515 research outputs found
Failed Gamma-Ray Bursts: Thermal UV/Soft X-ray Emission Accompanied by Peculiar Afterglows
We show that the photospheres of "failed" Gamma-Ray Bursts (GRBs), whose bulk
Lorentz factors are much lower than 100, can be outside of internal shocks. The
resulting radiation from the photospheres is thermal and bright in UV/Soft
X-ray band. The photospheric emission lasts for about one thousand seconds with
luminosity about several times 10^46 erg/s. These events can be observed by
current and future satellites. It is also shown that the afterglows of failed
GRBs are peculiar at the early stage, which makes it possible to distinguish
failed GRBs from ordinary GRBs and beaming-induced orphan afterglows.Comment: 19 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Detection of high-power two-mode squeezing by sum-frequency generation
We introduce sum-frequency generation (SFG) as an effective physical
two-photon detector for high power two-mode squeezed coherent states with
arbitrary frequency separation, as produced by parametric oscillators well
above the threshold. Using a formalism of "collective modes", we describe both
two-mode squeezing and degenerate squeezing on equal footing and derive simple
relations between the input degree of squeezing and the measured SFG quadrature
noise. We compare the proposed SFG detection to standard homodyne measurement,
and show advantages in robustness to detection inefficiency (loss of SFG
photons) and acceptance bandwidth.Comment: 5 pages, 3 figure
GRB060218 as a Tidal Disruption of a White Dwarf by an Intermediate Mass Black Hole
A highly unusual pair of a gamma-ray burst (GRB) GRB060218 and an associated
supernova SN2006aj has puzzled theorists for years. A supernova shock breakout
and a jet from a newborn stellar mass compact object were put forward to
explain its multiwavelength signature. We propose that the source is naturally
explained by another channel, a tidal disruption of a white dwarf (WD) by an
intermediate mass black hole (IMBH). The tidal disruption is accompanied by a
tidal pinching, which leads to the ignition of a WD and a supernova. Some
debris falls back onto the IMBH, forms a disk, which quickly amplifies the
magnetic field, and launches a jet. We successfully fit soft X-ray spectrum
with the Comptonized blackbody emission from a jet photosphere. The optical/UV
emission is consistent with self-absorbed synchrotron from the expanding jet
front. The accretion rate temporal dependence Mdot(t) in a tidal disruption
provides a good fit to soft X-ray lightcurve. The IMBH mass is found to be
about 10^4Msun in three independent estimates: (1) fitting tidal disruption
Mdot(t) to soft X-ray lightcurve; (2) computing the jet base radius in a jet
photospheric emission model; (3) inferring the central BH mass based on a host
dwarf galaxy stellar mass. The supernova position is consistent with the center
of the host galaxy, while low supernova ejecta mass is consistent with a WD
mass. High expected rate of tidal disruptions in dwarf galaxies is consistent
with one source observed by Swift satellite over several years at GRB060218
distance of 150Mpc. The encounters with the WDs provide a lot of fuel for IMBH
growth.Comment: 15 pages, 5 figures, accepted to ApJ, minor change
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
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
Observational evidence of dissipative photospheres in gamma-ray bursts
The emission from a gamma-ray burst (GRB) photosphere can give rise to a
variety of spectral shapes. The spectrum can retain the shape of a Planck
function or it can be broadened and have the shape of a Band function. This
fact is best illustrated by studying GRB090902B: The main gamma-ray spectral
component is initially close to a Planck function, which can only be explained
by emission from the jet photosphere. Later, the same component evolves into a
broader Band function. This burst thus provides observational evidence that the
photosphere can give rise to a non-thermal spectrum. We show that such a
broadening is most naturally explained by subphotospheric dissipation in the
jet. The broadening mainly depends on the strength and location of the
dissipation, on the magnetic field strength, and on the relation between the
energy densities of thermal photons and of the electrons. We suggest that the
evolution in spectral shape observed in GRB090902B is due to a decrease of the
bulk Lorentz factor of the flow, leading to the main dissipation becoming
subphotospheric. Such a change in the flow parameters can also explain the
correlation observed between the peak energy of the spectrum and low-energy
power law slope, alpha, a correlation commonly observed in GRBs. We conclude
that photospheric emission could indeed be a ubiquitous feature during the
prompt phase in GRBs and play a decisive role in creating the diverse spectral
shapes and spectral evolutions that are observed.Comment: Submitted to MNRAS, 14 pages, 7 figure
The Environments of the Most Energetic Gamma-Ray Bursts
We analyze the properties of a sample of long gamma-ray bursts (LGRBs)
detected by the Fermi satellite that have a spectroscopic redshift and good
follow-up coverage at both X-ray and optical/nIR wavelengths. The evolution of
LGRB afterglows depends on the density profile of the external medium, enabling
us to separate wind or ISM-like environments based on the observations. We do
this by identifying the environment that provides the best agreement between
estimates of , the index of the underlying power-law distribution of
electron energies, as determined by the behavior of the afterglow in different
spectral/temporal regimes. At 11 rest-frame hours after trigger, we find a
roughly even split between ISM-like and wind-like environments. We further find
a 2 separation in the prompt emission energy distributions of wind-like
and ISM-like bursts. We investigate the underlying physical parameters of the
shock, and calculate the (degenerate) product of density and magnetic field
energy (). We show that must be to avoid
implied densities comparable to the intergalactic medium. Finally, we find that
the most precisely constrained observations disagree on by more than would
be expected based on observational errors alone. This suggests additional
sources of error that are not incorporated in the standard afterglow theory.
For the first time, we provide a measurement of this intrinsic error which can
be represented as an error in the estimate of of magnitude .
When this error is included in the fits, the number of LGRBs with an identified
environment drops substantially, but the equal division between the two types
remains.Comment: 31 pages (+14 appendix), 9 figures, 6 tables. Accepted for
publication in Ap
The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and Gamma-ray Sources
Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with the
impulsive phase of solar flares have been the subject of renewed interest in
recent years. They have been interpreted in terms of thin-target, nonthermal
bremsstrahlung emission. This interpretation has led to rather extreme physical
requirements in some cases. For example, in one case, essentially all of the
electrons in the source must be accelerated to nonthermal energies to account
for the coronal HXR source. In other cases, the extremely hard photon spectra
of the coronal continuum gamma-ray emission suggest that the low energy cutoff
of the electron energy distribution lies in the MeV energy range. Here we
consider the role of inverse Compton scattering (ICS) as an alternate emission
mechanism in both the ultra- and mildly relativistic regimes. It is known that
relativistic electrons are produced during powerful flares; these are capable
of up-scattering soft photospheric photons to HXR and gamma-ray energies.
Previously overlooked is the fact that mildly relativistic electrons, generally
produced in much greater numbers in flares of all sizes, can up-scatter EUV/SXR
photons to HXR energies. We also explore ICS on anisotropic electron
distributions and show that the resulting emission can be significantly
enhanced over an isotropic electron distribution for favorable viewing
geometries. We briefly review results from bremsstrahlung emission and
reconsider circumstances under which nonthermal bremsstrahlung or ICS would be
favored. Finally, we consider a selection of coronal HXR and gamma-ray events
and find that in some cases the ICS is a viable alternative emission mechanism
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