322 research outputs found
Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
We present observations of two LMC supernova remnants (SNRs), DEM L238 and
DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central
emission, surrounded by a faint shell, is present in both remnants. The central
emission has an entirely thermal spectrum dominated by strong Fe L-shell lines,
with the deduced Fe abundance in excess of solar and not consistent with the
LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and
DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission
originates in gas swept up and heated by the blast wave. A standard Sedov
analysis implies about 50 solar masses in both swept-up shells, SNR ages
between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and
subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich
supernova ejecta are close to collisional ionization equilibrium. Their
presence is unexpected, because standard Type Ia SNR models predict faint
ejecta emission with short ionization ages. Both SNRs belong to a previously
unrecognized class of Type Ia SNRs characterized by bright interior emission.
Denser than expected ejecta and/or a dense circumstellar medium around the
progenitors are required to explain the presence of Fe-rich ejecta in these
SNRs. Substantial amounts of circumstellar gas are more likely to be present in
explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar
SNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myr
old) progenitors.Comment: 24 pages, 8 figures, ApJ, in pres
Supernova Remnants in the Magellanic Clouds. VI. The DEML316 Supernova Remnants
The DEML316 system contains two shells, both with the characteristic
signatures of supernova remnants (SNRs). We analyze Chandra and XMM-Newton data
for DEML316, investigating its spatial and spectral X-ray features. Our Chandra
observations resolve the structure of the northeastern SNR (Shell A) as a
bright inner ring and a set of "arcs" surrounded by fainter diffuse emission.
The spectrum is well fit by a thermal plasma model with temperature ~1.4 keV;
we do not find significant spectral differences for different regions of this
SNR. The southwestern SNR (Shell B) exhibits an irregular X-ray outline, with a
brighter interior ring of emission including a bright knot of emission. Overall
the emission of the SNR is well described by a thermal plasma of temperature
~0.6 keV. The Bright Knot, however, is spectrally distinct from the rest of the
SNR, requiring the addition of a high-energy spectral component consistent with
a power-law spectrum of photon index 1.6--1.8.
We confirm the findings of Nishiuchi et al. (2001) that the spectra of these
shells are notably different, with Shell A requiring a high iron abundance for
a good spectral fit, implying a Type Ia origin. We further explicitly compare
abundance ratios to model predictions for Type Ia and Type II supernovae. The
low ratios for Shell A (O/Fe of 1.5 and Ne/Fe of 0.2) and the high ratios for
Shell B (O/Fe of 30--130 and Ne/Fe of 8--16) are consistent with Type Ia and
Type II origins, respectively. The difference between the SNR progenitor types
casts some doubt on the suggestion that these SNRs are interacting with one
another.Comment: Accepted for ApJ v. 635 (December issue
Observational Constraints on Superbubble X-ray Energy Budgets
The hot, X-ray-emitting gas in superbubbles imparts energy and enriched
material to the interstellar medium (ISM) and generates the hot ionized medium,
the ISM's high-temperature component. The evolution of superbubble energy
budgets is not well understood, however, and the processes responsible for
enhanced X-ray emission in superbubbles remain a matter of debate. We present
Chandra ACIS-S observations of two X-ray-bright superbubbles in the Large
Magellanic Cloud (LMC), DEM L50 (N186) and DEM L152 (N44), with an emphasis on
disentangling the true superbubble X-ray emission from non-related diffuse
emission and determining the spatial origin and spectral variation of the X-ray
emission. An examination of the superbubble energy budgets shows that on the
order of 50% of the X-ray emission comes from regions associated with supernova
remnant (SNR) impacts. We find some evidence of mass-loading due to swept-up
clouds and metallicity enrichment, but neither mechanism provides a significant
contribution to the X-ray luminosities. We also find that one of the
superbubbles, DEM L50, is likely not in collisional ionization equilibrium. We
compare our observations to the predictions of the standard Weaver et al. model
and to 1-D hydrodynamic simulations including cavity supernova impacts on the
shell walls. Our observations show that mass-loading due to thermal evaporation
from the shell walls and SNR impacts are the dominant source of enhanced X-ray
luminosities in superbubbles. These two processes should affect most
superbubbles, and their contribution to the X-ray luminosity must be considered
when determining the energy available for transport to the ISM.Comment: 25 pages, 11 figures, accepted for publication in Ap
A Study of the Populations of X-ray Sources in the Small Magellanic Cloud with ASCA
The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made
multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images
in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately
constructed, and the latter provides the first hard X-ray view of the SMC. We
extract 39 sources from the two-band images with a criterion of S/N>5, and
conduct timing and spectral analyses for all of these sources. Coherent
pulsations are detected from 12 X-ray sources; five of which are new
discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux
variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the
other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because
their spectra exhibit emission lines from highly ionized atoms. We find that
XBPs and thermal SNRs in the SMC can be clearly separated by their hardness
ratio (the ratio of the count rate between the hard and soft bands). Using this
empirical grouping, we find many XBP candidates in the SMC, although no
pulsations have yet been detected from these sources. Possible implications on
the star-formation history and evolution of the SMC are presented by a
comparison of the source populations in the SMC and our Galaxy.Comment: 11 pages, 39 Figures, to be published in ApJ Supplement. Tables (body
and figures also) are available at
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miRNA independent hepacivirus variants suggest a strong evolutionary pressure to maintain miR-122 dependence
Hepatitis C virus (HCV) requires the liver specific micro-RNA (miRNA), miR-122, to replicate. This was considered unique among RNA viruses until recent discoveries of HCV-related hepaciviruses prompting the question of a more general miR-122 dependence. Among hepaciviruses, the closest known HCV relative is the equine non-primate hepacivirus (NPHV). Here, we used Argonaute cross-linking immunoprecipitation (AGO-CLIP) to confirm AGO binding to the single predicted miR-122 site in the NPHV 5âUTR in vivo. To study miR-122 requirements in the absence of NPHV-permissive cell culture systems, we generated infectious NPHV/HCV chimeric viruses with the 5â end of NPHV replacing orthologous HCV sequences. These chimeras were viable even in cells lacking miR-122, although miR-122 presence enhanced virus production. No other miRNAs bound this region. By random mutagenesis, we isolated HCV variants partially dependent on miR-122 as well as robustly replicating NPHV/HCV variants completely independent of any miRNAs. These miRNA independent variants even replicate and produce infectious particles in non-hepatic cells after exogenous delivery of apolipoprotein E (ApoE). Our findings suggest that miR-122 independent HCV and NPHV variants have arisen and been sampled during evolution, yet miR-122 dependence has prevailed. We propose that hepaciviruses may use this mechanism to guarantee liver tropism and exploit the tolerogenic liver environment to avoid clearance and promote chronicity
The Milky Way in X-rays for an outside observer: Log(N)-Log(S) and Luminosity Function of X-ray binaries from RXTE/ASM data
We study the Log(N)-Log(S) and X-ray luminosity function in the 2-10 keV
energy band, and the spatial (3-D) distribution of bright, log(L_X) > 34-35
erg/s, X-ray binaries in the Milky Way. In agreement with theoretical
expectations and earlier results we found significant differences between the
spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The
volume density of LMXB sources peaks strongly at the Galactic Bulge. HMXBs tend
to avoid the inner 3-4 kpc of the Galaxy, HMXBs are more concentrated towards
the Galactic Plane and show clear signatures of the spiral structure in their
spatial distribution. LMXB sources have a flatter Log(N)-Log(S) distribution
and luminosity function than HMXBs. The integrated 2-10 keV luminosities of
X-ray binaries, averaged over 1996--2000, are 2-3 * 10^39 (LMXB) and 2-3 *
10^38 (HMXB) erg/s. Normalised to the stellar mass and the star formation rate,
respectively, these correspond to 5 * 10^28 erg/s/M_sol for LMXBs and 5 * 10^37
erg/s/(M_sol/yr) for HMXBs. Due to the shallow slopes of the luminosity
functions the integrated emission of X-ray binaries is dominated by the 5-10
most luminous sources which determine the appearance of the Milky Way in the
standard X-ray band for an outside observer. In particular variability of
individual sources or an outburst of a bright transient source can increase the
integrated luminosity of the Milky Way by as much as a factor of ~2. Although
the average LMXB luminosity function shows a break near the Eddington
luminosity for a 1.4 M_sol neutron star, at least 11 sources showed episodes of
super-Eddington luminosity during ASM observations. We provide the maps of
distribution of X-ray binaries in the Milky Way in various projections, which
can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.Comment: 23 pages, 19 figures, accepted by A&
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X-ray emission from stainless steel foils irradiated by femtosecond petawatt laser pulses
We report about nonlinear growth of x-ray emission intensity emitted from plasma generated by femtosecond petawatt laser pulses irradiating stainless steel foils. X-ray emission intensity increases as ⌠I 4.5 with laser intensity I on a target. High spectrally resolved x-ray emission from front and rear surfaces of 5 ÎŒm thickness stainless steel targets were obtained at the wavelength range 1.7-2.1 Ă
, for the first time in experiments at femtosecond petawatt laser facility J-KAREN-P. Total intensity of front x-ray spectra three times dominates to rear side spectra for maximum laser intensity I â 3.21021 W/cm2. Growth of x-ray emission is mostly determined by contribution of bremsstrahlung radiation that allowed estimating bulk electron plasma temperature for various magnitude of laser intensity on target
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