1,155 research outputs found
Time dependent numerical model for the emission of radiation from relativistic plasma
We describe a numerical model constructed for the study of the emission of
radiation from relativistic plasma under conditions characteristic, e.g., to
gamma-ray bursts (GRB's) and active galactic nuclei (AGN's). The model solves
self consistently the kinetic equations for e^\pm and photons, describing
cyclo-synchrotron emission, direct Compton and inverse Compton scattering, pair
production and annihilation, including the evolution of high energy
electromagnetic cascades. The code allows calculations over a wide range of
particle energies, spanning more than 15 orders of magnitude in energy and time
scales. Our unique algorithm, which enables to follow the particle
distributions over a wide energy range, allows to accurately derive spectra at
high energies, >100 \TeV. We present the kinetic equations that are being
solved, detailed description of the equations describing the various physical
processes, the solution method, and several examples of numerical results.
Excellent agreement with analytical results of the synchrotron-SSC model is
found for parameter space regions in which this approximation is valid, and
several examples are presented of calculations for parameter space regions
where analytic results are not available.Comment: Minor changes; References added, discussion on observational status
added. Accepted for publication in Ap.
Closure Relations for Electron-Positron Pair-Signatures in Gamma-Ray Bursts
We present recipes to diagnose the fireball of gamma-ray bursts (GRBs) by
combining observations of electron-positron pair-signatures (the
pair-annihilation line and the cutoff energy due to the pair-creation process).
Our recipes are largely model-independent and extract information even from the
non-detection of either pair-signature. We evaluate physical quantities such as
the Lorentz factor, optical depth and pair-to-baryon ratio, only from the
observable quantities. In particular, we can test whether the prompt emission
of GRBs comes from the pair/baryonic photosphere or not. The future-coming
Gamma-Ray Large Area Space Telescope (GLAST) satellite will provide us with
good chances to use our recipes by detecting or non-detecting pair-signatures.Comment: 7 pages, 4 figures, accepted for publication in ApJ, with extended
discussions. Conclusions unchange
Witnessing the gradual slow-down of powerful extragalactic jets: The X-ray -- optical -- radio connection
A puzzling feature of the {\it Chandra}--detected quasar jets is that their
X-ray emission decreases faster along the jet than their radio emission,
resulting to an outward increasing radio to X-ray ratio. In some sources this
behavior is so extreme that the radio emission peak is located clearly
downstream of that of the X-rays. This is a rather unanticipated behavior given
that the inverse
Compton nature of the X-rays and the synchrotron radio emission are
attributed to roughly the same electrons of the jet's non-thermal electron
distribution. In this note we show that this morphological behavior can result
from the gradual deceleration of a relativistic flow and that the offsets in
peak emission at different wavelengths carry the imprint of this deceleration.
This notion is consistent with another recent finding, namely that the jets
feeding the terminal hot spots of powerful radio galaxies and quasars are still
relativistic with Lorentz factors . The picture of the
kinematics of powerful jets emerging from these considerations is that they
remain relativistic as they gradually decelerate from Kpc scales to the hot
spots, where, in a final collision with the intergalactic medium, they
slow-down rapidly to the subrelativistic velocities of the hot spot advance
speed.Comment: Submitted in ApJ Letters on Jan. 14, 200
Post-natal erythromycin exposure and risk of infantile hypertrophic pyloric stenosis: a systematic review and meta-analysis
PURPOSE: Macrolide antibiotics, erythromycin, in particular, have been linked to the development of infantile hypertrophic pyloric stenosis (IHPS). Our aim was to conduct a systematic review of the evidence of whether post-natal erythromycin exposure is associated with subsequent development of IHPS. METHODS: A systematic review of postnatal erythromycin administration and IHPS was performed. Papers were included if data were available on development (yes/no) of IHPS in infants exposed/unexposed to erythromycin. Data were meta-analysed using Review Manager 5.3. A random effects model was decided on a priori due to heterogeneity of study design; data are odds ratio (OR) with 95 % CI. RESULTS: Nine papers reported data suitable for analysis; two randomised controlled trials and seven retrospective studies. Overall, erythromycin exposure was significantly associated with development of IHPS [OR 2.45 (1.12-5.35), p = 0.02]. However, significant heterogeneity existed between the studies (I (2) = 84 %, p < 0.0001). Data on erythromycin exposure in the first 14 days of life was extracted from 4/9 studies and identified a strong association between erythromycin exposure and subsequent development IHPS [OR 12.89 (7.67-2167), p < 0.00001]. CONCLUSION: This study demonstrates a significant association between post-natal erythromycin exposure and development of IHPS, which seems stronger when exposure occurs in the first 2 weeks of life
Radiation mechanisms and geometry of Cygnus X-1 in the soft state
We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition
from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in
1996 May and June. The spectra consist of a dominant soft component below ~2
keV and a power-law-like continuum extending to at least ~800 keV. We interpret
them as emission from an optically-thick, cold accretion disc and from an
optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total
available power is dissipated in the corona. We model the soft component by
multi-colour blackbody disc emission taking into account the torque-free
inner-boundary condition. If the disc extends down to the minimum stable orbit,
the ASCA/RXTE data yield the most probable black hole mass of about 10 solar
masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft
state in the upper part of the stable, gas-pressure dominated, accretion-disc
solution branch. The spectrum of the corona is well modelled by repeated
Compton scattering of seed photons from the disc off electrons with a hybrid,
thermal/non-thermal distribution. The electron distribution can be
characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50
keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The
compactness of the corona is between 2 and 7, and a presence of a significant
population of electron-positron pairs is ruled out. We find strong signatures
of Compton reflection from a cold and ionized medium, presumably an accretion
disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum
is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted
to MNRA
Constraints on the Physical Parameters of TeV Blazars
We consider the constraints on the physical parameters of a homogeneous SSC
model that can be derived from the spectral shape and variability of TeV
blazars. Assuming that the relativistic electron spectrum is a broken power
law, where the break energy is a free parameter, we write the
analytical formulae that allow to connect the physical parameters of the model
to observable quantities. The constraints can be summarized in a plane where
the coordinates are the Doppler factor and the magnetic field. The consistency
between the break energy and the balance between cooling and escape and the
interpretation of the soft photon lags measured in some sources as radiative
cooling times are treated as additional independent constraints.
We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk
421 and Mrk 501.Comment: 36 pages, incl. 6 figures in PS format, AAS LaTeX, to be published in
ApJ, Dec 199
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