67 research outputs found
On the formation of TeV radiation in LS 5039
The recent detections of TeV gamma-rays from compact binary systems show that
relativistic outflows (jets or winds) are sites of effective acceleration of
particles up to multi-TeV energies. In this paper, we discuss the conditions of
acceleration and radiation of ultra-relativistic electrons in LS 5039, the
gamma-ray emitting binary system for which the highest quality TeV data are
available. Assuming that the gamma-ray emitter is a jet-like structure, we
performed detailed numerical calculations of the energy spectrum and
lightcurves accounting for the acceleration efficiency, the location of the
accelerator, the speed of the emitting flow, the inclination angle of the
system, as well as specific features related to anisotropic inverse Compton
scattering and pair production. We conclude that the accelerator should not be
deep inside the binary system unless we assume a very efficient acceleration
rate. We show that within the IC scenario both the gamma-ray spectrum and flux
are strongly orbital phase dependent. Formally, our model can reproduce, for
specific sets of parameter values, the energy spectrum of gamma-rays reported
by HESS for wide orbital phase intervals. However, the physical properties of
the source can be constrained only by observations capable of providing
detailed energy spectra for narrow orbital phase intervals ().Comment: 14 pages, 26 figures, accepted for publication in MNRAS, submitted on
July 11, 200
Gamma-Ray Emission in the Seyfert Galaxy NGC 4151: Investigating the Role of Jet and Coronal Activities
NGC 4151, a nearby Seyfert galaxy, has recently been reported to emit gamma
rays in the GeV range, posing an intriguing astrophysical mystery. The star
formation rate of NGC 4151 is too low to explain the observed GeV flux, but the
galaxy is known for its coronal activity in X-ray and jet activity in radio. We
propose that either the combination of these two activities or the jet activity
alone can account for the gamma-ray spectrum. An energy-dependent variability
search would allow us to distinguish between the two scenarios, as the coronal
component can only contribute at energies of GeV. Our analysis also
indicates that the expected neutrino flux from the coronal component is likely
to be undetectable by IceCube.Comment: 6 pages, 3 figure
Simple analytical approximations for treatment of inverse Compton scattering of relativistic electrons in the black-body radiation field
The inverse Compton (IC) scattering of relativistic electrons is one of the
major gamma-ray production mechanisms in different environments. Often the
target photons for the IC scattering are dominated by black (or grey) body
radiation. In this case, the precise treatment of the characteristics of IC
radiation requires numerical integrations over the Planckian distribution.
Formally, analytical integrations are also possible but they result in series
of several special functions; this limits the efficiency of usage of these
expressions. The aim of this work is the derivation of approximate analytical
presentations which would provide adequate accuracy for the calculations of the
energy spectra of up-scattered radiation, the rate of electron energy losses,
and the mean energy of emitted photons. Such formulae have been obtained by
merging the analytical asymptotic limits. The coefficients in these expressions
are calculated via the least square fitting of the results of numerical
integrations. The simple analytical presentations, obtained for both the
isotropic and anisotropic target radiation fields, provide adequate (as good as
) accuracy for broad astrophysical applications.Comment: 16 pages, 11 figures, accepted for publication in Ap
Unraveling the high-energy emission components of gamma-ray binaries
The high and very-high energy spectrum of gamma-ray binaries has become a
challenge for all theoretical explanations since the detection of powerful,
persistent GeV emission from LS 5039 and LS I +61 303 by Fermi/LAT. The
spectral cutoff at a few GeV indicates that the GeV component and the fainter,
hard TeV emission above 100 GeV are not directly related. We explore the
possible origins of these two emission components in the framework of a young,
non-accreting pulsar orbiting the massive star, and initiating the non-thermal
emission through the interaction of the stellar and pulsar winds. The
pulsar/stellar wind interaction in a compact orbit binary gives rise to two
potential locations for particle acceleration: the shocks at the head-on
collision of the winds and the termination shock caused by Coriolis forces on
scales larger than the binary separation. We explore the suitability of these
two locations to host the GeV and TeV emitters, respectively, through the study
of their non-thermal emission along the orbit. We focus on the application of
this model to LS 5039 given its well determined stellar wind with respect to
other gamma-ray binaries. The application of the proposed model to LS 5039
indicates that these two potential emitter locations provide the necessary
conditions for reproduction of the two-component high-energy gamma-ray spectrum
of LS 5039. In addition, the ambient postshock conditions required at each of
the locations are consistent with recent hydrodynamical simulations. The
scenario based on the interaction of the stellar and pulsar winds is compatible
with the GeV and TeV emission observed from gamma-ray binaries with unknown
compact objects, such as LS 5039 and LS I +61 303.Comment: Version as published in A&
Monte-Carlo Simulations of Radio Emitting Secondaries in Gamma-Ray Binaries
Several binary systems that contain a massive star have been detected in both
the radio band and at very high energies. In the dense stellar photon field of
these sources, gamma-ray absorption and pair creation are expected to occur,
and the radiation from these pairs may contribute significantly to the observed
radio emission. We aim at going deeper in the study of the properties, and in
particular the morphology, of the pair radio emission in gamma-ray binaries. We
apply a Monte-Carlo code that computes the creation location, the spatial
trajectory and the energy evolution of the pairs produced in the binary system
and its surroundings. The radio emission produced by these pairs, with its
spectral, variability and spatial characteristics, is calculated as it would be
seen from a certain direction. A generic case is studied first, and then the
specific case of LS 5039 is also considered. We find that, confirming previous
results, the secondary radio emission should appear as an extended radio
structure of a few milliarcseconds size. This radiation would be relatively
hard, with fluxes up to ~ 10 mJy. Modulation is expected depending on the
gamma-ray production luminosity, system eccentricity, and wind ionization
fraction, and to a lesser extent on the magnetic field structure. In gamma-ray
binaries in general, the pairs created due to photon-photon interactions can
contribute significantly to the core, and generate an extended structure. In
the case of LS 5039, the secondary radio emission is likely to be a significant
fraction of the detected core flux, with a marginal extension.Comment: 13 pages, 12 Figures, 2 Tables, to be published in PASJ (Publications
of the Astronomical Society of Japan), in pres
Fast moving pulsars as probes of interstellar medium
Pulsars moving through ISM produce bow shocks detected in hydrogen H
line emission. The morphology of the bow shock nebulae allows one to probe the
properties of ISM on scales pc and smaller. We performed 2D RMHD
modeling of the pulsar bow shock and simulated the corresponding H
emission morphology. We find that even a mild spatial inhomogeneity of ISM
density, , leads to significant variations of the shape
of the shock seen in H line emission. We successfully reproduce the
morphology of the Guitar Nebula. We infer quasi-periodic density variations in
the warm component of ISM with a characteristic length of ~pc.
Structures of this scale might be also responsible for the formation of the
fine features seen at the forward shock of Tycho SNR in X-rays. Formation of
such short periodic density structures in the warm component of ISM is
puzzling, and bow-shock nebulae provide unique probes to study this phenomenon.Comment: 12 pages, 7 figures, accepted MNRA
On the properties of inverse Compton spectra generated by up-scattering a power-law distribution of target photons
Relativistic electrons are an essential component in many astrophysical
sources, and their radiation may dominate the high-energy bands. Inverse
Compton (IC) emission is the radiation mechanism that plays the most important
role in these bands. The basic properties of IC, such as the total and
differential cross sections, have long been studied; the properties of the IC
emission depend strongly not only on the emitting electron distribution but
also on the properties of the target photons. This complicates the
phenomenological studies of sources, where target photons are supplied from a
broad radiation component. We study the spectral properties of IC emission
generated by a power-law distribution of electrons on a power-law distribution
of target photons. We approximate the resulting spectrum by a broken-power-law
distribution and show that there can be up to three physically motivated
spectral breaks. If the target photon spectrum extends to sufficiently low
energies, ( and
are electron mass and speed of light, respectively;
and are the minimum/maximum
energies of target photons and electrons, respectively), then the high energy
part of the IC component has a spectral slope typical for the Thomson regime
with an abrupt cutoff close to . The spectra typical for the
Klein-Nishina regime are formed above . If
the spectrum of target photons features a cooling break, i.e., a change of the
photon index by at , then the transition to
the Klein-Nishina regime proceeds through an intermediate change of the photon
index by at .Comment: 12 pages, 7 figure
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