lags for all
combinations of the standard Swift hard x-ray energy bands: 15-25 keV, 25-50
keV, 50-100 keV and 100-200 keV and plotted the time dilation corrected lag as
a function of isotropic peak luminosity. The mean value of the correlation
coefficient for various channel combinations is -0.68 with a chance probability
of ~ 0.7 x 10^{-3}. In addition, the mean value of the power-law index is 1.4
+/- 0.3. Hence, our study lends support for the existence of a lag-luminosity
correlation, albeit with large scatter.

http://arxiv.org/abs/0908.2370

A. de Ugarte Postigo, P. Goldoni, C.C. Thöne, S.D. Vergani, V. D'Elia, S. Piranomonte, D. Malesani, S. Covino, H. Flores, J.P.U. Fynbo, J. Hjorth, R.A.M.J. Wijers, S. D'Odorico, F. Hammer, L. Kaper, P. Kjærgaard, S. Randich, M.I. Andersen, L.A. Antonelli, L. Christensen, P. D'Avanzo, F. Fiore, P.J. Groot, E. Maiorano, E. Palazzi, E. Pian, G. Tagliaferri, M.E. van den Ancker, J. Vernet

Context. X-shooter is the first second-generation instrument to become
operative at the ESO Very Large Telescope (VLT). It is a broad-band
medium-resolution spectrograph designed with gamma-ray burst (GRB) afterglow
spectroscopy as one of its main science drivers.

Aims. During the first commissioning night on sky with the instrument fully
assembled, X-shooter observed the afterglow of GRB 090313 as a demonstration of
the instrument's capabilities.

Methods. GRB 090313 was observed almost two days after the burst onset, when
the object had already faded to R~21.6. Furthermore, the 90% illuminated Moon
was just 30 degrees away from the field. In spite of the adverse conditions, we
obtained a spectrum that, for the first time in GRB research, covers
simultaneously the range from 5700 to 23000 Angstroms.

Results. The spectrum shows multiple absorption features at a redshift of
3.3736, the redshift of the GRB. These features are composed of 3 components
with different ionisation levels and velocities. Some of the features have
never been observed before in a GRB at such a high redshift. Furthermore, we
detect two intervening systems at redshifts of 1.8005 and 1.9597.

Conclusions. These results demonstrate the potential of X-shooter in the GRB
field, as it was capable of observing a GRB down to a magnitude limit that
would include 72% of long GRB afterglows 2 hours after the burst onset. Coupled
with the rapid response mode available at VLT, allowing reaction times of just
a few minutes, X-shooter constitutes an important leap forward on medium
resolution spectroscopic studies of GRBs, their host galaxies and intervening
systems, probing the early history of the Universe.

http://arxiv.org/abs/0911.3901

Nathaniel R. Butler, Joshua S. Bloom, Dovi Poznanski

We calculate durations and spectral parameters for 207 Swift bursts detected
by the BAT instrument from April 2007 to August 2009, including 67 events with
measured redshifts. This is the first supplement to our catalog of 425 Swift
GRBs (147 with redshifts) starting from GRB041220. This complete and extensive
data set, analyzed with a unified methodology, allows us to conduct an accurate
census of intrinsic GRB energetics, hardnesses, durations, and redshifts. The
GRB world model we derive reproduces well the observables from both Swift and
pre-Swift satellites. Comparing to the cosmic star formation rate, we estimate
that only about 0.1% of massive stars explode as bright GRBs. There is strong
evidence for evolution in the Swift population at intermediate and high-z, and
we can rule out (at the 5-sigma level) that this is due to evolution in the
luminosity function of GRBs. Instead, the Swift sample suggests a modest
propensity for low-metallicity, evidenced by an increase in the rate density
with redshift. Treating the multivariate data and selection effects rigorously,
we find a real, intrinsic correlation between E_iso and E_pk (and possibly also
duration); however, the correlation {\it is not} a narrow log-log relation and
its observed appearance is strongly detector-dependent. We also estimate the
high-z rate (3–9% of GRBs at z beyond 5) and discuss the extent of a large
missing population of low-E_pk XRFs as well as a potentially large missing
population of short-duration GRBs that will be probed by EXIST.

http://arxiv.org/abs/0910.3341