112 research outputs found
ASCA Observations of the Twin Supernova Remnants in the Large Magellanic Cloud, DEM L316
We report results from an ASCA X-ray study of DEM L316, an emission nebula in
the Large Magellanic Cloud (LMC) consisting of two closely-spaced supernova
remnants (SNRs). The SIS image shows separate X-ray sources located at the
positions of the two radio- and optically-emitting SNR shells, 054769.7A and
054769.7B (hereafter, shell A and B). The individual X-ray spectrum of each
shell is well described by optically-thin thermal emission, although the
characteristics of the emission differ in important details between them. Shell
A exhibits strong iron L emission, which we attribute to the presence of
iron-rich ejecta leading to the suggestion that this SNR originates from a Type
Ia SN, an explosion of a moderate mass progenitor. Shell B, on the other hand,
has a chemical composition similar to that of the interstellar medium of the
LMC and so its X-ray emission is dominated by swept-up matter. The different
spectral features strongly disfavor the hypothesis that the two shells are due
to one supernova explosion into an interconnected bubble. We could not obtain
the evidence which positively supports the collision between two SNRs.Comment: 5 pages, 1 table, 7 figures Also available at
http://www-cr.scphys.kyoto-u.ac.jp/MC
Review of Discrete X-Ray Sources in the Small Magellanic Cloud: Summary of the ASCA Results and Implication on the Recent Star Forming Activity
We made 22 observations on the Small Magellanic Cloud (SMC) and covered full
regions by the end of the ASCA mission. We detected 106 discrete sources with a
criterion of S/N > 5 and performed systematic analyses on all of the sources.
We determined the source positions with an ~40'' error radius (90% confidence)
for sources detected in the central 20' radius of the GIS. We detected coherent
pulsations from 17 sources. Among them, eight were newly discovered during this
study. We classified most of these pulsars as X-ray binary pulsars (XBPs) based
on their properties, such as the flux variability and the existence of an
optical counterpart. We detected X-ray emission from eight supernova remnants
(SNRs). Among them, five SNRs showed emission lines in their spectra, hence we
regarded the five as thermal SNRs. We found that XBPs and thermal SNRs in the
SMC can be clearly separated by their spectral hardness ratio. Applying this
empirical law to faint (thus unclassified) sources, we found 19 XBP candidates
and four thermal SNR candidates. We also found several tens of candidates for
active galactic nuclei, both from the hardness ratio and the logN--logS
relation of extragalactic sources. Based on these ASCA results and further
information from other sattelites, we compiled comprehensive catalogues of
discrete X-ray sources in the Small Magellanic Cloud. Using the catalogues, we
derived the spatial distributions of XBPs and SNRs. XBPs and SNRs were found to
be concentrated in the main body and eastern wing, which resembles the
distribution of young stars with ages of ~2e7yr. By comparing the source
populations in the SMC and our Galaxy, we suggest that the star-forming rate
(per unit mass) in the SMC was much higher than the Galaxy 1e7yr ago. We also
discuss the recent change of the star-forming rate in the SMC.Comment: 61 pages, 19 figures, to be published in PASJ. Also available at
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job/ (with high-resolution
images
Centrally Peaked X-Ray Supernova Remnants in the Small Magellanic Cloud Studied with ASCA and ROSAT
This paper presents ASCA/SIS and ROSAT/HRI results of three supernova
remnants (SNRs) in the Small Magellanic Cloud: 0103-726, 0045-734, and
0057-7226. The ROSAT/HRI images of these SNRs indicate that the most of the
X-ray emissions are concentrated in the center region. Only from 0103-726 are
faint X-rays along the radio shell also detected. The ASCA/SIS spectra of
0103-726 and 0045-734 exhibit strong emission lines from highly ionized metals.
The spectra were well-fitted with non-equilibrium ionization (NEI) plasma
models. The metal abundances are found to be larger than the mean chemical
compositions in the interstellar medium (ISM) of the SMC. Thus, X-rays from
these two SNRs are attributable to the ejecta gas, although the ages estimated
from the ionization timescale are significantly large, ~> 10^4 yr. The chemical
compositions are roughly consistent with the type-II supernova origin of a
progenitor mass ~< 20 M_solar. The SIS spectrum of 0057-7226 was also fitted
with an NEI model of an estimated age ~> 6 x 10^3 yr. Although no constraint on
the metal abundances was obtained, the rather weak emission lines are
consistent with the low metal abundances in the ISM of the SMC. A possible
scenario for the evolution of the morphologies and spectra of SNRs is proposed.Comment: 8 pages, 8 figures, to be published in PASJ. Also available at
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job/ (with high-resolution
images
Discovery of Non Thermal X-Rays from the Northwest Shell of the New SNR RX J1713.7-3946: The Second SN1006 ?
We report ASCA results of a featureless X-ray spectrum from RX J1713.7-3946,
a new shell-like SNR discovered with the ROSAT all sky survey. The northwest
part of RX J1713.7-3946 was in the field of the ASCA Galactic Plane Survey
Project and was found to exhibit a shell-like structure. The spectrum, however
shows neither line emission nor any signature of a thermal origin. Instead, a
power-law model with a photon index of 2.4-2.5 gives reasonable fit to the
spectrum, suggesting a non-thermal origin. Together with the similarity to
SN1006, we propose that RX J1713.7-3946 is the second example, after SN1006, of
a synchrotron X-ray radiation from a shell of SNRs. Since the synchrotron
X-rays suggest existence of extremely high energy charged particles in the SNR
shell, our discovery should have strong impact on the origin of the cosmic
X-rays.Comment: 4 pages, 3 postscript figure
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
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job
A Histone-Like Protein of Mycobacteria Possesses Ferritin Superfamily Protein-Like Activity and Protects against DNA Damage by Fenton Reaction
Iron is an essential metal for living organisms but its level must be strictly controlled in cells, because ferrous ion induces toxicity by generating highly active reactive oxygen, hydroxyl radicals, through the Fenton reaction. In addition, ferric ion shows low solubility under physiological conditions. To overcome these obstacles living organisms possess Ferritin superfamily proteins that are distributed in all three domains of life: bacteria, archaea, and eukaryotes. These proteins minimize hydroxyl radical formation by ferroxidase activity that converts Fe2+ into Fe3+ and sequesters iron by storing it as a mineral inside a protein cage. In this study, we discovered that mycobacterial DNA-binding protein 1 (MDP1), a histone-like protein, has similar activity to ferritin superfamily proteins. MDP1 prevented the Fenton reaction and protects DNA by the ferroxidase activity. The Km values of the ferroxidase activity by MDP1 of Mycobacterium bovis bacillus Calmette-Guérin (BCG-3007c), Mycobacterium tuberculosis (Rv2986c), and Mycobacterium leprae (ML1683; ML-LBP) were 0.292, 0.252, and 0.129 mM, respectively. Furthermore, one MDP1 molecule directly captured 81.4±19.1 iron atoms, suggesting the role of this protein in iron storage. This study describes for the first time a ferroxidase-iron storage protein outside of the ferritin superfamily proteins and the protective role of this bacterial protein from DNA damage
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