2,877 research outputs found
Prompt and Delayed High-Energy Emission from Cosmological Gamma-Ray Bursts
In the cosmological blast-wave model for gamma ray bursts (GRBs), high energy
(> 10 GeV) gamma-rays are produced either through Compton scattering of soft
photons by ultrarelativistic electrons, or as a consequence of the acceleration
of protons to ultrahigh energies. We describe the spectral and temporal
characteristics of high energy gamma-rays produced by both mechanisms, and
discuss how these processes can be distinguished through observations with
low-threshold Cherenkov telescopes or GLAST. We propose that Compton scattering
of starlight photons by blast wave electrons can produce delayed flares of GeV
-- TeV radiation.Comment: to appear in Proceedings of VERITAS Workshop on TeV Astrophysics of
Extragalactic Sources, eds. M. Catanese, J. Quinn, T. Weeke
Timing Signatures of the Internal-Shock Model for Blazars
We investigate the spectral and timing signatures of the internal-shock model
for blazars. For this purpose, we develop a semi-analytical model for the
time-dependent radiative output from internal shocks arising from colliding
relativistic shells in a blazar jet. The emission through synchrotron and
synchrotron-self Compton (SSC) radiation as well as Comptonization of an
isotropic external radiation field are taken into account. We evaluate the
discrete correlation function (DCF) of the model light curves in order to
evaluate features of photon-energy dependent time lags and the quality of the
correlation, represented by the peak value of the DCF. The almost completely
analytic nature of our approach allows us to study in detail the influence of
various model parameters on the resulting spectral and timing features. This
paper focuses on a range of parameters in which the gamma-ray production is
dominated by Comptonization of external radiation, most likely appropriate for
gamma-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL
Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability
of the optical emission is highly correlated with the X-ray and high-energy
(HE: > 100 MeV) gamma-ray emission. Our baseline model predicts a lead of the
optical variability with respect to the higher-energy bands by 1 - 2 hours and
of the HE gamma-rays before the X-rays by about 1 hour. We show that variations
of certain parameters may lead to changing signs of inter-band time lags,
potentially explaining the lack of persistent trends of time lags in most
blazars.Comment: Accepted for publication in Ap
Boundary correlation function of fixed-to-free bcc operators in square-lattice Ising model
We calculate the boundary correlation function of fixed-to-free boundary
condition changing operators in the square-lattice Ising model. The correlation
function is expressed in four different ways using block Toeplitz
determinants. We show that these can be transformed into a scalar Toeplitz
determinant when the size of the matrix is even. To know the asymptotic
behavior of the correlation function at large distance we calculate the
asymptotic behavior of this scalar Toeplitz determinant using the Szeg\"o's
theorem and the Fisher-Hartwig theorem. At the critical temperature we confirm
the power-law behavior of the correlation function predicted by conformal field
theory
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Feller Processes: The Next Generation in Modeling. Brownian Motion, L\'evy Processes and Beyond
We present a simple construction method for Feller processes and a framework
for the generation of sample paths of Feller processes. The construction is
based on state space dependent mixing of L\'evy processes.
Brownian Motion is one of the most frequently used continuous time Markov
processes in applications. In recent years also L\'evy processes, of which
Brownian Motion is a special case, have become increasingly popular.
L\'evy processes are spatially homogeneous, but empirical data often suggest
the use of spatially inhomogeneous processes. Thus it seems necessary to go to
the next level of generalization: Feller processes. These include L\'evy
processes and in particular Brownian motion as special cases but allow spatial
inhomogeneities.
Many properties of Feller processes are known, but proving the very existence
is, in general, very technical. Moreover, an applicable framework for the
generation of sample paths of a Feller process was missing. We explain, with
practitioners in mind, how to overcome both of these obstacles. In particular
our simulation technique allows to apply Monte Carlo methods to Feller
processes.Comment: 22 pages, including 4 figures and 8 pages of source code for the
generation of sample paths of Feller processe
The Hard VHE Gamma-ray Emission in High-Redshift TeV Blazars: Comptonization of Cosmic Microwave Background Radiation in an Extended Jet?
Observations of very-high-energy (VHE, E > 250 GeV) gamma-ray emission from
several blazars at z > 0.1 have placed stringent constraints on the elusive
spectrum and intensity of the intergalactic infrared background radiation
(IIBR). Correcting their observed VHE spectrum for gamma-gamma absorption even
by the lowest plausible level of the IIBR provided evidence for a very hard
(photon spectral index Gamma_{ph} < 2) intrinsic source spectrum out to TeV
energies. Such a hard VHE gamma-ray spectrum poses a serious challenge to the
conventional synchrotron-self-Compton interpretation of the VHE emission of TeV
blazars and suggests the emergence of a separate emission component beyond a
few hundred GeV. Here we propose that such a very hard, slowly variable VHE
emission component in TeV blazars may be produced via Compton upscattering of
Cosmic Microwave Background (CMB) photons by shock-accelerated electrons in an
extended jet. For the case of 1ES 1101-232, this component could dominate the
bolometric luminosity of the extended jet if the magnetic fields are of the
order of typical intergalactic magnetic fields B ~ 10 micro-Gauss and electrons
are still being accelerated out to TeV energies gamma > 4 X 10^6) on kiloparsec
scales along the jet.Comment: Accepted for publication in ApJ Letter
Gamma-Ray Studies of Blazars: Synchro-Compton Analysis of Flat Spectrum Radio Quasars
We extend a method for modeling synchrotron and synchrotron self-Compton
radiations in blazar jets to include external Compton processes. The basic
model assumption is that the blazar radio through soft X-ray flux is nonthermal
synchrotron radiation emitted by isotropically-distributed electrons in the
randomly directed magnetic field of outflowing relativistic blazar jet plasma.
Thus the electron distribution is given by the synchrotron spectrum, depending
only on the Doppler factor and mean magnetic field , given
that the comoving emission region size scale R_b^\prime \lesssim c \dD
t_v/(1+z), where is variability time and is source redshift.
Generalizing the approach of Georganopoulos, Kirk, and Mastichiadis (2001) to
arbitrary anisotropic target radiation fields, we use the electron spectrum
implied by the synchrotron component to derive accurate Compton-scattered
-ray spectra throughout the Thomson and Klein-Nishina regimes for
external Compton scattering processes. We derive and calculate accurate
-ray spectra produced by relativistic electrons that Compton-scatter
(i) a point source of radiation located radially behind the jet, (ii) photons
from a thermal Shakura-Sunyaev accretion disk and (iii) target photons from the
central source scattered by a spherically-symmetric shell of broad line region
(BLR) gas. Calculations of broadband spectral energy distributions from the
radio through -ray regimes are presented, which include self-consistent
absorption on the same radiation fields that provide target
photons for Compton scattering. Application of this baseline flat spectrum
radio/-ray quasar model is considered in view of data from -ray
telescopes and contemporaneous multi-wavelength campaigns.Comment: Accepted by ApJ. 22 pages, 12 figures, 2 tables. Minor revisions to
figures and tex
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