35 research outputs found
Afterglows of Gamma-Ray Bursts: Short vs. Long GRBs
We compiled a large sample of Swift-era photometric data on long (Type II)
and short (Type I) GRB afterglows. We compare the luminosity and energetics of
the different samples to each other and to the afterglows of the pre-Swift era.
Here, we present the first results of these studies.Comment: Conference Proceedings, "Gamma-Ray Bursts 2007", Santa Fe, shortened
poster presentation; 4 pages, 3 figures; for full updated papers, go here to
arXiv:0712.2186 and also here to arXiv:0804.195
Follow-up observations from observatories based in Spain
We present a review of the follow-up observations carried out from
observatories located in Spain; Calar-Alto, Izanha and Roque de Los Muchachos.
It summarizes the observations carried out by our group for 27 GRBs occurred in
the period 1999-2000, spanning from GRB 990123 to GRB 001007.Comment: 3 pages, no figures. To appear in the proceedings of the October 2000
Rome Workshop on ``Gamma-Ray Bursts in the Afterglow Era'
The Discovery of an Embedded Cluster of High-Mass Stars Near SGR 1900+14
Deep I-band imaging to approximately I = 26.5 of the soft gamma-ray repeater
SGR 1900+14 region has revealed a compact cluster of massive stars located only
a few arcseconds from the fading radio source thought to be the location of the
SGR (Frail, Kulkarni, & Bloom 1999). This cluster was previously hidden in the
glare of the pair of M5 supergiant stars (whose light was removed by PSF
subtraction) proposed by Vrba et al. (1996) as likely associated with the SGR
1900+14. The cluster has at least 13 members within a cluster radius of
approximately 0.6 pc, based on an estimated distance of 12-15 kpc. It is
remarkably similar to a cluster found associated with SGR 1806-20 (Fuchs et al.
1999). That similar clusters have now been found at or near the positions of
the two best-studied SGRs suggests that young neutron stars, thought to be
responsible for the SGR phenomenon, have their origins in proximate compact
clusters of massive stars.Comment: 5 pages, 3 figures, accepted Astrophysical Journal Letter
Clustering of galaxies around GRB sight-lines
There is evidence of an overdensity of strong intervening MgII absorption
line systems distributed along the lines of sight towards GRB afterglows
relative to quasar sight-lines. If this excess is real, one should also expect
an overdensity of field galaxies around GRB sight-lines, as strong MgII tends
to trace these sources. In this work, we test this expectation by calculating
the two point angular correlation function of galaxies within
120 ( at ) of GRB afterglows. We compare the Gamma-ray burst Optical and
Near-infrared Detector (GROND) GRB afterglow sample -- one of the largest and
most homogeneous samples of GRB fields -- with galaxies and AGN found in the
COSMOS-30 photometric catalog. We find no significant signal of anomalous
clustering of galaxies at an estimated median redshift of around GRB
sight-lines, down to . This result is contrary to the
expectations from the MgII excess derived from GRB afterglow spectroscopy,
although many confirmed galaxy counterparts to MgII absorbers may be too faint
to detect in our sample -- especially those at . We note that the addition
of higher sensitivity Spitzer IRAC or HST WFC3 data for even a subset of our
sample would increase this survey's depth by several orders of magnitude,
simultaneously increasing statistics and enabling the investigation of a much
larger redshift space.}Comment: 10 pages, 6 figures. A&A accepte
Gamma-Ray Bursts Trace UV Metrics of Star Formation over 3 < z < 5
We present the first uniform treatment of long duration gamma-ray burst (GRB)
host galaxy detections and upper limits over the redshift range 3<z<5, a key
epoch for observational and theoretical efforts to understand the processes,
environments, and consequences of early cosmic star formation. We contribute
deep imaging observations of 13 GRB positions yielding the discovery of eight
new host galaxies. We use this dataset in tandem with previously published
observations of 31 further GRB positions to estimate or constrain the host
galaxy rest-frame ultraviolet (UV; 1600 A) absolute magnitudes M_UV. We then
use the combined set of 44 M_UV estimates and limits to construct the M_UV
luminosity function (LF) for GRB host galaxies over 3<z<5 and compare it to
expectations from Lyman break galaxy (LBG) photometric surveys with the Hubble
Space Telescope. Adopting standard prescriptions for the luminosity dependence
of galaxy dust obscuration (and hence, total star formation rate), we find that
our LF is compatible with LBG observations over a factor of 600x in host
luminosity, from M_UV = -22.5 mag to >-15.6 mag, and with extrapolations of the
assumed Schechter-type LF well beyond this range. We review proposed
astrophysical and observational biases for our sample, and find they are for
the most part minimal. We therefore conclude, as the simplest interpretation of
our results, that GRBs successfully trace UV metrics of cosmic star formation
over the range 3<z<5. Our findings suggest GRBs are providing an accurate
picture of star formation processes from z ~3 out to the highest redshifts.Comment: publ. ApJ 809 (2015) 76; 14 figures; replacement to reflect changes
to v1 (rounding effects, diff. LF from Bouwens
GRB 091127: The cooling break race on magnetic fuel
Using high-quality, broad-band afterglow data for GRB 091127, we investigate
the validity of the synchrotron fireball model for gamma-ray bursts, and infer
physical parameters of the ultra-relativistic outflow. We used multi-wavelength
follow-up observations obtained with GROND and the XRT onboard the Swift
satellite. The resulting afterglow light curve is of excellent accuracy, and
the spectral energy distribution is well-sampled over 5 decades in energy.
These data present one of the most comprehensive observing campaigns for a
single GRB afterglow and allow us to test several proposed emission models and
outflow characteristics in unprecedented detail. Both the multi-color light
curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a
cooling break moving from high to lower energies. The early light curve is well
described by a broken power-law, where the initial decay in the optical/NIR
wavelength range is considerably flatter than at X-rays. Detailed fitting of
the time-resolved SED shows that the break is very smooth with a sharpness
index of 2.2 +- 0.2, and evolves towards lower frequencies as a power-law with
index -1.23 +- 0.06. These are the first accurate and contemporaneous
measurements of both the sharpness of the spectral break and its time
evolution. The measured evolution of the cooling break (nu_c propto t^-1.2) is
not consistent with the predictions of the standard model, wherein nu_c propto
t^-0.5 is expected. A possible explanation for the observed behavior is a time
dependence of the microphysical parameters, in particular the fraction of the
total energy in the magnetic field epsilon_B. This conclusion provides further
evidence that the standard fireball model is too simplistic, and time-dependent
micro-physical parameters may be required to model the growing number of
well-sampled afterglow light curves.Comment: accepted to A&A, 13 pages, 5 figure
Evolution of the Polarization of the Optical Afterglow of the Gamma-ray Burst GRB 030329
The association of a supernova with GRB 0303291,2 strongly supports the collapsar model3 of -ray bursts (GRBs), where a relativistic jet4 forms after 1 the progenitor star collapses. Such jets cannot be spatially resolved because of their cosmological distances. Their existence is conjectured based on breaks in GRB afterglow light curves and the theoretical desire to reduce the GRB energy requirements. Temporal evolution of polarization5,6,7 may provide independent evidence for the jet structure of the relativistic outflow. Small-level polarization ( 1-3%)8−17 has been reported for a few bursts, but the temporal evolution of polarization properties could not be established. Here, we report polarimetric observations of the afterglow of GRB 030329 with high signal-to-noise and high sampling frequency. We establish the polarization light curve, detect sustained polarization at the percent level, and find significant variability. The data imply that the afterglow magnetic field has small coherence length and is mostly random, probably generated by turbulence, in contrast with the high polarization detected in the prompt -rays from GRB 02120618. Our results suggest a different structure and origin of the magnetic field in the prompt vs. afterglow emission regions
The host galaxy of the short GRB 050709
The host of the short gamma-ray burst (GRB) 050709 is a morphologically disturbed low-luminous galaxy. At a redshift of z = 0.16, it belongs to one of the cosmologically nearest short-GRB hosts identified to date. Consequently, it represents a promising target for sensitive, spatially resolved observational studies. We have used the Multi Unit Spectroscopic Explorer (MUSE) mounted at the Very Large Telescope to characterize the GRB host galaxy. In addition, we performed deep radio-continuum observations of the host with the Australia Telescope Compact Array (ATCA) and with ALMA at 1.3 mm. Moreover, we made use of archival Spitzer Space Telescope 24 μm and Hubble Space Telescope/F814W imaging data of this galaxy. The spatially resolved MUSE data reveal that the entire host is a source of strong line emission, in particular from Hα and [O II