910 research outputs found
Line emission from gamma-ray burst environments
The time and angle dependent line and continuum emission from a dense torus
around a cosmological gamma-ray burst source is simulated, taking into account
photoionization, collisional ionization, recombination, and electron heating
and cooling due to various processes. The importance of the hydrodynamical
interaction between the torus and the expanding blast wave is stressed. Due to
the rapid deceleration of the blast wave as it interacts with the dense torus,
the material in the torus will be illuminated by a drastically different photon
spectrum than observable through a low-column-density line of sight, and will
be heated by the hydrodynamical interaction between the blast wave and the
torus. A model calculation to reproduce the Fe K-alpha line emission observed
in the X-ray afterglow of GRB 970508 is presented. The results indicate that ~
10^{-4} solar masses of iron must be concentrated in a region of less than
10^{-3} pc. The illumination of the torus material due to the hydrodynamic
interaction of the blast wave with the torus is the dominant heating and
ionization mechanism leading to the formation of the iron line. These results
suggest that misaligned GRBs may be detectable as X-ray flashes with pronounced
iron emission line features.Comment: Accepted for publication in ApJ. Updated recombination rate data;
discussion on element abundances added; references update
Spherical magnetic nanoparticles: magnetic structure and interparticle interaction
The interaction between spherical magnetic nanoparticles is investigated from
micromagnetic simulations and ananlysed in terms of the leading dipolar
interaction energy between magnetic dipoles. We focus mainly on the case where
the particles present a vortex structure. In a first step the local magnetic
structure in the isolated particle is revisited. For particles bearing a
uniaxial magnetocrystaline anisotropy, it is shown that the vortex core
orientation relative to the easy axis depends on both the particle size and the
anisotropy constant. When the particles magnetization present a vortex
structure, it is shown that the polarization of the particles by the dipolar
field of the other one must be taken into account in the interaction. An
analytic form is deduced for the interaction which involves the vortex core
magnetization and the magnetic susceptibility which are obtained from the
magnetic properties of the isolated particle.Comment: 20 pages, 10 figures Published in Journal of Applied Physics. To be
found at: http://link.aip.org/link/?jap/105/07391
Photon-Photon Absorption of Very High Energy Gamma-Rays from Microquasars: Application to LS 5039
Very high energy (VHE) gamma-rays have recently been detected from the
Galactic black-hole candidate and microquasar LS 5039. A plausible site for the
production of these VHE gamma-rays is the region close to the mildly
relativistic outflow. However, at distances comparable to the binary
separation, the intense photon field of the stellar companion will lead to
substantial gamma-gamma absorption of VHE gamma-rays. If the system is viewed
at a substantial inclination (i > 0), this absorption feature will be modulated
on the orbital period of the binary as a result of a phase-dependent
stellar-radiation intensity and pair-production threshold. We apply our results
to LS 5039 and find that (1) gamma-gamma absorption effects will be substantial
if the photon production site is located at a distance from the central compact
object of the order of the binary separation (~ 2.5e12 cm) or less; (2) the
gamma-gamma absorption depth will be largest at a few hundred GeV, leading to a
characteristic absorption trough; (3) the gamma-gamma absorption feature will
be strongly modulated on the orbital period of the binary, characterized by a
spectral hardening accompanying periodic dips of the VHE gamma-ray flux; and
(4) gamma rays can escape virtually unabsorbed, even from within ~ 10^{12} cm,
when the star is located behind the production site as seen by the observer.Comment: Submitted to ApJ Letters. AASTeX, 12 ms pages, including 4 eps
figure
Questions on uncertainties in parton distributions
A discussion is presented of the manner in which uncertainties in parton distributions
and related quantities are determined. One of the central problems
is the criteria used to judge what variation of the parameters describing a set
of partons is acceptable within the context of a global fit. Various ways of
addressing this question are outlined
Polarizable molecular interactions in condensed phase and their equivalent nonpolarizable models
Earlier, using phenomenological approach, we showed that in some cases
polarizable models of condensed phase systems can be reduced to nonpolarizable
equivalent models with scaled charges. Examples of such systems include ionic
liquids, TIPnP-type models of water, protein force fields, and others, where
interactions and dynamics of inherently polarizable species can be accurately
described by nonpolarizable models. To describe electrostatic interactions, the
effective charges of simple ionic liquids are obtained by scaling the actual
charges of ions by a factor of 1/sqrt(eps_el), which is due to electronic
polarization screening effect; the scaling factor of neutral species is more
complicated. Here, using several theoretical models, we examine how exactly the
scaling factors appear in theory, and how, and under what conditions,
polarizable Hamiltonians are reduced to nonpolarizable ones. These models allow
one to trace the origin of the scaling factors, determine their values, and
obtain important insights on the nature of polarizable interactions in
condensed matter systems.Comment: 43 pages, 3 figure
Broadband Spectral Analysis of PKS 0528+134: A Report on Six Years of EGRET Observations
The multiwavelength spectra of PKS 0528+134 during six years of observations
by EGRET have been analyzed using synchrotron self-Compton (SSC) and external
radiation Compton (ERC) models. We find that a two-component model, in which
the target photons are produced externally to the gamma-ray emitting region,
but also including an SSC component, is required to suitably reproduce the
spectral energy distributions of the source. Our analysis indicates that there
is a trend in the observed properties of PKS 0528+134, as the source goes from
a gamma-ray low state to a flaring state. We observe that during the higher
gamma-ray states, the bulk Lorentz factor of the jet increases and the ERC
component dominates the high-energy emission. Our model calculations indicate
the trend that the energies of the electrons giving rise to the synchrotron
peak decreases, and the power-ratio of the gamma-ray and low energy spectral
components increases, as the source goes from a low to a high gamma-ray state.Comment: 36 pages, 13 figures, final version accepted for publication in ApJ;
includes minor modification
Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039
Recent HESS observations show that microquasars in high-mass systems are
sources of VHE gamma-rays. A leptonic jet model for microquasar gamma-ray
emission is developed. Using the head-on approximation for the Compton cross
section and taking into account angular effects from the star's orbital motion,
we derive expressions to calculate the spectrum of gamma rays when nonthermal
jet electrons Compton-scatter photons of the stellar radiation field.
Calculations are presented for power-law distributions of nonthermal electrons
that are assumed to be isotropically distributed in the comoving jet frame, and
applied to -ray observations of LS 5039. We conclude that (1) the TeV
emission measured with HESS cannot result only from Compton-scattered stellar
radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a
combination of CSSR and SSC; (2) fitting both the HESS data and the EGRET data
associated with LS 5039 requires a very improbable leptonic model with a very
hard electron spectrum. Because the gamma rays would be variable in a leptonic
jet model, the data sets are unlikely to be representative of a simultaneously
measured gamma-ray spectrum. We therefore attribute EGRET gamma rays primarily
to CSSR emission, and HESS gamma rays to SSC emission. Detection of periodic
modulation of the TeV emission from LS 5039 would favor a leptonic SSC or
cascade hadron origin of the emission in the inner jet, whereas stochastic
variability alone would support a more extended leptonic model. The puzzle of
the EGRET gamma rays from LS 5039 will be quickly solved with GLAST. (Abridged)Comment: 17 pages, 11 figures, ApJ, in press, June 1, 2006, corrected eq.
Statistical Mechanics of Logarithmic REM: Duality, Freezing and Extreme Value Statistics of Noises generated by Gaussian Free Fields
We compute the distribution of the partition functions for a class of
one-dimensional Random Energy Models (REM) with logarithmically correlated
random potential, above and at the glass transition temperature. The random
potential sequences represent various versions of the 1/f noise generated by
sampling the two-dimensional Gaussian Free Field (2dGFF) along various planar
curves. Our method extends the recent analysis of Fyodorov Bouchaud from the
circular case to an interval and is based on an analytical continuation of the
Selberg integral. In particular, we unveil a {\it duality relation} satisfied
by the suitable generating function of free energy cumulants in the
high-temperature phase. It reinforces the freezing scenario hypothesis for that
generating function, from which we derive the distribution of extrema for the
2dGFF on the interval. We provide numerical checks of the circular and
the interval case and discuss universality and various extensions. Relevance to
the distribution of length of a segment in Liouville quantum gravity is noted.Comment: 25 pages, 12 figures Published version. Misprint corrected,
references and note adde
Multiwavelength Observations of GX 339-4 in 1996. III. Keck Spectroscopy
As part of our multiwavelength campaign of observations of GX 339-4 in 1996
we present our Keck spectroscopy performed on May 12 UT. At this time, neither
the ASM on the RXTE nor BATSE on the CGRO detected the source. The optical
emission was still dominated by the accretion disk with V approximately 17 mag.
The dominant emission line is H alpha, and for the first time we are able to
resolve a double peaked profile. The peak separation Delta v = 370 +/- 40 km/s.
Double peaked H alpha emission lines have been seen in the quiescent optical
counterparts of many black hole X-ray novae. However, we find that the peak
separation is significantly smaller in GX 339-4, implying that the optical
emission comes from a larger radius than in the novae. The H alpha emission
line may be more akin to the one in Cygnus X-1, where it is very difficult to
determine if the line is intrinsically double peaked because absorption and
emission lines from the companion star dominate.Comment: Submitted to Astrophysical Journal. 10 pages. 2 figure
Time-Dependent Synchrotron and Compton Spectra from Jets of Microquasars
Jet models for the high-energy emission of Galactic X-ray binary sources have
regained significant interest with detailed spectral and timing studies of the
X-ray emission from microquasars, the recent detection by the HESS
collaboration of very-high-energy gamma-rays from the microquasar LS~5039, and
the earlier suggestion of jet models for ultraluminous X-ray sources observed
in many nearby galaxies. Here we study the synchrotron and Compton signatures
of time-dependent electron injection and acceleration, adiabatic and radiative
cooling, and different jet geometries in the jets of Galactic microquasars.
Synchrotron, synchrotron-self-Compton, and external-Compton radiation processes
with soft photons provided by the companion star and the accretion disk are
treated. An analytical solution is presented to the electron kinetic equation
for general power-law geometries of the jets for Compton scattering in the
Thomson regime. We pay particular attention to predictions concerning the rapid
flux and spectral variability signatures expected in a variety of scenarios,
making specific predictions concerning possible spectral hysteresis, similar to
what has been observed in several TeV blazars. Such predictions should be
testable with dedicated monitoring observations of Galactic microquasars and
ultraluminous X-ray sources using Chandra and/or XMM-Newton.Comment: Accepted for publication in ApJ; 37 manuscript pages, including 10
eps figures; uses AASTeX macro
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