55 research outputs found
Discovery of Spectral Transitions from Two Ultra-Luminous Compact X-Ray Sources in Ic342
Two {\it ASCA} observations were made of two ultra-luminous compact X-ray
sources (ULXs), Source 1 and Source 2, in the spiral galaxy IC 342. In the 1993
observation, Source 2 showed a 0.5--10 keV luminosity of
ergs s (assuming a distance of 4.0 Mpc), and a hard power-law spectrum
of photon index . As already reported, Source 1 was times
brighter on that occasion, and exhibited a soft spectrum represented by a
multi-color disk model of inner-disk temperature keV. The second
observation made in February 2000 revealed that Source 1 had made a transition
into a hard spectral state, while Source 2 into a soft spectral state. The ULXs
are therefore inferred to exhibit two distinct spectral states, and sometimes
make transitions between them. These results significantly reinforce the
scenario which describes ULXs as mass-accreting black holes.Comment: 11 pages, 3 figures; acceoted for ApJ
Manipulating flexible parts using a teleoperated system with time delay: An experiment
This paper reports experiments involving the handling of flexible parts (e.g. wires) when using a teleoperated system with time delay. The task is principally a peg-in-hole task involving the wrapping of a wire around two posts on the task-board. It is difficult to estimate the effects of the flexible parts; therefore, on-line teleoperation is indispensable for this class of unpredictable task. We first propose a teleoperation system based on the predictive image display, then describe an experimental teleoperation testbed with a four second transmission time delay. Finally, we report on wire handling operations that were performed to evaluate the performance of this system. Those experiments will contribute to future advanced experiments for the MITI ETS-7 mission
Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources
Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and
Galactic superluminal jet sources share the common spectral characteristic that
they have unusually high disk temperatures which cannot be explained in the
framework of the standard optically thick accretion disk in the Schwarzschild
metric. On the other hand, the standard accretion disk around the Kerr black
hole might explain the observed high disk temperature, as the inner radius of
the Kerr disk gets smaller and the disk temperature can be consequently higher.
However, we point out that the observable Kerr disk spectra becomes
significantly harder than Schwarzschild disk spectra only when the disk is
highly inclined. This is because the emission from the innermost part of the
accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen
for a face-on disk. The Galactic superluminal jet sources are known to be
highly inclined systems, thus their energy spectra may be explained with the
standard Kerr disk with known black hole masses. For ULXs, on the other hand,
the standard Kerr disk model seems implausible, since it is highly unlikely
that their accretion disks are preferentially inclined, and, if edge-on Kerr
disk model is applied, the black hole mass becomes unreasonably large (> 300
M_solar). Instead, the slim disk (advection dominated optically thick disk)
model is likely to explain the observed super-Eddington luminosities, hard
energy spectra, and spectral variations of ULXs. We suggest that ULXs are
accreting black holes with a few tens of solar mass, which is not unexpected
from the standard stellar evolution scenario, and that their X-ray emission is
from the slim disk shining at super-Eddington luminosities.Comment: ApJ, accepte
Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC 1313
Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313,
named X-1 and X-2, were observed with Suzaku on 2005 September 15. During the
observation for a net exposure of 28~ks (but over a gross time span of 90~ks),
both objects varied in intensity by about 50~%. The 0.4--10 keV X-ray
luminosity of X-1 and X-2 was measured as
and , respectively, with the former the
highest ever reported for this ULX. The spectrum of X-1 can be explained by a
sum of a strong and variable power-law component with a high energy cutoff, and
a stable multicolor blackbody with an innermost disk temperature of
keV. These results suggest that X-1 was in a ``very high'' state, where the
disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1
is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best
represented, in its fainter phase, by a multicolor blackbody model with the
innermost disk temperature of 1.2--1.3 keV, and becomes flatter as the source
becomes brighter. Hence X-2 is interpreted to be in a slim-disk state. These
results suggest that the two ULXs have black hole masses of a few tens to a few
hundreds solar masses.Comment: accepted for publication in PAS
Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (NGC 5194)
We observed the nearby galaxy M~51 (NGC 5194) with BeppoSAX. The X-ray
properties of the nucleus below 10 keV are almost the same as the ASCA results
regarding the hard component and the neutral Fe K line, but the
intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS
data, we detected a bright hard X-ray emission component which dominates above
10 keV. The 10 -- 100 keV flux and luminosity of this component are
respectively erg s cm and erg
s. These are about 10 times higher than the extrapolation from the soft
X-ray band, and similar to the flux observed with Ginga, which found a bright
power law component in 2 -- 20 keV band. Considering other wavelength
properties and the X-ray luminosity, together with strong neutral Fe K line,
the hard X-ray emission most likely arises from a low luminosity active
nucleus, which is obscured with a column density of cm.
This suggests that hidden low luminosity AGNs may well be present in other
nearby galaxies. We interpret the discrepancy between Ginga and other X-ray
satellites to be due to a large variability of absorption column density toward
the line of sight over several years, suggesting that the Compton thick
absorption material may be present on a spatial scale of a parsec. Apart from
the nucleus, several ultra-luminous off-nuclear X-ray sources detected in M~51
exhibit long-term time variability, suggesting the state transition similar to
that observed in Galactic black hole candidates.Comment: 18 pages, 8 figures, accepted for A&
Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC 1313
A study was made of two ultraluminous X-ray soures (ULXs) in the nearby face-on, late-type Sb galaxy NGC 1313 using data from Suzaku, the 5th Japanese X-ray satellite. Within the 90 ks observation, both sources named X-1 and X-2 exhibited luminosity change by about 50%. The 0.4-10 keV X-ray luminosity was measured to be 2.5 x 10(exp 40) erg per second and 5.8 x 10 erg per second for X-1 and X-2, respectively, requiring a black hole of 50-200 solar mass in order not to exceed the Eddingtion limit. For X-1: the spectrum exhibited a strong power-law component with a high energy cutoff which is thought to arise from strong Comptonization by a disk corona, suggesting the source was in a very high state. Absorption line features with equivalent widths of 40-80 eV found at 7.0 keV and 7.8 keV in the X-1 spectrum support the presence of a highly ionized plasma and a high mass accretion rate on the system. Oxygen abundance of the NGC 1313 circumstellar matter toward X-1 was found to be subsolar, viz. O/H = (5.0 plus or minus 1.0) x 10(exp -4). The spectrum of X-2 in fainter phase is best represented by a multicolor disk blackbody model with T (sub in) = 1.2-1.3 keV and becomes flatter as the flux increases; the source is interpreted to be in a slim disk state
Hard X-ray Detector (HXD) on Board Suzaku
The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from
10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators.
The HXD is designed to achieve an extremely low in-orbit back ground based on a
combination of new techniques, including the concept of well-type active shield
counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV,
the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2}
keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2}
keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for
the phoswich counter. Tight active shielding of the HXD results in a large
array of guard counters surrounding the main detector parts. These
anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large
effective area for sub-MeV to MeV gamma-rays. They work as an excellent
gamma-ray burst monitor with limited angular resolution (~5 degree). The
on-board signal-processing system and the data transmitted to the ground are
also described.Comment: 35 pages, 25 figures and 4 tables; acceted for Publication of the
Astronomical Society of Japa
Microflares and the Statistics of X-ray Flares
This review surveys the statistics of solar X-ray flares, emphasising the new
views that RHESSI has given us of the weaker events (the microflares). The new
data reveal that these microflares strongly resemble more energetic events in
most respects; they occur solely within active regions and exhibit
high-temperature/nonthermal emissions in approximately the same proportion as
major events. We discuss the distributions of flare parameters (e.g., peak
flux) and how these parameters correlate, for instance via the Neupert effect.
We also highlight the systematic biases involved in intercomparing data
representing many decades of event magnitude. The intermittency of the
flare/microflare occurrence, both in space and in time, argues that these
discrete events do not explain general coronal heating, either in active
regions or in the quiet Sun.Comment: To be published in Space Science Reviews (2011
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