astro-grb

A. O. Jaunsen, E. Rol, D. J. Watson, D. Malesani, J. P. U. Fynbo, B. Milvang-Jensen, J. Hjorth, P. M. Vreeswijk, J. -E. Ovaldsen, K. Wiersema, N. R. Tanvir, J. Gorosabel, A. J. Levan, M. Schirmer, A. J. Castro-Tirado

We report on the highly extinguished afterglow of GRB 070306 and the
properties of the host galaxy. An optical afterglow was not detected at the
location of the burst, but in near-infrared a doubling in brightness during the
first night and later power-law decay in the K band provided a clear detection
of the afterglow. The host galaxy is relatively bright, R ~ 22.8. An optical
low resolution spectrum revealed a largely featureless host galaxy continuum
with a single emission line. Higher resolution follow-up spectroscopy shows
this emission to be resolved and consisting of two peaks separated by 7 AA,
suggesting it to be [O II] at a redshift of z = 1.49594 +- 0.00006. The
infrared color H-K = 2 directly reveals significant reddening. By modeling the
optical/X-ray spectral energy distribution at t = 1.38 days with an
extinguished synchrotron spectrum, we derive A_V = 5.5 +- 0.6 mag. This is
among the largest values ever measured for a GRB afterglow and visual
extinctions exceeding unity are rare. The importance of early NIR observations
is obvious and may soon provide a clearer view into the once elusive 'dark
bursts'.

http://arxiv.org/abs/0803.4017

N. R. Tanvir, A. J. Levan, E. Rol, R. L. C. Starling, J. Gorosabel, R. S. Priddey, D. Malesani, P. Jakobsson, P. T. O'Brien, A. O. Jaunsen, J. Hjorth, J. P. U. Fynbo, A. Melandri, A. Gomboc, B. Milvang-Jensen, A. S. Fruchter, M. Jarvis, C. A. C. Fernandes, T. Wold

Gamma-ray bursts are powerful probes of the early universe, but locating and
identifying very distant GRBs remains challenging. We report here the discovery
of the K-band afterglow of Swift GRB 060923A, imaged within the first hour
post-burst, and the faintest so far found. It was not detected in any bluer
bands to deep limits, making it a candidate very high redshift burst (z>11).
However, our later-time optical imaging and spectroscopy reveal a faint galaxy
coincident with the GRB position which, if it is the host, implies a more
moderate redshift (most likely z<2.8) and therefore that dust is the likely
cause of the very red afterglow colour. This being the case, it is one of the
few instances so far found of a GRB afterglow with high dust extinction.

http://arxiv.org/abs/0803.4100

Alexander Tchekhovskoy (1), Jonathan C. McKinney (2), Ramesh Narayan (1) ((1) Harvard CfA/ITC, (2) Stanford University/KIPAC)

Gamma-ray bursts (GRBs) require an engine capable of driving a jet of plasma
to ultrarelativistic bulk Lorentz factors of up to several hundred and into
narrow opening angles of a few degrees. We test whether the popular
magnetic-driving paradigm can generate the required Lorentz factors and opening
angles by finding global axisymmetric stationary solutions of
magnetically-dominated (electromagnetically pure) ultrarelativistic jets. Our
global solutions are obtained via time-dependent ideal relativistic
magnetodynamical numerical simulations that follow the jet from the central
engine to beyond six orders of magnitude in radius. Our model is primarily
motivated by the collapsar model, in which a jet is produced by a spinning
black hole or neutron star and then propagates through a massive stellar
envelope. However, the results can be applied to other GRB models and other
systems with relativistic jets. We find that the size of (and pressures within)
the presupernova progenitor star can determine the terminal Lorentz factor and
opening angle of the jet independent of the jet's initial magnetization. At the
radius where the jet breaks out of the star, our fiducial model generates a
Lorentz factor $\gamma\sim 400$ and a half-opening angle $\theta_j\sim
2^\circ$, consistent with observations of most long-duration GRBs. Other models
give $\gamma$ in the range 100 to 5000 and $\theta_j$ from $0.1^\circ$ to
$10^\circ$, thus reproducing the range of properties inferred for GRB jets. A
potentially observable feature of our solutions is that the maximum Poynting
flux in the jet is found at $\theta \sim \theta_j$ with the jet power
concentrated in a hollow cone, while the maximum in the Lorentz factor occurs
at an angle $\theta$ much smaller than $\theta_j$ [abridged].

http://arxiv.org/abs/0803.3807

Kevin Heng, Davide Lazzati, Rosalba Perna, Peter Garnavich, Alberto Noriega-Crespo, David Bersier, Thomas Matheson, Michael Pahre

We present a proof-of-concept study that dust extinction curves can be
extracted from the infrared (IR), optical, ultraviolet (UV) and X-ray afterglow
observations of GRBs without assuming known extinction laws. We focus on GRB
050525A (z = 0.606), for which we also present IR observations from the Spitzer
Space Telescope at about 2.3 days post-burst. We construct the spectral energy
distribution (SED) of the afterglow and use it to derive the dust extinction
curve of the host galaxy in 7 optical/UV wavebands. By comparing our derived
extinction curve to known templates, we see that the Galactic or Milky Way
extinction laws are disfavored versus the Small and Large Magellanic Cloud (SMC
and LMC) ones, but that we cannot rule out the presence of a LMC-like 2175
angstrom bump in our extinction curve. The dust-to-gas ratio present within the
host galaxy of GRB 050525A is similar to that found in the LMC, while about 10
to 40% more dust is required if the SMC template is assumed. Our method is
useful to observatories that are capable of simultaneously observing GRB
afterglows in multiple wavebands from the IR to the X-ray.

http://arxiv.org/abs/0803.2879

J.S. Bloom (1,2), D. A. Perley (1), W. Li (1), N. R. Butler (1), A. A. Miller (1), D. Kocevski (1), D. A. Kann (3), R. J. Foley (1), H.-W. Chen (4), A. V. Filippenko (1), D. L. Starr (1,5), B. Macomber (1), J. X. Prochaska (6), R. Chornock (1), D. Poznanski (1), S. Klose (3) (1. UCB, 2. Sloan, 3. Tautenburg, 4. U of Chicago, 5. LCOGT, 6. UCO/Lick Observatory)

The first gamma-ray burst (GRB) confirmed to be bright enough to be seen with
the naked eye, GRB 080319B, allowed for exquisite follow-up across the
electromagnetic spectrum. We present our detailed optical and infrared
observations of the afterglow, consisting of over 5000 images starting 122 s
after the GRB trigger, in concert with our own analysis of the Swift UVOT, BAT,
and XRT data. The event is extreme not only in observed properties but
intrinsically: it was the most luminous ever recorded at optical wavelengths
and had an exceedingly high isotropic-equivalent energy release in gamma-rays.
At early times, the afterglow evolution is broadly consistent with being
reverse-shock dominated, then is subsumed by a forward shock at around 1000 s.
Analysis of the forward shock suggests that the remarkable energetics of this
burst may be owed largely to extreme collimation. The spectral energy
distribution, spanning from ultraviolet through near-infrared, shows no
evidence for a significant amount of dust extinction in the host frame. We do
find significant color evolution in the optical afterglow: starting at about
1000 s the index shifts blueward before apparently shifting back to the red at
late times. Finally, we examine the detectability of such events with current
and future facilities and find that such an event could be detected in
gamma-rays by BAT out to z = 4.9 (8 sigma), while the nominal EXIST sensitivity
would allow detection to z = 12.2. At K-band this source would have been easily
detected with meter-class telescopes to z = 17.

http://arxiv.org/abs/0803.3215

Francesca Matteucci (Dipartimento di Astronomia, Universita di Trieste; INAF, Trieste)

I will review the role of massive stars in galactic evolution both from the
nucleosynthesis and energetics point of view. In particular, I will highlight
some important observational facts explained by means of massive stars in
galaxies of different morphological type: the Milky Way, ellipticals and dwarf
spheroidals. I will describe first the time-delay model and its interpretation
in terms of abundance ratios in galaxies, then I will discuss the importance of
mass loss in massive stars to reproduce the data in the Galactic bulge and
disk. I will discuss also how massive stars can be important producers of
primary nitrogen if rotation in stellar models is taken into account.
Concerning elliptical galaxies, I will show that to reproduce the observed
[Mg/Fe] versus Mass relation in these galaxies it is necessary to assume a more
important role of massive stars in more massive galaxies and that this can be
achieved by means of downsizing in star formation. I will discuss how massive
stars are responsible in triggering galactic winds both in ellipticals and
dwarf spheroidals. These latter systems show a low overabundance of
alpha-elements relative to Fe with respect to Galactic stars of the same
[Fe/H]: this is interpreted as due to a slow star formation coupled with very
efficient galactic winds. Finally, I will show a comparison between the
predicted Type Ib/c rates in galaxies and the observed GRB rate and how we can
impose constraints on the mechanism of galaxy formation by studying the GRB
rate at high redshift.

http://arxiv.org/abs/0803.3016

Kevin Heng, Davide Lazzati, Rosalba Perna, Peter Garnavich, Alberto Noriega-Crespo, David Bersier, Thomas Matheson, Michael Pahre

We present a proof-of-concept study that dust extinction curves can be
extracted from the infrared (IR), optical, ultraviolet (UV) and X-ray afterglow
observations of GRBs without assuming known extinction laws. We focus on GRB
050525A (z = 0.606), for which we also present IR observations from the Spitzer
Space Telescope at about 2.3 days post-burst. We construct the spectral energy
distribution (SED) of the afterglow and use it to derive the dust extinction
curve of the host galaxy in 7 optical/UV wavebands. By comparing our derived
extinction curve to known templates, we see that the Galactic or Milky Way
extinction laws are disfavored versus the Small and Large Magellanic Cloud (SMC
and LMC) ones, but that we cannot rule out the presence of a LMC-like 2175
angstrom bump in our extinction curve. The dust-to-gas ratio present within the
host galaxy of GRB 050525A is similar to that found in the LMC, while about 10
to 40% more dust is required if the SMC template is assumed. Our method is
useful to observatories that are capable of simultaneously observing GRB
afterglows in multiple wavebands from the IR to the X-ray.

http://arxiv.org/abs/0803.2879

S. Savaglio (MPE), K. Glazebrook (Swinburne University), D. Le Borgne (CEA/Saclay)

We present the most extensive and complete study of the properties for the
largest sample (46 objects) of gamma-ray burst (GRB) host galaxies. The
redshift interval and the mean redshift of the sample are 0<z<6.3 and z=0.96
(look-back time: 7.2 Gyrs), respectively; 89% of the hosts are at z < 1.6.
Optical-NIR photometry and spectroscopy are used to derive stellar masses, star
formation rates, dust extinctions and metallicities. The average stellar mass
is 10^9.3 M_sun, with a 1 sigma dispersion of 0.8 dex. The average metallicity
for a subsample of 17 hosts is about 1/4 solar and the dust extinction in the
visual band (for a subsample of 10 hosts) is A_V=0.5. We obtain new relations
to derive SFR from [OII] or UV fluxes, when Balmer emission lines are not
available. SFRs, corrected for dust extinction, aperture slit loss and stellar
Balmer absorption, are in the range 0.01-36 M_sun yr^-1. The median SFR per
unit stellar mass (specific star formation rate) is 0.8 Gyr^-1. Equivalently
the inverse quantity, the median formation time scale is 1.3 Gyr. Most GRBs are
associated with the death of young massive stars, more common in star-forming
galaxies. Therefore GRBs are an effective tool to detect star-forming galaxies
in the universe. Star-forming galaxies at z<1.6 are a faint and low-mass
population, hard to detect for the deepest conventional optical-NIR surveys
possible today, unless a GRB event occurs. There is no compelling evidence that
GRB hosts are peculiar galaxies. More data on the sub-class of short GRB are
necessary to establish the nature of their hosts.

http://arxiv.org/abs/0803.2718

Keiichi Maeda, Koji Kawabata, Paolo A. Mazzali, Masaomi Tanaka, Stefano Valenti, Ken'ichi Nomoto, Takashi Hattori, Jinsong Deng, Elena Pian, Stefan Taubenberger, Masanori Iye, Thomas Matheson, Alexei V. Filippenko, Kentaro Aoki, George Kosugi, Youichi Ohyama, Toshiyuki Sasaki, Tadafumi Takata

Core-collapse supernovae (CC-SNe) are the explosions that announce the death
of massive stars. Some CC-SNe are linked to long-duration gamma-ray bursts
(GRBs) and are highly aspherical. One important question is to what extent
asphericity is common to all CC-SNe. Here we present late-time spectra for a
number of CC-SNe from stripped-envelope stars, and use them to explore any
asphericity generated in the inner part of the exploding star, near the site of
collapse. A range of oxygen emission-line profiles is observed, including a
high incidence of double-peaked profiles, a distinct signature of an aspherical
explosion. Our results suggest that all CC-SNe from stripped-envelope stars are
aspherical explosions and that SNe accompanied by GRBs exhibit the highest
degree of asphericity.

http://arxiv.org/abs/0801.1100

J. M. Castro Cerón (1), M. J. Michałowski (1), J. Hjorth (1), D. Malesani (1), J. Gorosabel (2), D. Watson (1), J. P. U. Fynbo (1); (1. Dark Cosmology Centre (NBI) Copenhagen; 2. Instituto de Astrofísica de Andalucía (CSIC) Granada)

We analyse Spitzer images of 30 long-duration gamma-ray burst (GRB) host
galaxies. We estimate their total stellar masses (M*) based on the rest-frame
K-band luminosities and constrain their unobscured star-formation rates (SFR)
based on the rest-frame UV continuua. Further, we compute a mean value for
M*/L_Krest = 0.45 Msun/Lsun. We find that the hosts are low M*, star-forming
systems. The median M* in our sample (<M*> = 10^9.7 Msun) is lower than that of
“field” galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M* is
10^7 Msun < M* < 10^11 Msun, while the range spanned by the unobscured SFR is
10^-2 Msun yr^-1 < SFR < 10 Msun yr^-1. There is no evidence for intrinsic
evolution in the distribution of M* with redshift. We show that extinction by
dust must be present in at least 25% of the GRB hosts in our sample, and
suggest that this is a way to reconcile our lower, UV-based, specific SFR (PHI
= SFR/M*) with previous claims that GRBs have some of the highest PHI values.
We also examine the effect that the inability to resolve the star-forming
regions in the hosts has on PHI.

http://arxiv.org/abs/0803.2235