959 research outputs found
Ionization, Magneto-rotational, and Gravitational Instabilities in Thin Accretion Disks Around Supermassive Black Holes
We consider the combined role of the thermal ionization, magneto-rotational
and gravitational instabilities in thin accretion disks around supermassive
black holes. We find that in the portions of the disk unstable to the
ionization instability, the gas remains well coupled to the magnetic field even
on the cold, neutral branch of the thermal limit cycle. This suggests that the
ionization instability is not a significant source of large amplitude
time-dependent accretion in AGN. We also argue that, for accretion rates
greater than 10^{-2} solar masses per year, the gravitationally unstable and
magneto-rotationally unstable regions of the accretion disk overlap; for lower
accretion rates they may not. Some low-luminosity AGN, e.g. NGC 4258, may thus
be in a transient phase in which mass is building up in a non-accreting
gravitationally and magneto-rotationally stable ``dead zone.'' We comment on
possible implications of these findings.Comment: Accepted for publication in Ap
ASCA PV observations of the Seyfert 2 galaxy NGC 4388: the obscured nucleus and its X-ray emission
We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster
observed with ASCA during its PV phase. The 0.5-10 keV X-ray spectrum consists
of multiple components; (1) a continuum component heavily absorbed by a column
density NH = 4E23 cm-2 above 3 keV; (2) a strong 6.4 keV line (EW = 500 eV);
(3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray
emission below 1 keV. The detection of strong absorption for the hard X-ray
component is firm evidence for an obscured active nucleus in this Seyfert 2
galaxy. The absorption corrected X-ray luminosity is about 2E42 erg/s. This is
the first time that the fluorescent iron-K line has been detected in this
object. The flat spectrum in the intermediate energy range may be a scattered
continuum from the central source. The soft X-ray emission below 1 keV can be
thermal emission from a temperature kT = 0.5 keV, consistent with the spatially
extended emission observed by ROSAT HRI. However, the low abundance (0.05 Zs)
and high mass flow rate required for the thermal model and an iron-K line
stronger than expected from the obscuring torus model are puzzling. An
alternative consistent solution can be obtained if the central source was a
hundred times more luminous over than a thousand years ago. All the X-ray
emission below 3 keV is then scattered radiation.Comment: 9 pages, 5 Postscript figures, to be published in MNRA
Is the black hole in GX 339-4 really spinning rapidly?
The wide-band Suzaku spectra of the black hole binary GX 339-4, acquired in
2007 February during the Very High state, were reanalyzed. Effects of event
pileup (significant within ~ 3' of the image center) and telemetry saturation
of the XIS data were carefully considered. The source was detected up to ~ 300$
keV, with an unabsorbed 0.5--200 keV luminosity of ~3.8 10^{38} erg/s at 8 kpc.
The spectrum can be approximated by a power-law of photon index 2.7, with a
mild soft excess and a hard X-ray hump. When using the XIS data outside 2' of
the image center, the Fe-K line appeared extremely broad, suggesting a high
black hole spin as already reported by Miller et al. (2008) based on the Suzaku
data and other CCD data. When the XIS data accumulation is further limited to
>3' to avoid event pileup, the Fe-K profile becomes narrower, and there appears
a marginally better solution that suggests the inner disk radius to be 5-14
times the gravitational radius (1-sigma), though a maximally spinning black
hole is still allowed by the data at the 90% confidence level. Consistently,
the optically-thick accretion disk is inferred to be truncated at a radius 5-32
times the gravitational radius. Thus, the Suzaku data allow an alternative
explanation without invoking a rapidly spinning black hole. This inference is
further supported by the disk radius measured previously in the High/Soft
state.Comment: 5 pages, figures, Suzaku results on GX 339-4, accepted to APJL. Nov.
11, 2009, accepted to ApJ
X-ray Measurements of the Gravitational Potential Profile in the Central Region of the Abell 1060 Cluster of Galaxies
X-ray spectral and imaging data from ASCA and ROSAT were used to measure the
total mass profile in the central region of Abell 1060, a nearby and relatively
poor cluster of galaxies. The ASCA X-ray spectra, after correcting for the
spatial response of the X-ray telescope, show an isothermal distribution of the
intra-cluster medium (ICM) within at least 12' (or kpc;
km sMpc) in radius of the cluster center. The
azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a
central excess above an isothermal model. The ring-sorted ASCA GIS
spectra and the radial surface brightness distribution from the ROSAT PSPC were
simultaneously utilized to constrain the gravitational potential profile. Some
analytic models of the total mass density profile were examined. The ICM
density profile was also specified by analytic forms. The ICM temperature
distribution was constrained to satisfy the hydrostatic equilibrium, and to be
consistent with the data. Then, the total mass distribution was found to be
described better by the universal dark halo profile proposed by Navarro, Frenk,
and White (1996;1997) than by a King-type model with a flat density core. A
profile with a central cusp together with a logarithmic radial slope of was also consistent with the data. Discussions are made concerning the
estimated dark matter distribution around the cluster center.Comment: 32 pages. Accepted: ApJ 2000, 35 pages, Title was correcte
The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026
using observations made with the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. The mass function obtained of ~16Mo together with the
detection of an extended near-total eclipse confirm that the primary star is a
supergiant as predicted. The orbital eccentricity is found to be very low with
a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be
6.0724 +/- 0.0009 days using an improved and extended light curve obtained with
RXTE's All Sky Monitor. Observations with the ASCA satellite provide an
improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0"
(2000) with an estimated systematic uncertainty of less than 12". A
serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA
observations.Comment: Accepted for publication in the Astrophysical Journa
A search for cyclotron resonance features with INTEGRAL
We present an INTEGRAL observation of the Cen-Crux region in order to search
the electron cyclotron resonance scattering features from the X-ray binary
pulsars. During the AO1 200ks observation, we clearly detected 4 bright X-ray
binaries, 1 Seyfert Galaxy, and 4 new sources in the field of view. Especially
from GX301-2, the cyclotron resonance feature is detected at about 37 keV, and
width of 3--4 keV. In addition, the depth of the resonance feature strongly
depends on the X-ray luminosity. This is the first detection of luminosity
dependence of the resonance depth. The cyclotron resonance feature is
marginally detected from 1E1145.1-6141. Cen X-3 was very dim during the
observation and poor statistics disable us to detect the resonance
features.These are first INTEGRAL results of searching for the cyclotron
resonance feature.Comment: 4pages, 8figures, To be published in the Proceedings of the 5th
INTEGRAL Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munic
The Structure of the X-Ray Emitting Gas in the Hydra-A Cluster of Galaxies
The temperature and abundance structure in the intracluster medium (ICM) of
the Hydra-A cluster of galaxies is studied with ASCA and ROSAT. The effect of
the large extended outskirts in the point-spread function of the X-Ray
Telescope on ASCA is included in this analysis. In the X-ray brightness
profile, the strong central excess above a single beta-model, identified in the
Einstein and ROSAT data, is also found in the harder energy band (>4keV). A
simultaneous fit of five annular spectra taken with the GIS instrument shows a
radial distribution of the temperature and metal abundance. A significant
central enhancement in the abundance distribution is found, while the
temperature profile suggests that the ICM is approximately isothermal with the
temperature of ~3.5keV. The ROSAT PSPC spectrum in the central 1'.5 region
indicates a significantly lower temperature than the GIS result. A joint
analysis of the GIS and PSPC data reveals that the spectra can be described by
a two temperature model as well as by a cooling flow model. In both cases, the
hot phase gas with the temperature of ~3.5keV occupies more than 90% of the
total emission measure within 1'.5 from the cluster center. The estimated mass
of the cooler (0.5-0.7keV) component is ~2-6 x 10^9 M_solar, which is
comparable to the mass of hot halos seen in non-cD ellipticals. The cooling
flow model gives the mass deposition rate of 60+-30 M_solar/yr, an order of
magnitude lower than the previous estimation.Comment: 27 pages, 14 figures, AAS LATEX macros v4.0, to appear in The
Astrophysical Journa
The Declined Activity in the Nucleus of NGC 1316
NGC 1316 (Fornax A) is a radio galaxy with prototypical double lobes, where
the magnetic field intensity is accurately measured via the inverse-Compton
technique. The radio-emitting electrons in the lobes are inferred to have a
synchrotron life time of 0.1 Gyr. Considering the lobe energetics, we estimate
the past nuclear X-ray luminosity of NGC 1316 to be at least 4 times 10^{34} W
(4 times 10^{41} erg s^{-1}). Thus, the nucleus was rather active at least 0.1
Gyr ago. In contrast, we confirmed with ASCA and ROSAT that the nucleus of NGC
1316 is very faint in X-rays at present, with the 2--10 keV luminosity of any
AGN-like hard component being < 2 times 10^{33} W (2 times 10^{40} erg s^{-1})
even assuming a nuclear obscuration up to 10^{28} m^{-2} (10^{24} cm^{-2}).
This is at least an order of magnitude lower than the estimated past activity,
indicating that the nucleus is presently very inactive. From these two results,
we conclude that the nucleus of NGC 1316 has become dormant during the last 0.1
Gyr. This suggests the possible abundance of ``dormant'' quasars in nearby
galaxies.Comment: 7 pages, 2 figures, to be published in the Astrophysical Journal
Letter
Recovery of the orbital parameters and pulse evolution of V0332+53 during a huge outburst
The high mass X-ray binary (HMXB) V0332+53 became active at the end of 2004
and the outburst was observed at hard X-rays by RXTE and INTEGRAL. Based on
these hard X-ray observations, the orbital parameters are measured through
fitting the Doppler-shifted spin periods. The derived orbital period and
eccentricity are consistent with those of Stella et al. (1985) obtained from
EXOSAT observations, whereas the projected semimajor axis and the periastron
longitude are found to have changed from 484 to 86 lt-s and
from 31310 to 28314, respectively. This would
indicate an angular speed of 1.50.8 yr for
rotation of the orbit over the past 21 years. The periastron passage time of
MJD 533671 is just around the time when the intensity reached maximum and
an orbital period earlier is the time when the outburst started. This
correlation resembles the behavior of a Type I outburst. During outburst the
source spun up with a rate of 8.01 s
day. The evolution of pulse profile is highly intensity dependent. The
separation of double pulses remained almost constant ( 0.47) when the
source was bright, and dropped to 0.37 within 3 days as the source
became weaker. The pulse evolution of V0332+53 may correlate to the change in
dominance of the emission between fan-beam and pencil-beam mechanisms.Comment: 13 pages, 3 figures, accepted for publication in ApJ
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