147 research outputs found
On cosmic-ray production efficiency at supernova remnant shocks propagating into realistic diffuse interstellar medium
Using three-dimensional magnetohydrodynamics simulations, we show that the
efficiency of cosmic-ray (CR) production at supernova remnants (SNRs) is
over-predicted if it could be estimated based on proper motion measurements of
H filaments in combination with shock-jump conditions. Density
fluctuations of upstream medium make shock waves rippled and oblique almost
everywhere. The kinetic energy of the shock wave is transferred into that of
downstream turbulence as well as thermal energy which is related to the shock
velocity component normal to the shock surface. Our synthetic observation shows
that the CR acceleration efficiency as estimated from a lower downstream plasma
temperature, is overestimated by 10-40%, because rippled shock does not
immediately dissipate all upstream kinetic energy.Comment: 7 pages, 3 figures, 1 table, accepted for publication in ApJ; the
paper with full resolution images is
http://www.phys.aoyama.ac.jp/~ryo/papers/shimoda2015.pd
Evolution of Synchrotron X-rays in Supernova Remnants
A systematic study of the synchrotron X-ray emission from supernova remnants
(SNRs) has been conducted. We selected a total of 12 SNRs whose synchrotron
X-ray spectral parameters are available in the literature with reasonable
accuracy, and studied how their luminosities change as a function of radius. It
is found that the synchrotron X-ray luminosity tends to drop especially when
the SNRs become larger than ~5 pc, despite large scatter. This may be explained
by the change of spectral shape caused by the decrease of the synchrotron
roll-off energy. A simple evolutionary model of the X-ray luminosity is
proposed and is found to reproduce the observed data approximately, with
reasonable model parameters. According to the model, the total energy of
accelerated electrons is estimated to be 10^(47-48) ergs, which is well below
the supernova explosion energy. The maximum energies of accelerated electrons
and protons are also discussed.Comment: 6 pages, 2 figures, ApJ, in pres
Suzaku Observation of the RCW86 Northeastern Shell
This paper reports the Suzaku results on the northeast shell of RCW 86. With
the spatial and spectral analysis, we separated the X-rays into three distinct
components; low (kT_e~0.3keV) and high (kT_e~1.8keV) temperature plasmas and a
non-thermal component, and discovered their spatial distributions are different
from each other. The low temperature plasma is dominated at the east rim,
whereas the non-thermal emission is the brightest at the northeast rim which is
spatially connected from the east rim. The high temperature plasma, found to
contain the ~6.42keV line (K alpha of low-ionized iron), is enhanced at the
inward region with respect to the east rim and has no spatial correlation with
the non-thermal X-ray (the northeast). The Fe-Kalpha line, therefore, is not
related to the non-thermal emission but originates from Fe-rich ejecta heated
to the high temperatures by the reverse shock. Since the metal abundances of
the low temperature plasma are sub-solar, the most possible origin of this
component is interstellar medium heated by a blast wave. The non-thermal X-ray,
which has a power-law index of ~2.8, is likely to be synchrotron emission. A
possible scenario to explain these morphologies and spectra is: A fast moving
blast wave in a thin cavity of OB association collided with a dense
interstellar medium or cloud at the east region very recently. As the result,
the reverse shock in this interior decelerated, and arrived at the Fe-rich
region of the ejecta and heated it. In the northeast rim, on the other hand,
the blast wave is still moving fast, and accelerated high energy electrons to
emit synchrotron X-rays.Comment: 13 pages, 5 figures (9 figure files), accepted for publication in
PAS
Observational constraints on the maximum energies of accelerated particles in supernova remnants
Supernova remnants (SNRs) are thought to be the most plausible sources of
Galactic cosmic rays. One of the principal questions is whether they are
accelerating particles up to the maximum energy of Galactic cosmic rays
(PeV). In this paper, we summarize our recent studies on
gamma-ray-emitting SNRs. We first evaluated the reliability of SNR age
estimates to quantitatively discuss time dependence of their acceleration
parameters. Then we systematically modeled their gamma-ray spectra to constrain
the acceleration parameters. The current maximum energy estimates were found to
be well below PeV for most sources. The basic time dependence of the maximum
energy assuming the Sedov evolution () cannot be
explained with the simplest acceleration condition (Bohm limit) and requires
shock-ISM (interstellar medium) interaction. The inferred maximum energies
during lifetime averaged over the sample can be expressed as TeV
( with being the age at the
maximum, which reaches PeV only if yr. The
maximum energies during lifetime are suggested to have a variety of 1-2 orders
of magnitude from object to object on the other hand. This variety will reflect
the dependence on environments.Comment: 6 pages, 4 figures, proceedings of 7th Heidelberg International
Symposium on High-Energy Gamma-Ray Astronomy (Gamma 2022
TeV Gamma-Rays from Old Supernova Remnants
We study the emission from an old supernova remnant (SNR) with an age of
around 10^5 yrs and that from a giant molecular cloud (GMC) encountered by the
SNR. When the SNR age is around 10^5 yrs, proton acceleration is efficient
enough to emit TeV gamma-rays both at the shock of the SNR and that in the GMC.
The maximum energy of primarily accelerated electrons is so small that TeV
gamma-rays and X-rays are dominated by hadronic processes, pi^0-decay and
synchrotron radiation from secondary electrons, respectively. However, if the
SNR is older than several 10^5 yrs, there are few high-energy particles
emitting TeV gamma-rays because of the energy loss effect and/or the wave
damping effect occurring at low-velocity isothermal shocks. For old SNRs or
SNR-GMC interacting systems capable of generating TeV gamma-ray emitting
particles, we calculated the ratio of TeV gamma-ray (1-10 TeV) to X-ray (2-10
keV) energy flux and found that it can be more than ~10^2. Such a source
showing large flux ratio may be a possible origin of recently discovered
unidentified TeV sources.Comment: 10 pages, 6 figures, 2 tables, MNRAS in pres
X-Ray Observation of Very High Energy Gamma-Ray Source, HESS J1745-303, with Suzaku
Suzaku observations of a TeV unidentified (unID) source, HESS J1745-303, are
presented. A possible excess of neutral iron line emission is discovered, and
is likely associated with the main part of HESS J1745-303, named "region A". It
may be an X-ray reflection nebula where the X-rays from previous Galactic
Center (GC) activity are reflected by a molecular cloud. This result further
strengthens the assumption that the molecular cloud which is spatially
coincident with region A of HESS J1745-303 is located in the GC region. The TeV
emission from molecular clouds is reminiscent of the diffuse TeV gamma-rays
from the GC giant molecular clouds, and it could have the same emission
mechanism. With deep exposure mapping observations by Suzaku, a tight
upper-limit on the 2-10 keV continuum diffuse emission from region A is
obtained, as 2.1x10^-13ergs s^-1cm^-2. The flux ratio between 1-10 TeV and 2-10
keV is larger than 4. Possible scenarios to reproduce wide-band spectra from
keV to TeV are examined. Thermal X-rays from nearby two old supernova remnants,
G359.0-0.9 and G359.1-0.5, are detected, and their emission properties are well
determined in the present study with deep exposure.Comment: 18 pages, 14 figures, ApJ accepte
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