607 research outputs found
High-efficiency photospheric emission of long-duration gamma-ray burst jets: the effect of the viewing angle
We present the results of a numerical investigation of the spectra and light
curves of the emission from the photospheres of long-duration gamma-ray burst
jets. We confirm that the photospheric emission has high efficiency and we show
that the efficiency increases slightly with the off-axis angle. We show that
the peak frequency of the observed spectrum is proportional to the square root
of the photosphere's luminosity, in agreement with the Amati relation. However,
a quantitative comparison reveals that the thermal peak frequency is too small
for the corresponding total luminosity. As a consequence, the radiation must be
out of thermal equilibrium with the baryons in order to reproduce the
observations. Finally, we show that the spectrum integrated over the emitting
surface is virtually indistinguishable from a Planck law, and therefore an
additional mechanism has to be identified to explain the non-thermal behavior
of the observed spectra at both high and low frequencies.Comment: 6 pages, 8 figures, ApJ in press (few changes to figures
Population III Gamma Ray Bursts
We discuss a model of Poynting-dominated gamma-ray bursts from the collapse
of very massive first generation (pop. III) stars. From redshifts of order 20,
the resulting relativistic jets would radiate in the hard X-ray range around 50
keV and above, followed after roughly a day by an external shock component
peaking around a few keV. On the same timescales an inverse Compton component
around 75 GeV may be expected, as well as a possible infra-red flash. The
fluences of these components would be above the threshold for detectors such as
Swift and Fermi, providing potentially valuable information on the formation
and properties of what may be the first luminous objects and their black holes
in the high redshift Universe.Comment: 12 pages; Apj, subm. 12/10/2009; accepted 04/12/201
Bioinformatics tools in predictive ecology: Applications to fisheries
This article is made available throught the Brunel Open Access Publishing Fund - Copygith @ 2012 Tucker et al.There has been a huge effort in the advancement of analytical techniques for molecular biological data over the past decade. This has led to many novel algorithms that are specialized to deal with data associated with biological phenomena, such as gene expression and protein interactions. In contrast, ecological data analysis has remained focused to some degree on off-the-shelf statistical techniques though this is starting to change with the adoption of state-of-the-art methods, where few assumptions can be made about the data and a more explorative approach is required, for example, through the use of Bayesian networks. In this paper, some novel bioinformatics tools for microarray data are discussed along with their ‘crossover potential’ with an application to fisheries data. In particular, a focus is made on the development of models that identify functionally equivalent species in different fish communities with the aim of predicting functional collapse
A search for thermal X-ray signatures in Gamma-Ray Bursts I: Swift bursts with optical supernovae
The X-ray spectra of Gamma-Ray Bursts can generally be described by an
absorbed power law. The landmark discovery of thermal X-ray emission in
addition to the power law in the unusual GRB 060218, followed by a similar
discovery in GRB 100316D, showed that during the first thousand seconds after
trigger the soft X-ray spectra can be complex. Both the origin and prevalence
of such spectral components still evade understanding, particularly after the
discovery of thermal X-ray emission in the classical GRB 090618. Possibly most
importantly, these three objects are all associated with optical supernovae,
begging the question of whether the thermal X-ray components could be a result
of the GRB-SN connection, possibly in the shock breakout. We therefore
performed a search for blackbody components in the early Swift X-ray spectra of
11 GRBs that have or may have associated optical supernovae, accurately
recovering the thermal components reported in the literature for GRBs 060218,
090618 and 100316D. We present the discovery of a cooling blackbody in GRB
101219B/SN2010ma, and in four further GRB-SNe we find an improvement in the fit
with a blackbody which we deem possible blackbody candidates due to
case-specific caveats. All the possible new blackbody components we report lie
at the high end of the luminosity and radius distribution. GRB 101219B appears
to bridge the gap between the low-luminosity and the classical GRB-SNe with
thermal emission, and following the blackbody evolution we derive an expansion
velocity for this source of order 0.4c. We discuss potential origins for the
thermal X-ray emission in our sample, including a cocoon model which we find
can accommodate the more extreme physical parameters implied by many of our
model fits.Comment: 16 pages, 6 figures, accepted for MNRA
Extremely narrow spectrum of GRB110920A: further evidence for localised, subphotospheric dissipation
Much evidence points towards that the photosphere in the relativistic outflow
in GRBs plays an important role in shaping the observed MeV spectrum. However,
it is unclear whether the spectrum is fully produced by the photosphere or
whether a substantial part of the spectrum is added by processes far above the
photosphere. Here we make a detailed study of the ray emission from
single pulse GRB110920A which has a spectrum that becomes extremely narrow
towards the end of the burst. We show that the emission can be interpreted as
Comptonisation of thermal photons by cold electrons in an unmagnetised outflow
at an optical depth of . The electrons receive their energy by a
local dissipation occurring close to the saturation radius. The main spectral
component of GRB110920A and its evolution is thus, in this interpretation,
fully explained by the emission from the photosphere including localised
dissipation at high optical depths.Comment: 14 pages, 11 figures, accepted to MNRA
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