127 research outputs found
New Identification of the Mixed-Morphology Supernova Remnant G298.6-0.0 with Possible Gamma-ray Association
We present an X-ray analysis on the Galactic supernova remnant (SNR)
G298.6-0.0 with Suzaku. The X-ray image shows a center-filled structure inside
the radio shell, implying this SNR is categorized as a mixed-morphology (MM)
SNR. The spectrum is well reproduced by a single temperature plasma model in
ionization equilibrium, with a temperature of 0.78 (0.70-0.87) keV. The total
plasma mass of 30 solar mass indicates that the plasma has interstellar medium
origin. The association with a GeV gamma-ray source 3FGL J1214.0-6236 on the
shell of the SNR is discussed, in comparison with other MM SNRs with GeV
gamma-ray associations. It is found that the flux ratio between
absorption-corrected thermal X-rays and GeV gamma-rays decreases as the MM SNRs
evolve to larger physical sizes. The absorption-corrected X-ray flux of
G298.6-0.0 and the GeV gamma-ray flux of 3FGL J1214.0-6236 closely follow this
trend, implying that 3FGL J1214.0-6236 is likely to be the GeV counterpart of
G298.6-0.0.Comment: 11 pages, 6 figures, PASJ, in pres
X-ray Measurements of the Particle Acceleration Properties at Inward Shocks in Cassiopeia A
We present new evidence that the bright non-thermal X-ray emission features
in the interior of the Cassiopeia A supernova remnant (SNR) are caused by
inward moving shocks based on Chandra and NuSTAR observations. Several bright
inward-moving filaments were identified using monitoring data taken by Chandra
in 2000-2014. These inward-moving shock locations are nearly coincident with
hard X-ray (15-40 keV) hot spots seen by NuSTAR. From proper motion
measurements, the transverse velocities were estimated to be in the range
2,100-3,800 km s for a distance of 3.4 kpc. The shock velocities
in the frame of the expanding ejecta reach values of 5,100-8,700 km
s, slightly higher than the typical speed of the forward shock.
Additionally, we find flux variations (both increasing and decreasing) on
timescales of a few years in some of the inward-moving shock filaments. The
rapid variability timescales are consistent with an amplified magnetic field of
0.5-1 mG. The high speed and low photon cut-off energy of the
inward-moving shocks are shown to imply a particle diffusion coefficient that
departs from the Bohm regime ( 3-8) for the few
simple physical configurations we consider in this study. The maximum electron
energy at these shocks is estimated to be 8-11 TeV, smaller than the
values of 15-34 TeV inferred for the forward shock. Cassiopeia A is
dynamically too young for its reverse shock to appear to be moving inward in
the observer frame. We propose instead that the inward-moving shocks are a
consequence of the forward shock encountering a density jump of 5-8
in the surrounding material.Comment: 16 pages, 8 figures, accepted for publication in Ap
Discovery of X-ray Emission from the Galactic Supernova Remnant G32.8-0.1 with Suzaku
We present the first dedicated X-ray study of the supernova remnant (SNR)
G32.8-0.1 (Kes 78) with Suzaku. X-ray emission from the whole SNR shell has
been detected for the first time. The X-ray morphology is well correlated with
the emission from the radio shell, while anti-correlated with the molecular
cloud found in the SNR field. The X-ray spectrum shows not only conventional
low-temperature (kT ~ 0.6 keV) thermal emission in a non-equilibrium ionization
state, but also a very high temperature (kT ~ 3.4 keV) component with a very
low ionization timescale (~ 2.7e9 cm^{-3}s), or a hard non-thermal component
with a photon index Gamma~2.3. The average density of the low-temperature
plasma is rather low, of the order of 10^{-3}--10^{-2} cm^{-3}, implying that
this SNR is expanding into a low-density cavity. We discuss the X-ray emission
of the SNR, also detected in TeV with H.E.S.S., together with multi-wavelength
studies of the remnant and other gamma-ray emitting SNRs, such as W28 and RCW
86. Analysis of a time-variable source, 2XMM J185114.3-000004, found in the
northern part of the SNR, is also reported for the first time. Rapid time
variability and a heavily absorbed hard X-ray spectrum suggest that this source
could be a new supergiant fast X-ray transient.Comment: 20 pages, 14 figures, ApJ, in pres
Two-Dimensional particle-in-cell simulations of the nonresonant, cosmic-ray driven instability in SNR shocks
In supernova remnants, the nonlinear amplification of magnetic fields
upstream of collisionless shocks is essential for the acceleration of cosmic
rays to the energy of the "knee" at 10^{15.5}eV. A nonresonant instability
driven by the cosmic ray current is thought to be responsible for this effect.
We perform two-dimensional, particle-in-cell simulations of this instability.
We observe an initial growth of circularly polarized non-propagating magnetic
waves as predicted in linear theory. It is demonstrated that in some cases the
magnetic energy density in the growing waves, can grow to at least 10 times its
initial value. We find no evidence of competing modes, nor of significant
modification by thermal effects. At late times we observe saturation of the
instability in the simulation, but the mechanism responsible is an artefact of
the periodic boundary conditions and has no counterpart in the supernova-shock
scenario.Comment: 18 pages, 6 figures, accepted for publication in Ap
Suzaku Detection of Diffuse Hard X-Ray Emission outside Vela X
Vela X is a large, 3x2 degrees, radio-emitting pulsar wind nebula (PWN)
powered by the Vela pulsar in the Vela supernova remnant. Using four Suzaku/XIS
observations pointed just outside Vela X, we find hard X-ray emission extending
throughout the fields of view. The hard X-ray spectra are well represented by a
power-law. The photon index is measured to be constant at Gamma~2.4, similar to
that of the southern outer part of Vela X. The power-law flux decreases with
increasing distance from the pulsar. These properties lead us to propose that
the hard X-ray emission is associated with the Vela PWN. The larger X-ray
extension found in this work strongly suggests that distinct populations
relativistic electrons form the X-ray PWN and Vela X, as was recently inferred
from multiwavelength spectral modeling of Vela X.Comment: 18 pages, 7 figures, accepted for publication in PASJ (Suzaku Special
Issue
Cosmological magnetic field survival
It is widely believed that primordial magnetic fields are dramatically
diluted by the expansion of the universe. As a result, cosmological magnetic
fields with residual strengths of astrophysical relevance are generally sought
by going outside standard cosmology, or by extending conventional
electromagnetic theory. Nevertheless, the survival of strong B-fields of
primordial origin is possible in spatially open Friedmann universes without
changing conventional electromagnetism. The reason is the hyperbolic geometry
of these spacetimes, which slows down the adiabatic magnetic decay-rate and
leads to their superadiabatic amplification on large scales. So far, the effect
has been found to operate on Friedmannian backgrounds containing either
radiation or a slow-rolling scalar field. We show here that the superadiabatic
amplification of large-scale magnetic fields, generated by quantum fluctuations
during inflation, is essentially independent of the type of matter that fills
the universe and appears to be a generic feature of open Friedmann spacetimes.
We estimate the late-time strength of any residual field in a marginally open
universe and show that it can easily meet the requirements for the dynamo
generation of the magnetic fields observed in galaxies today.Comment: Equations streamlined, references updated. MNRAS in pres
Nonthermal X-Rays from Supernova Remnant G330.2+1.0 and the Characteristics of its Central Compact Object
We present results from our X-ray data analysis of the SNR G330.2+1.0 and its
CCO, CXOU J160103.1--513353 (J1601). Using our XMM-Newton and Chandra
observations, we find that the X-ray spectrum of J1601 can be described by
neutron star atmosphere models (T ~ 2.5--3.7 MK). Assuming the distance of d ~
5 kpc for J1601 as estimated for SNR G330.2+1.0, a small emission region of R ~
1--2 km is implied. X-ray pulsations previously suggested by Chandra are not
confirmed by the XMM-Newton data, and are likely not real. However, our timing
analysis of the XMM-Newton data is limited by poor photon statistics, and thus
pulsations with a relatively low amplitude (i.e., an intrinsic pulsed-fraction
< 40%) cannot be ruled out. Our results indicate that J1601 is a CCO similar to
that in the Cassiopeia A SNR.X-ray emission from SNR G330.2+1.0 is dominated by
power law continuum (Gamma ~ 2.1--2.5) which primarily originates from thin
filaments along the boundary shell. This X-ray spectrum implies synchrotron
radiation from shock-accelerated electrons with an exponential roll-off
frequency ~ 2--3 x 10^17 Hz. For the measured widths of the X-ray filaments (D
~ 0.3 pc) and the estimated shock velocity (v_s ~ a few x 10^3 km s^-1), a
downstream magnetic field B ~ 10--50 G is derived. The estimated maximum
electron energy E_max ~ 27--38 TeV suggests that G330.2+1.0 is a candidate TeV
gamma-ray source. We detect faint thermal X-ray emission in G330.2+1.0. We
estimate a low preshock density n_0 ~ 0.1 cm^-3, which suggests a dominant
contribution from an inverse Compton mechanism (than the proton-proton
collision) to the prospective gamma-ray emission. Follow-up deep radio, X-ray,
and gamma-ray observations will be essential to reveal the details of the shock
parameters and the nature of particle accelerations in this SNR.Comment: 26 pages, 3 tables, 7 figures (4 color figures), Accepted by Ap
Discovery of New Interacting Supernova Remnants in the Inner Galaxy
OH(1720 MHz) masers are excellent signposts of interaction between supernova
remnants(SNRs) and molecular clouds. Using the GBT and VLA we have surveyed 75
SNRs and six candidates for maser emission. Four new interacting SNRs are
detected with OH masers: G5.4-1.2, G5.7-0.0, G8.7-0.1 and G9.7-0.0. The newly
detected interacting SNRs G5.7-0.0 and G8.7-0.1 have TeV gamma-ray counterparts
which may indicate a local cosmic ray enhancement. It has been noted that
maser-emitting SNRs are preferentially distributed in the Molecular Ring and
Nuclear Disk. We use the present and existing surveys to demonstrate that
masers are strongly confined to within 50 degrees Galactic longitude at a rate
of 15 percent of the total SNR population. All new detections are within 10
degrees Galactic longitude emphasizing this trend. Additionally, a substantial
number of SNR masers have peak fluxes at or below the detection threshold of
existing surveys. This calls into question whether maser surveys of Galactic
SNRs can be considered complete and how many maser-emitting remnants remain to
be detected in the Galaxy.Comment: Accepted to ApJ Letters, with 2 figures and 2 table
Measuring the Broad-band X-Ray Spectrum from 400 eV to 40 keV in the Southwest Part of the Supernova Remnant RX J1713.7-3946
We report on results from Suzaku broadband X-ray observations of the
southwest part of the Galactic supernova remnant (SNR) RX J1713.7-3946 with an
energy coverage of 0.4-40 keV. The X-ray spectrum, presumably of synchrotron
origin, is known to be completely lineless, making this SNR ideally suited for
a detailed study of the X-ray spectral shape formed through efficient particle
acceleration at high speed shocks. With a sensitive hard X-ray measurement from
the HXD PIN on board Suzaku, we determine the hard X-ray spectrum in the 12--40
keV range to be described by a power law with photon index Gamma = 3.2+/- 0.2,
significantly steeper than the soft X-ray index of Gamma = 2.4+/- 0.05 measured
previously with ASCA and other missions. We find that a simple power law fails
to describe the full spectral range of 0.4-40 keV and instead a power-law with
an exponential cutoff with hard index Gamma = 1.50+/- 0.09 and high-energy
cutoff epsilon_c = 1.2+/- 0.3 keV formally provides an excellent fit over the
full bandpass. If we use the so-called SRCUT model, as an alternative model, it
gives the best-fit rolloff energy of epsilon_{roll} = 0.95+/- 0.04 keV.
Together with the TeV gamma-ray spectrum ranging from 0.3 to 100 TeV obtained
recently by HESS observations, our Suzaku observations of RX J1713.7-3946
provide stringent constraints on the highest energy particles accelerated in a
supernova shock.Comment: 11 pages, 11 figures, accepted for publication in Publications of the
Astronomical Society of Japan (PASJ
A Suzaku Observation of the Low-Ionization Fe-Line Emission from RCW 86
The newly operational X-ray satellite Suzaku observed the southwestern
quadrant of the supernova remnant (SNR) RCW 86 in February 2006 to study the
nature of the 6.4 keV emission line first detected with the Advanced Satellite
for Cosmology and Astronomy (ASCA). The new data confirm the existence of the
line, localizing it for the first time; most of the line emission is adjacent
and interior to the forward shock and not at the locus of the continuum hard
emission. We also report the first detection of a 7.1 keV line that we
interpret as the K-beta emission from low-ionization iron. The Fe-K line
features are consistent with a non-equilibrium plasma of Fe-rich ejecta with
n_{e}t <~ 10^9 cm^-3 s and kT_{e} ~ 5 keV. This combination of low n_{e}t and
high kT_{e} suggests collisionless electron heating in an SNR shock. The Fe
K-alpha line shows evidence for intrinsic broadening, with a width of 47
(34--59) eV (99% error region). The difference of the spatial distributions of
the hard continuum above 3 keV and the Fe-K line emission support a synchrotron
origin for the hard continuum.Comment: 6 pages with 6 figures. Accepted for PASJ Suzaku Special Issue (vo.
58, sp.1
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