213 research outputs found
Suzaku investigation into the nature of the nearest ultraluminous X-ray source, M33 X-8
The X-ray spectrum of the nearest ultraluminous X-ray source, M33 X-8,
obtained by Suzaku during 2010 January 11 -- 13, was closely analyzed to
examine its nature. It is, by far, the only data with the highest signal
statistic in 0.4 -- 10 keV range. Despite being able to reproduce the X-ray
spectrum, Comptonization of the disk photons failed to give a physically
meaningful solution. A modified version of the multi-color disk model, in which
the dependence of the disk temperature on the radius is described as r^(-p)
with p being a free parameter, can also approximate the spectrum. From this
model, the innermost disk temperature and bolometric luminosity were obtained
as T_in = 2.00-0.05+0.06 keV and L_disk = 1.36 x 10^39 (cos i)^(-1) ergs/s,
respectively, where i is the disk inclination. A small temperature gradient of
p = 0.535-0.005+0.004, together with the high disk temperature, is regarded as
the signatures of the slim accretion disk model, suggesting that M33 X-8 was
accreting at high mass accretion rate. With a correction factor for the slim
disk taken into account, the innermost disk radius, R_in =81.9-6.5+5.9 (cos
i)^(-0.5) km, corresponds to the black hole mass of M \sim 10 M_sun (cos
i)^(-0.5). Accordingly, the bolometric disk luminosity is estimated to be about
80 (cos i)^(-0.5)% of the Eddington limit. A numerically calculated slim disk
spectrum was found to reach a similar result. Thus, the extremely
super-Eddington luminosity is not required to explain the nature of M33 X-8.
This conclusion is utilized to argue for the existence of intermediate mass
black holes with M > 100 M_sun radiating at the sub/trans-Eddington luminosity,
among ultraluminous X-ray sources with L_disk > 10^(40) ergs/s.Comment: 10 pages, 4 figures, PASJ accepte
ASCA Observations of Two Ultra-Luminous Compact X-Ray Sources in the Edge-on Spiral Galaxy NGC 4565
The edge-on spiral galaxy NGC 4565 was observed for 35 ks with ASCA in
the 0.5-10 keV energy band. The X-ray emission was dominated by two bright
sources, which can be identified with two point-like X-ray sources seen in the
ROSAT HRI image. The observed 0.5-10 keV fluxes of these sources, and , % % imply bolometric
luminosities of and ,
respectively. They exhibit similar spectra, which can be explained by emission
from optically thick accretion disks with the inner disk temperature of 1.4-1.6
keV. One of them, coincident in position with the nucleus, shows too low
absorption to be the active nucleus seen through the galaxy disk. Their spectra
and high luminosities suggest that they are both mass accreting black hole
binaries. However the black-hole mass required by the Eddington limit is rather
high (), and the observed disk temperature is too high to be
compatible with the high black-hole mass. Several attempts are made to solve
these problesms.Comment: 20page
Suzaku Observations of M82 X-1 : Detection of a Curved Hard X-ray Spectrum
A report is presented on Suzaku observations of the ultra-luminous X-ray
source X-1 in the starburst galaxy M82, made three time in 2005 October for an
exposure of ~ 30 ks each. The XIS signals from a region of radius 3 around the
nucleus defined a 2-10 keV flux of 2.1 x 10^-11 erg s-1 cm-2 attributable to
point sources. The 3.2-10 keV spectrum was slightly more convex than a
power-law with a photon index of 1.7. In all observations, the HXD also
detected signals from M82 up to ~ 20 keV, at a 12-20 keV flux of 4.4 x 10^-12
erg s-1 cm-2 . The HXD spectrum was steeper than that of the XIS. The XIS and
HXD spectra can be jointly reproduced by a cutoff power-law model, or similar
curved models. Of the detected wide-band signals, 1/3 to 2/3 are attributable
to X-1, while the remainder to other discrete sources in M82. Regardless of the
modeling of these contaminants, the spectrum attributable to X-1 is more curved
than a power-law, with a bolometric luminosity of (1.5 -3) x 10 ^40 erg s-1.
These results are interpreted as Comptonized emission from a black hole of
100-200 solar masses, radiating roughly at the Eddington luminosity.Comment: 19 pages, 9 figures, accepted in Publications of the Astronomical
Society of Japa
Spectral transitions of an ultraluminous X-ray source, NGC 2403 Source 3
Suzaku observation of an ultraluminous X-ray source, NGC 2403 Source 3,
performed on 2006 March 16--17, is reported. The Suzaku XIS spectrum of Source
3 was described with a multi-color black-body-like emission from an optically
thick accretion disk. The innermost temperature and radius of the accretion
disk was measured to be keV and km, respectively, where with being the disk inclination. The bolometric
luminosity of the source was estimated to be ergs s. Archival Chandra and XMM-Newton data of the source were
analyzed for long-term spectral variations. In almost all observations, the
source showed multi-color black-body-like X-ray spectra with parameters similar
to those in the Suzaku observation. In only one Chandra observation, however,
Source 3 was found to exhibit a power-law-like spectrum, with a photon index of
, when it was fainter by about than in the
Suzaku observation. The spectral behavior is naturally explained in terms of a
transition between the slim disk state and the "very high" states, both found
in Galactic black hole binaries when their luminosity approach the Eddington
limit. These results are utilized to argue that ultraluminous X-ray sources
generally have significantly higher black-hole masses than ordinary
stellar-mass black holes.Comment: Accepted for PASJ 3nd Suzaku special issu
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
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