24 research outputs found
The Burst Cluster: Dark Matter in a Cluster Merger Associated with the Short Gamma Ray Burst, GRB 050509B
We have identified a merging galaxy cluster with evidence of two distinct
sub-clusters. The X-ray and optical data suggest that the subclusters are
moving away from each other after closest approach. This cluster merger was
discovered from observations of the well localized short-duration gamma-ray
burst (GRB), GRB 050509B. The Swift/Burst Alert Telescope (BAT) source position
is coincident with a cluster of galaxies ZwCl 1234.0+02916. The subsequent
Swift/X-Ray Telescope (XRT) localization of the X-ray afterglow found the GRB
coincident with 2MASX J12361286+2858580, a giant red elliptical galaxy in the
cluster. Deep multi-epoch optical images were obtained to constrain the
evolution of the GRB afterglow, including a 27480s exposure in the F814W band
with Hubble Space Telescope Advanced Camera for Surveys (ACS), among the
deepest imaging ever obtained towards a known galaxy cluster in a single
passband. We perform a weak gravitational lensing analysis, including mapping
the total mass distribution of the merger system. Combined with Chandra X-ray
Observatory and Swift/XRT observations, we investigate the dynamical state of
the merger to probe the nature of the dark matter component. Our weak
gravitational lensing measurements reveal a separation of the X-ray centroid of
the western subcluster from the center of the mass and galaxy light
distributions, similar to that of the famous "Bullet cluster". We conclude that
the "Burst cluster" is another candidate merger system for determining the
nature of dark matter and for studying the environment of short GRBs. We
discuss connections between the cluster dynamical state and/or matter
composition and compact object mergers, the leading model for the origin of
short GRBs. Finally, we present results from a weak lensing survey based on
archival Very Large Telescope (VLT) images in the areas of 5 other short GRBs.Comment: 17 pages, 7 figures, accepted by Ap
Infrared and Optical Observations of GRB 030115 and its Extremely Red Host Galaxy: Implications for Dark Bursts
We present near-infrared (nIR) and optical observations of the afterglow of
GRB 030115. Discovered in an infrared search at Kitt Peak 5 hours after the
burst trigger, this afterglow is amongst the faintest observed in the R-band at
an early epoch, and exhibits very red colors, with . The
magnitude of the optical afterglow of GRB 030115 is fainter than many upper
limits for other bursts, suggesting that without early nIR observations it
would have been classified as a ``dark'' burst. Both the color and optical
magnitude of the afterglow are likely due to dust extinction and indicate that
at least some optical afterglows are very faint due to dust along the line of
sight. Multicolor {\it Hubble Space Telescope} observations were also taken of
the host galaxy and the surrounding field. Photometric redshifts imply that the
host, and a substantial number of faint galaxies in the field are at . The overdensity of galaxies is sufficiently great that GRB 030115 may
have occurred in a rich high-redshift cluster. The host galaxy shows extremely
red colors (R-K=5) and is the first GRB host to be classified as an Extremely
Red Object (ERO). Some of the galaxies surrounding the host also show very red
colors, while the majority of the cluster are much bluer, indicating ongoing
unobscured star formation. As it is thought that much of high redshift star
formation occurs in highly obscured environments it may be that GRB 030115
represent a transition object, between the relatively unobscured afterglows
seen to date and a population which are very heavily extinguished, even in the
nIR.Comment: 19 Pages. ApJ 2006, 647, 47
The LOFAR Transients Pipeline
Current and future astronomical survey facilities provide a remarkably rich
opportunity for transient astronomy, combining unprecedented fields of view
with high sensitivity and the ability to access previously unexplored
wavelength regimes. This is particularly true of LOFAR, a
recently-commissioned, low-frequency radio interferometer, based in the
Netherlands and with stations across Europe. The identification of and response
to transients is one of LOFAR's key science goals. However, the large data
volumes which LOFAR produces, combined with the scientific requirement for
rapid response, make automation essential. To support this, we have developed
the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image
data from LOFAR or other instruments and searches it for transients and
variables, providing automatic alerts of significant detections and populating
a lightcurve database for further analysis by astronomers. Here, we discuss the
scientific goals of the TraP and how it has been designed to meet them. We
describe its implementation, including both the algorithms adopted to maximize
performance as well as the development methodology used to ensure it is robust
and reliable, particularly in the presence of artefacts typical of radio
astronomy imaging. Finally, we report on a series of tests of the pipeline
carried out using simulated LOFAR observations with a known population of
transients.Comment: 30 pages, 11 figures; Accepted for publication in Astronomy &
Computing; Code at https://github.com/transientskp/tk
How Special Are Dark Gamma-Ray Bursts: A Diagnostic Tool
We present here a comprehensive study of the optical/near-infrared (IR) upper
limits for gamma-ray bursts that have an X-ray afterglow. We have extrapolated
the X-ray afterglows to optical wavelengths based on the physics of the
fireball blast wave model (e.g. Rees & Meszaors 1992; Meszaros & Rees 1997),
and compared these results with optical upper limits for a large sample of
bursts. We find a small set of only three bursts out of a sample of 20 for
which the upper limits are not compatible with their X-ray afterglow properties
within the context of any blast wave model. This sparse sample does not allow
us to conclusively determine the cause of this optical/near-IR deficit.
Extinction in the host galaxy is a likely cause, but high redshifts and
different afterglow mechanisms might also explain the deficit in some cases. We
note that the three bursts appear to have higher than average gamma-ray peak
fluxes. In a magnitude versus time diagram the bursts are separated from the
majority of bursts with a detected optical/near-IR afterglow. However, two GRBs
with an optical afterglow (one of which is highly reddened) also fall in this
region with dark bursts, making it likely that dark bursts are at the faint end
of the set of optically detected bursts, and therefore the dark bursts likely
form a continuum with the bursts with a detected optical afterglow. Our work
provides a useful diagnostic tool for follow-up observations for potentially
dark bursts; applied to the events detected with the Swift satellite, it will
significantly increase our sample of truly dark bursts and shed light upon
their nature.Comment: 19 pages; accepted to Ap
Prompt and Afterglow Emission Properties of Gamma-Ray Bursts with Spectroscopically Identified Supernovae
We present a detailed spectral analysis of the prompt and afterglow emission
of four nearby long-soft gamma-ray bursts (GRBs 980425, 030329, 031203, and
060218) that were spectroscopically found to be associated with type Ic
supernovae, and compare them to the general GRB population. For each event, we
investigate the spectral and luminosity evolution, and estimate the total
energy budget based upon broadband observations. The observational inventory
for these events has become rich enough to allow estimates of their energy
content in relativistic and sub-relativistic form. The result is a global
portrait of the effects of the physical processes responsible for producing
long-soft GRBs. In particular, we find that the values of the energy released
in mildly relativistic outflows appears to have a significantly smaller scatter
than those found in highly relativistic ejecta. This is consistent with a
picture in which the energy released inside the progenitor star is roughly
standard, while the fraction of that energy that ends up in highly relativistic
ejecta outside the star can vary dramatically between different events.Comment: 55 pages including 23 figures and 8 tables. Accepted for publication
in ApJ. Replaced with the accepted versio
A Deep Search with HST for Late Time Supernova Signatures in the Hosts of XRF 011030 and XRF 020427
X-ray Flashes (XRFs) are, like Gamma-Ray Bursts (GRBs), thought to signal the
collapse of massive stars in distant galaxies. Many models posit that the
isotropic equivalent energies of XRFs are lower than those for GRBs, such that
they are visible from a reduced range of distances when compared with GRBs.
Here we present the results of two epoch Hubble Space Telescope imaging of two
XRFs. These images taken approximately 45 and 200 days post burst reveal no
evidence for an associated supernova in either case. Supernovae such as SN
1998bw would have been visible out to z ~1.5 in each case, while fainter
supernovae such as SN 2002ap would have been visible to z ~ 1. If the XRFs lie
at such large distances, their energies would not fit the observed correlation
between the GRB peak energy and isotropic energy release, in which soft bursts
are less energetic. We conclude that, should these XRFs reside at low redshifts
(), either their line of sight is heavily extinguished, or they are
associated with extremely faint supernovae, or, unlike GRBs, these XRFs do not
have temporally coincident supernovae.Comment: 12 Pages, 4 Figures, accepted for publication in Ap
Very Early Optical Afterglows of Gamma-Ray Bursts: Evidence for Relative Paucity of Detection
Very early observations with the Swift satellite of gamma-ray burst (GRB)
afterglows reveal that the optical component is not detected in a large number
of cases. This is in contrast to the bright optical flashes previously
discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the
X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence
is used to quantify the seemingly deficient optical, and in some cases X-ray,
light at these early epochs. This comparison reveals that some of these bursts
appear to have higher than normal gamma-ray efficiencies. We discuss possible
mechanisms and their feasibility for explaining the apparent lack of early
optical emission. The mechanisms considered include: foreground extinction,
circumburst absorption, Ly-alpha blanketing and absorption due to high
redshift, low density environments, rapid temporal decay, and intrinsic
weakness of the reverse shock. Of these, foreground extinction, circumburst
absorption, and high redshift provide the best explanations for most of the
non-detections in our sample. There is tentative evidence of suppression of the
strong reverse shock emission. This could be because of a
Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse
shock.Comment: 22 pages, 5 figures. Accepted for publication in Ap
A very energetic supernova associated with the gamma-ray burst of 29 March 2003
Over the past five years evidence has mounted that long-duration (> 2 s)
gamma-ray bursts (GRBs)--the most brilliant of all astronomical
explosions--signal the collapse of massive stars in our Universe. This evidence
was originally based on the probable association of one unusual GRB with a
supernova, but now includes the association of GRBs with regions of massive
star formation in distant galaxies, the appearance of supernova-like 'bumps' in
the optical afterglow light curves of several bursts and lines of freshly
synthesized elements in the spectra of a few X-ray afterglows. These
observations support, but do not yet conclusively demonstrate, the idea that
long-duration GRBs are associated with the deaths of massive stars, presumably
arising from core collapse. Here we report evidence that a very energetic
supernova (a hypernova) was temporally and spatially coincident with a GRB at
redshift z = 0.1685. The timing of the supernova indicates that it exploded
within a few days of the GRB, strongly suggesting that core-collapse events can
give rise to GRBs, thereby favouring the 'collapsar' model.Comment: 19 pages, 3 figure