217 research outputs found
A Stellar-mass Black Hole in the Ultra-luminous X-ray Source M82 X-1?
We have analyzed the archival XMM-Newton data of the bright Ultra-Luminous
X-ray Source (ULX) M82 X-1 with an 105 ksec exposure when the source was in the
steady state. Thanks to the high photon statistics from the large effective
area and long exposure, we were able to discriminate different X-ray continuum
spectral models. Neither the standard accretion disk model (where the radial
dependency of the disk effective temperature is T(r) \propto r^-3/4) nor a
power-law model gives a satisfactory fit. In fact, observed curvature of the
M82 X-1 spectrum was just between those of the two models. When the exponent of
the radial dependence (p in T(r) \propto r^-p) of the disk temperature is
allowed to be free, we obtained p =0.61^+0.03_-0.02. Such a reduction of p from
the standard value 3/4 under extremely high mass accretion rates is predicted
from the accretion disk theory as a consequence of the radial energy advection.
Thus, the accretion disk in M82 X-1 is considered to be in the Slim disk state,
where an optically thick Advection Dominant Accretion Flow (ADAF) is taking
place. We have applied a theoretical slim disk spectral model to M82 X-1, and
estimated the black hole mass ~ 19-32 M_odot. We propose that M82 X-1 is a
relatively massive stellar black hole which has been produced through evolution
of an extremely massive star, shining at a super-Eddington luminosity by
several times the Eddington limit.Comment: 12 pages, 2 figures, Accepted for ApJ
ASCA Slew Survey
We are systematically analyzing ASCA GIS data taken during the satellite
attitude maneuver operation. Our motivation is to search for serendipitous hard
X-ray sources and make the ASCA Slew Survey catalog.
During its operational life from 1993 February to 2000 July, ASCA carried out
more than 2,500 maneuver operations, and total exposure time during the
maneuver was ~415 ksec after data screening. Preliminary results are briefly
reported.Comment: Proceedings for "X-ray surveys in the light of new observations",
Santander (Spain), 2002 September. 1 pag
A Variable Partial Covering Model for the Seyfert 1 Galaxy MCG-6-30-15
We propose a simple spectral model for the Seyfert 1 Galaxy MCG-6-30-15 that
can explain most of the 1 - 40 keV spectral variation by change of the partial
covering fraction, similar to the one proposed by Miller et al. (2008). Our
spectral model is composed of three continuum components; (1) a direct
power-law component, (2) a heavily absorbed power-law component by mildly
ionized intervening matter, and (3) a cold disk reflection component far from
the black hole with moderate solid-angle ({\Omega}/2{\pi} \approx 0.3)
accompanying a narrow fluorescent iron line. The first two components are
affected by the surrounding highly ionized thin absorber with N_H \approx
10^{23.4}cm-2 and log {\xi} \approx 3.4. The heavy absorber in the second
component is fragmented into many clouds, each of which is composed of radial
zones with different ionization states and column densities, the main body (N_H
\approx 10^24.2cm-2, log {\xi} \approx 1.6), the envelope (N_H \approx
10^22.1cm-2, log {\xi} \approx 1.9) and presumably a completely opaque core.
These parameters of the ionized absorbers, as well as the intrinsic spectral
shape of the X-ray source, are unchanged at all. The central X-ray source is
moderately extended, and its luminosity is not significantly variable. The
observed flux and spectral variations are mostly explained by variation of the
geometrical partial covering fraction of the central source from 0 (uncovered)
to \sim0.63 by the intervening ionized clouds in the line of sight. The ionized
iron K-edge of the heavily absorbed component explains most of the seemingly
broad line-like feature, a well-known spectral characteristic of MCG-6-30-15.
The direct component and the absorbed component anti-correlate, cancelling
their variations each other, so that the fractional spectral variation becomes
the minimum at the iron energy band; another observational characteristic of
MCG-6-30-15 is thus explained.Comment: Accepted to Publications of the Astronomical Society of Japa
Chandra Observations of a Non-Thermal Supernova Remnant Candidate AX J1843.8-0352 and its Surroundings
We present the Chandra results of AX J1843.8-0352, a supernova remnant (SNR)
recently identified with ASCA. Chandra spatially resolved two components from
this SNR: non-thermal and thermal ones. The morphology of the non-thermal
component is clumpy and elliptical, elongated from the north to the south with
a mean diameter of about 9 arcmin. The spectrum is fitted with a power-law
model of photon index 2.1 and the east rim is associated with the non-thermal
radio sources C and F (Helfand et al. 1989). Therefore the non-thermal
component is probably synchrotron X-rays by energetic electrons accelerated at
the shell of the SNR. The thermal component is the brightest clump located
within the non-thermal component and shows a spectrum of a thin plasma of about
0.7 keV temperature. Notable discovery is its peculiar morphology; a head of 50
arcsec x 30 arcsec size near the south-east rim of the SNR and a 30 arcsec-long
``jet'' pointing to the southwest. Although this emission is associated with
the west part of the radio source F, the absorption is twice larger than that
of the non-thermal X-rays, or the bulk of the SNR emission. Therefore, it is
unclear whether this peculiar plasma is a thermal component associated with AX
J1843.8-0352, a Galactic source located in the far side of our Galaxy, or an
extragalactic source.Comment: 19 pages, 5 figures; to appear in Ap
Suzaku X-Ray Observation of the Dwarf Nova Z Camelopardalis at the Onset of an Optical Outburst
We present the result of a Suzaku X-ray spectroscopic observation of the
dwarf nova Z Camelopardalis, which was conducted by chance at the onset of an
optical outburst. We used the X-ray Imaging Spectrometer (a 38 ks exposure) and
the Hard X-ray Detector (34 ks) to obtain a 0.35-40 keV spectrum
simultaneously. Spectral characteristics suggest that the source was in the
X-ray quiescent state despite being in the rising phase of an outburst in the
optical band. The spectrum shows a clear signature of circumstellar absorption
in excess of interstellar absorption and the reprocessed emission features of
Fe fluorescence and Compton scattering. The extra absorption is explained due
to partial coverage by either neutral or ionized matter. We found a spectral
change during the observation, which is attributable only to the change in the
circumstellar absorption. Such an X-ray spectral variation is reported for the
first time in dwarf novae. We speculate that the variation in the circumstellar
absorption is interpreted as a time-varying disk wind or geometrically flaring
disk around the white dwarf during the propagation of a heat wave inward along
the accretion disk at the beginning of the outburst, in which optical outburst
and X-ray quiescent states co-exist.Comment: 10 pages, 9 figures, accepted for publication in PAS
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