306 research outputs found
Super-Eddington accretion rates in Narrow Line Seyfert 1 galaxies
Using the BH masses deduced from the empirical relation of Kaspi et al.
(2000) and assuming that the optical luminosity is provided by the accretion
disc, we show that Narrow Line Seyfert Galaxies 1 (NLS1s) accrete at
super-Eddington rates, while their luminosity stays of the order of the
Eddington limit. We take into account the possibility of a non-viscous energy
release in the gravitationally unstable region of the disc. It leads to a
smaller accretion rate and to a redder continuum than a standard disc, which
agrees better with the observations. The observed bolometric luminosities
appear to saturate at a few times the Eddington luminosity for super-Eddington
accretion rates, as predicted by slim disc models. The accretion rate stays
always of the order of a few M/yr, indicating that the growing of the
BH is mass supply limited . Since the masses of the BH increases by one order
of magnitude in a few 10 years, it could explain why NLS1s appear to not
follow the same BH - bulge relation as other galaxies. NLS1s should thus play
an important role in shaping the mass function of local BHs. We discuss the
possibility that the masses could be systematically underestimated due to an
inclination effect, and we conclude that the accretion rates could thus be
strongly overestimated, but only in a small proportion of objects.Comment: 13 pages, 8 figures, accepted in A &
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
Slim Disk Model for Soft X-Ray Excess and Variability of Narrow-Line Seyfert 1 Galaxies
Narrow-line Seyfert 1 galaxies (NLS1s) exhibit extreme soft X-ray excess and
large variability. We argue that both features can be basically accounted for
by the slim disk model. We assume that a central black-hole mass in NLS1 is
relatively small, , and that a disk shines nearly at
the Eddington luminosity, . Then, the disk becomes a slim disk and
exhibits the following distinctive signatures: (1) The disk luminosity
(particularly of X-rays) is insensitive to mass-flow rates, , since the
generated energy is partly carried away to the black hole by trapped photons in
accretion flow. (2) The spectra are multi-color blackbody. The maximum
blackbody temperature is keV,
and the size of the blackbody emitting region is small, r_{\rm bb} \lsim 3
r_{\rm S} (with being Schwarzschild radius) even for a
Schwarzschild black hole. (3) All the ASCA observation data of NLS1s fall onto
the region of (with being the Eddington
luminosity) on the () plane, supporting our view that a
slim disk emits soft X-rays at in NLS1s. (4) Magnetic energy
can be amplified, at most, up to the equipartition value with the trapped
radiation energy which greatly exceeds radiation energy emitted from the disk.
Hence, energy release by consecutive magnetic reconnection will give rise to
substantial variability in soft X-ray emission.Comment: 9 pages LaTeX including 4 figures, accepted to PASJ. e-mail to
[email protected]
Slim Disk Model for Narrow-Line Seyfert 1 Galaxies
We argue that both the extreme soft X-ray excess and the large-amplitude
variability of Narrow-Line Seyfert 1 galaxies (NLS1s) can be explained in the
framework of the slim disk model. When the disk luminosity approaches the
Eddington luminosity, the disk becomes a slim disk, exhibiting a multi-color
blackbody spectrum with a maximum temperature, T(bb), of about 0.2 (M/1e5 solar
masses)e(-1/4) keV, and size of the X-ray emitting region, r(bb), of about R(S)
(the Schwarzschild radius). Furthermore, magnetic energy can be amplified up to
a level exceeding radiation energy emitted from the disk, causing substantial
variability in X-rays by consecutive magnetic flares.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
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