48 research outputs found
Discovery of K-Shell Emission Lines of Neutral Atoms in the Galactic Center Region
The K-shell emission line of neutral irons from the Galactic center (GC)
region is one of the key for the structure and activity of the GC. The origin
is still open question, but possibly due either to X-ray radiation or to
electron bombarding to neutral atoms. To address this issue, we analyzed the
Suzaku X-ray spectrum from the GC region of intense neutral iron line emission,
and report on the discovery of Kalpha lines of neutral argon, calcium, chrome,
and manganese atoms. The equivalent widths of these Kalpha lines indicate that
the metal abundances in the GC region should be ~1.6 and ~4 of solar value,
depending on the X-ray and the electron origins, respectively. On the other
hand, the metal abundances in the hot plasma in the GC region are found to be
~1-2 solar. These results favor that the origin of the neutral Kalpha lines are
due to X-ray irradiation.Comment: 7 pages, 5 figures, accepted for publication in PASJ (Vol.62, No.2,
pp.423--429
Formation of intermediate-mass black holes in circumnuclear regions of galaxies
Recent high-resolution X-ray imaging studies have discovered possible
candidates of intermediate-mass black holes with masses of M_\bullet \sim
10^{2-4} \MO in circumnuclear regions of many (disk) galaxies. It is known
that a large number of massive stars are formed in a circumnuclear giant H {\sc
ii} region. Therefore, we propose that continual merger of compact remnants
left from these massive stars is responsible for the formation of such an
intermediate-mass black hole within a timescale of years. A
necessary condition is that several hundreds of massive stars are formed in a
compact region with a radius of a few pc.Comment: 11 pages, PASJ in pres
Suzaku Observation Adjacent to the South End of the Radio Arc
Suzaku observed the Galactic center region near the Radio Arc at ~20'
southeast of Sagittarius A*.In the 18'x18' field of view, we found four
distinct X-ray sources: a bright star and a diffuse source associated with the
star clusters in the soft band (0.5-2.0 keV), a small clump in a higher energy
band (4-6 keV), and a peculiar clump in the 6.4 keV line band.The latter two
clumps are located at the south end of the Radio Arc. This paper reports on the
results, and discusses the origin of these X-ray sources, with a particular
emphasis on small clumps.Comment: 16 pages, 9figures, accepted for publication in PASJ (Vol.61 No.3
An X-Ray Face-on View of the Sgr B Molecular Clouds Observed with Suzaku
We present a new methodology to derive the positions of the Sgr B molecular
clouds (MCs) along the line of sight, as an application study of the Galactic
center diffuse X-rays (GCDX). The GCDX is composed of hot plasma emission of
about 7 keV and 1 keV temperatures, and non-thermal continuum emission
including the 6.4 keV line from neutral irons. The former, the Galactic center
plasma emission (GCPE), is uniformly distributed over 1 degree in longitude,
while the latter is clumpy emission produced by Thomson scattering and
fluorescence from MCs irradiated by external X-rays (the X-ray reflection
nebula emission: XRNE). We examined the Suzaku X-ray spectra of the GCPE and
XRNE near to the Sgr B MC complex, and found that the spectra suffer from two
different absorptions of N_H (Abs1) >10^23 H cm^-2 and N_H (Abs2) ~6 x 10^22 H
cm^-2. Abs1 is proportional to the 6.4 keV-line flux, and hence is due to the
MCs, while Abs2 is typical of interstellar absorption toward the Galactic
center. Assuming that the GCPE plasma is spherically-extended around Sgr A*
with a uniform density and the same angular distribution of the two temperature
components, we quantitatively estimated the line-of-sight positions of the MCs
from the flux ratio the GCPE spectrum suffered by Abs1 and that with no Abs1.
The results suggest that the Sgr B MCs are located at the near side of Sgr A*
in the GCPE.Comment: 11 pages, 8 figures, accepted for publication in PASJ (Publications
of the Astronomical Society of Japan
Peculiar Chemical Abundances in the Starburst Galaxy M82 and Hypernova Nucleosynthesis
X-ray observations have shown that the chemical abundance in the starburst
galaxy M82 is quite rich in Si and S compared with oxygen. Such an abundance
pattern cannot be explained with any combination of conventional Type I and II
supernova yields. Also the energy to heavy element mass ratio of the observed
hot plasma is much higher than the value resulted from normal supernovae. We
calculate explosive nucleosynthesis in core-collapse hypernovae and show that
the abundance pattern and the large ratio between the energy and the heavy
element mass can be explained with the hypernova nucleosynthesis. Such
hypernova explosions are expected to occur for stars more massive than >~ 20-25
Msun, and likely dominating the starburst, because the age after the starburst
in M82 is estimated to be as short as ~ 10^6 - 10^7 yr. We also investigate
pair-instability supernovae (~ 150-300 Msun) and conclude that the energy to
heavy element mass ratio in these supernovae is too small to explain the
observation.Comment: 11 pages, 6 figures, To appear in the Astrophysical Journal 578, 200
Radiation-Induced Degradation Mechanism of X-ray SOI Pixel Sensors with Pinned Depleted Diode Structure
The X-ray Silicon-On-Insulator (SOI) pixel sensor named XRPIX has been
developed for the future X-ray astronomical satellite FORCE. XRPIX is capable
of a wide-band X-ray imaging spectroscopy from below 1 keV to a few tens of keV
with a good timing resolution of a few tens of s. However, it had a major
issue with its radiation tolerance to the total ionizing dose (TID) effect
because of its thick buried oxide layer due to the SOI structure. Although new
device structures introducing pinned depleted diodes dramatically improved
radiation tolerance, it remained unknown how radiation effects degrade the
sensor performance. Thus, this paper reports the results of a study of the
degradation mechanism of XRPIX due to radiation using device simulations. In
particular, mechanisms of increases in dark current and readout noise are
investigated by simulation, taking into account the positive charge
accumulation in the oxide layer and the increase in the surface recombination
velocity at the interface between the sensor layer and the oxide layer. As a
result, it is found that the depletion of the buried p-well at the interface
increases the dark current, and that the increase in the sense-node capacitance
increases the readout noise.Comment: 7 pages, 10 figures, accepted for publication in IEEE-TN