856 research outputs found
GRB 060218: The nature of the optical-UV component
The optical-UV component in GRB 060218 is assumed to be due to optically
thick cyclotron emission. The key aspect of this model is the high temperature
of the absorbing electrons. The heat input derives from nuclei accelerated in
semi-relativistic internal shocks, like in ordinary gamma-ray bursts. Coulomb
collisions transfer part of that energy to electrons. Inverse Compton cooling
on the X-ray photons leads to electron temperatures around 100 keV. Such a high
brightness temperature for the optical-UV emission implies an emitting area
roughly equal to that of the thermal X-ray component. This suggests a model in
which the radio, optical-UV and thermal X-ray emission are closely related:
Although the optical-UV and thermal X-ray emission are two separate spectral
components, it is argued that they both come from the photosphere of a
quasi-spherical, continuous outflow, whose interaction with the circumstellar
medium gives rise to the radio emission. The properties of GRB 060218, as
measured in the co-moving frame, are similar to those of ordinary gamma-ray
burst; i.e., the main difference is the much lower value of the bulk Lorentz
factor in GRB 060218. The cyclotron absorption implies a magnetic field in
rough equipartition with the matter energy density in the outflow. Hence, the
magnetic field could have a dynamically important role, possibly with a
magnetar as the central engine.Comment: 16 pages, 1 figure, accepted to Ap
The X-ray and radio emission from SN 2002ap: The importance of Compton scattering
The radio and X-ray observations of the Type Ic supernova SN 2002ap are
modeled. We find that inverse Compton cooling by photospheric photons explains
the observed steep radio spectrum, and also the X-ray flux observed by XMM.
Thermal emission from the shock is insufficient to explain the X-ray flux. The
radio emitting region expands with a velocity of, roughly, 70,000 km/s. From
the ratio of X-ray to radio emission we find that the energy densities of
magnetic fields and relativistic electrons are close to equipartion.Comment: 15 pages, 2 figures, ApJ accepte
The radio spectra of SN 2020oi: Effects of radiative cooling on the deduced source properties
Observations of radiative cooling in a synchrotron source offer a possibility
to further constrain its properties. Inverse Compton cooling is indicated in
the radio spectra during the early phases of SN\,2020oi. It is shown that
contrary to previous claims, observations are consistent with equipartition
between relativistic electrons and magnetic field as well as a constant
mass-loss rate of the progenitor star prior to the supernova explosion. The
reason for this difference is the need to include cooling directly in the
fitting procedure rather than estimating its effects afterward. It is
emphasized that the inferred properties of the supernova ejecta are sensitive
to the time evolution of the synchrotron self-absorption frequency; hence,
great care should be taken when modeling spectra for which cooling and/or
inhomogeneities are indicated. Furthermore, it is noted that the energies of
the relativistic electrons in the radio emission regions in supernovae are
likely too low for first-order Fermi acceleration to be effective.Comment: Accepted to ApJ; 21 pages including one appendi
Boomerang returns unexpectedly
Experimental study of the anisotropy in the cosmic microwave background (CMB)
is gathering momentum. The eagerly awaited Boomerang results have lived up to
expectations. They provide convincing evidence in favor of the standard
paradigm: the Universe is close to flat and with primordial fluctuations which
are redolent of inflation. Further scrutiny reveals something even more
exciting however -- two hints that there may be some unforeseen physical
effects. Firstly the primary acoustic peak appears at slightly larger scales
than expected. Although this may be explicable through a combination of mundane
effects, we suggest it is also prudent to consider the possibility that the
Universe might be marginally closed. The other hint is provided by a second
peak which appears less prominent than expected. This may indicate one of a
number of possibilities, including increased damping length or tilted initial
conditions, but also breaking of coherence or features in the initial power
spectrum. Further data should test whether the current concordance model needs
only to be tweaked, or to be enhanced in some fundamental way.Comment: 11 pages, 3 figures, final version accepted by Ap
Self-Organized Criticality in Compact Plasmas
Compact plasmas, that exist near black-hole candidates and in gamma ray burst
sources, commonly exhibit self-organized non-linear behavior. A model that
simulates the non-linear behavior of compact radiative plasmas is constructed
directly from the observed luminosity and variability. The simulation shows
that such plasmas self organize, and that the degree of non-linearity as well
as the slope of the power density spectrum increase with compactness. The
simulation is based on a cellular automaton table that includes the properties
of the hot (relativistic) plasmas, and the magnitude of the energy
perturbations. The plasmas cool or heat up, depending on whether they release
more or less than the energy of a single perturbation. The energy release
depends on the plasmas densities and temperatures, and the perturbations
energy. Strong perturbations may cool the previously heated plasma through
shocks and/or pair creation.
New observations of some active galactic nuclei and gamma ray bursters are
consistent with the simulationComment: 9 pages, 5 figures, AASTeX, Submitted to ApJ
Bone disease in patients with primary sclerosing cholangitis.
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Increased prevalence of sleep disturbances and daytime sleepiness in subjects with bronchial asthma: a population study of young adults in three European countries
To access publisher full text version of this article. Please click on the hyperlink in Additional Links fieldThe aim of this study was to investigate whether asthma is associated with decreased quality of sleep and increased daytime sleepiness. The study involved a random population of 2,202 subjects supplemented by 459 subjects with suspected asthma, aged 20-45 yrs. The subjects were from Reykjavik (Iceland), Uppsala and Göteborg (Sweden) and Antwerp (Belgium), and participated in the European Community Respiratory Health Survey. The investigation included a structured interview, methacholine challenge, skinprick tests and a questionnaire on sleep disturbances. Participants in Iceland and Sweden also estimated their sleep times and made peak expiratory flow (PEF) recordings during a period of 1 week. Asthma was defined as self-reported physician-diagnosed asthma with current asthma-related symptoms (n = 267). Difficulties inducing sleep (DIS) and early morning awakenings (EMA) were about twice as common, and daytime sleepiness 50% more common, in asthmatics compared with subjects without asthma. After adjusting for possible confounders, a positive association was found between asthma and: DIS (odds ratio (OR) = 1.8); EMA (OR = 2.0); daytime sleepiness (OR = 1.6); snoring (OR = 1.7); and self reported apnoeas (OR = 3.7). Allergic rhinitis, which was reported by 71% of subjects with asthma, was independently related to DIS (OR = 2.0) and daytime sleepiness (OR = 1.3). A significant correlation was found between the number of asthma-related symptoms and sleep disturbances (p < 0.001). Asthma is associated with decreased subjective quality of sleep and increased daytime sleepiness. Concurrent allergic rhinitis may be an important underlying cause of sleep impairment in asthmatic patients
Gamma-Ray Burst Spectral Correlations: Photospheric and Injection Effects
We present a physical framework that can account for most of the observed
spectral properties of the prompt gamma-ray burst emission. This includes the
variety of spectral shapes, shape evolutions, and spectral correlations between
flux and spectral peak, within bursts described by Borgonovo & Ryde, and among
bursts described by Amati/Ghirlanda. In our proposed model the spectral peak is
given by the photospheric emission from a relativistic outflow for which the
horizon length is much smaller than the radial width. The observed duration of
the thermal flash will be given by the radial light-crossing time. This then
gives that the typical emission site is at ~10e11 cm from the center, with a
Lorentz factor of ~300. This emission is accompanied by non-thermal emission
from dissipation locations outside the photosphere. The relative strength of
these two components depend on injection effects at the central engine leading
to varying relative location of the saturation and photospheric radii. The
total emission can then reproduce the observed variety. The spectral
correlations are found by assuming that the amount of energy dissipated depends
non-linearly on the averaged particle density. Beside the spectral correlations
this also gives a description of how the relative strength of the thermal
component varies with temperature within a burst.Comment: ApJ accepted, acknowledgement adde
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