1,945 research outputs found
Modeling of the Radio Emission from the Vela Supernova Remnant
Supernova remnants (SNRs) are widely considered to be sites of Galactic
cosmic ray (CR) acceleration. Vela is one of the nearest Galactic composite
SNRs to Earth accompanied by the Vela pulsar and its pulsar wind nebula (PWN)
Vela X. The Vela SNR is one of the most studied remnants and it benefits from
precise estimates of various physical parameters such as distance and age.
Therefore, it is a perfect object for a detailed study of physical processes in
SNRs. The Vela SNR expands into the highly inhomogeneous cloudy interstellar
medium (ISM) and its dynamics is determined by the heating and evaporation of
ISM clouds. It features an asymmetrical X-ray morphology which is explained by
the expansion into two media with different densities. This could occur if the
progenitor of the Vela SNR exploded close to the edge of the stellar wind
bubble of the nearby Wolf-Rayet star Velorum and hence one part of
the remnant expands into the bubble. The interaction of the ejecta and the main
shock of the remnant with ISM clouds causes formation of secondary shocks at
which additional particle acceleration takes place. This may lead to the close
to uniform distribution of relativistic particles inside the remnant. We
calculate the synchrotron radio emission within the framework of the new
hydrodynamical model which assumes the supernova explosion at the edge of the
stellar wind bubble. The simulated radio emission agrees well with both the
total radio flux from the remnant and the complicated radio morphology of the
source.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in A&
Probing cluster environments of blazars through gamma-gamma absorption
Most blazars are known to be hosted in giant elliptic galaxies, but their
cluster environments have not been thoroughly investigated. Cluster
environments may contain radiation fields of low-energy photons created by
nearby galaxies and/or stars in the intracluster medium that produce diffuse
intracluster light. These radiation fields may absorb very high energy gamma
rays ( GeV; VHE) and trigger pair cascades with further production
of subsequent generations of gamma rays with lower energies via inverse Compton
scattering on surrounding radiation fields leaving a characteristic imprint in
the observed spectral shape. The change of the spectral shape of the blazar
reflects the properties of its ambient medium. We show, however, that neither
intracluster light nor the radiation field of an individual nearby galaxy can
cause substantial gamma-gamma absorption. Substantial gamma-gamma absorption is
possible only in the case of multiple, , luminous nearby galaxies.
This situation is not found in the local Universe, but may be possible at
larger redshifts (). Since VHE gamma rays from such distances are
expected to be strongly absorbed by the extragalactic background light, we
consider possible signatures of gamma-ray induced pair cascades by calculating
the expected GeV flux which appears to be below the Fermi sensitivity even for
nearby galaxies.Comment: 5 pages, 5 figures, accepted for publication in A&
Nonthermal emission model of isolated X-ray pulsar RX J0420.0-5022
In the present paper an alternative theoretical interpretation to the
generally assumed thermal emission models of the observed X-ray spectrum of
isolated pulsar RX J0420.0-5022 is presented. It is well known that the
distribution function of relativistic particles is one-dimensional at the
pulsar surface. However, cyclotron instability causes an appearance of
transverse momenta of relativistic electrons, which as a result, start to
radiate in the synchrotron regime. On the basis of the Vlasov's kinetic
equation we study the process of the quasi-linear diffusion (QLD) developed by
means of the cyclotron instability. This mechanism provides generation of
optical and X-ray emission on the light cylinder lengthscales. The analysis of
the three archival XMM-Newton observations of RX J0420.0-5022 is performed.
Considering a different approach of the synchrotron emission theory, the
spectral energy distribution is obtained that is in a good agreement with the
observational data. A fit to the X-ray spectrum is performed using both the
present synchrotron emission model spectrum absorbed by cold interstellar
matter and generally assumed absorbed black-body model.Comment: 7 pages, 1 figures; accepted to Advances in Astronomy and Space
Physic
Gamma-gamma absorption in the -ray binary system PSR B1259-63/LS 2883
The observed TeV light curve from the -ray binary PSR B1259-63/LS
2883 shows a decrease in the flux at periastron which has not been fully
explained by emission mechanisms alone. This observed decrease can, however, be
explained by gamma-gamma absorption due to the stellar and disk photons. We
calculate the gamma-gamma absorption in PSR B1259-63/LS 2883 taking into
account photons from both the circumstellar disk and star, assuming the
rays originate at the position of the pulsar. The gamma-gamma
absorption due to the circumstellar disk photons produces a
decrease in the flux, and there is a total decrease of ( TeV)
within a few days before periastron, accompanied by a hardening of the
-ray photon index. While the gamma-gamma absorption alone is not
sufficient to explain the full complexity of the H.E.S.S. -ray light
curve it results in a significant decrease in the predicted flux, which is
co-incident with the observed decrease. In addition, we have calculated an
upper-limit on the gamma-gamma absorption, assuming that the emission is
produced at the apex of the bow shock. Future observations with CTA during the
2021 periastron passage may be able to confine the location of the emission
based on the degree of gamma-gamma absorption as well as measure the hardening
of the spectrum around periastron.Comment: 9 pages, 7 figures, 1 table. Accepted for publication in Ap
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