151 research outputs found
X-ray Variability Characteristics of the Seyfert 1 Galaxy NGC 3783
We have characterized the energy-dependent X-ray variability properties of
the Seyfert~1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-ray Timing
Explorer data. The high-frequency fluctuation power spectral density function
(PSD) slope is consistent with flattening towards higher energies. Light curve
cross correlation functions yield no significant lags, but peak coefficients
generally decrease as energy separation of the bands increases on both short
and long timescales. We have measured the coherence between various X-ray bands
over the temporal frequency range of 6e-8 to 1e-4 Hz; this range includes the
temporal frequency of the low-frequency power spectral density function (PSD)
break tentatively detected by Markowitz et al. and includes the lowest temporal
frequency over which coherence has been measured in any AGN to date. Coherence
is generally near unity at these temporal frequencies, though it decreases
slightly as energy separation of the bands increases. Temporal
frequency-dependent phase lags are detected on short time scales; phase lags
are consistent with increasing as energy separation increases or as temporal
frequency decreases. All of these results are similar to those obtained
previously for several Seyfert galaxies and stellar-mass black hole systems.
Qualitatively, these results are consistent with the variability models of
Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly
propagating variations in the local mass accretion rate.Comment: Accepted for publication in The Astrophysical Journal, 2005, vol.
635, p. 180; version 2 has minor grammatical changes; 23 pages; uses
emulateapj
ASCA Observations of the Jet Source XTE J1748-288
XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was
observed with ASCA on 1998/09/06 and 1998/09/26, 100 days after the onset of
the radio-X-ray outburst. The spectra were fitted with an attenuated power-law
model, and the 2-6-keV flux was 4.6 * 10^{-11} erg s^{-1} cm^{-2} and 2.2 *
10^{-12} on 09/06 and 09/26, respectively. The light curve showed that the
steady exponential decay with an e-folding time of 14 days lasted over 100 days
and 4 orders of magnitude from the peak of the outburst. The celestial region
including the source had been observed with ASCA on 1993/10/01 and 1994/09/22,
years before the discovery. In those period, the flux was < 10^{-13} erg s^{-1}
cm^{-2}, below ASCA's detection limit. The jet blob colliding to the
environmental matter was supposedly not the X-ray source, although the emission
mechanism has not been determined. A possible detection of a K line from highly
ionized iron is discussed.Comment: 11 pages, 4 figures, submitted to ApJL. Fig2 is replaced with correct
on
Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field
Force due to the self-gravity of the disc in the vertical direction is
considered to study its possible effects on the structure of a magnetized
advection-dominated accretion disc. We present steady-sate self similar
solutions for the dynamical structure of such a type of the accretion flows.
Our solutions imply reduced thickness of the disc because of the self-gravity.
It also imply that the thickness of the disc will increase by adding the
magnetic field strength.Comment: Accepted for publication in Astrophysics and Space Science
Where Are All The Fallback Disks? Constraints on Propeller Systems
Fallback disks are expected to form around new-born neutron stars following a
supernova explosion. In almost all cases, the disk will pass through a
propeller stage. If the neutron star is spinning rapidly (initial period ms) and has an ordinary magnetic moment ( G cm), the
rotational power transferred to the disk by the magnetic field of the neutron
star will exceed the Eddington limit by many orders of magnitude, and the disk
will be rapidly disrupted. Fallback disks can thus survive only around
slow-born neutron stars and around black holes, assuming the latter do not
torque their surrounding disks as strongly as do neutron stars. This might
explain the apparent rarity of fallback disks around young compact objects.Comment: Submitted to Astrophysical Journal Letter
First Detection of 12CO (1--0) Emission from Two Narrow-Line Seyfert 1 Galaxies
In order to investigate how the growth of galactic bulges progresses with the
growth of central black holes (BHs), we observed molecular gas (fuel for the
coming star formation) in possibly young active galaxies, narrow-line Seyfert 1
galaxies (NLS1s). We present the results of radio observations of 12CO(1--0)
using the Nobeyama Millimeter Array (with 2--4 kpc spatial resolution) for two
FIR-bright NLS1s, yielding the first detection of their CO emission.
Corresponding molecular--gas masses M(H2) of (1-3) X 109 Msun are the 2nd and
4th largest ones among NLS1s. By estimating dynamical masses and bulge masses
M(bulge) for these two NLS1s using CO channel map and CO line widths, we found
M(H2) amount to 0.13--0.35 of these masses. Taking account the star formation
efficiency (~ 0.1), the increase in M(bulge) in those NLS1s in the near future
(~< 10^{7.5} yr) is expected not to be a huge fraction (1--5% of the
preexisting stars). Bulge growth may have finished before BH growth, or
bulge--BH coevolution may proceed with many, occasional discrete events, where
one coevolution event produces only a small amount of mass growth of BHs and of
bulges. We also discuss the ratios of star-formation rate--to--gas accretion
rate onto BHs, finding that two NLS1s have very small ratios (~ 1) compared
with the M(bulge)/M(BH) ratios found in active and inactive galaxies (~ 700).
This huge difference suggests either the non-overlapped coevolution, long star
formation duration or temporarily high accretion rate during NLS1 phase.Comment: 29 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
An Incoherent Dynamo in Accretion Disks
We use the mean-field dynamo equations to show that an incoherent alpha
effect in mirror-symmetric turbulence in a shearing flow can generate a large
scale, coherent magnetic field. We illustrate this effect with simulations of a
few simple systems. In accretion disks, this process can lead to axisymmetric
magnetic domains whose radial and vertical dimensions will be comparable to the
disk height. This process may be responsible for observations of dynamo
activity seen in simulations of dynamo-generated turbulence involving, for
example, the Balbus-Hawley instability. In this case the magnetic field
strength will saturate at times the ambient pressure in real
accretion disks. The resultant dimensionless viscosity will be of the same
order. In numerical simulations the azimuthal extent of the simulated annulus
should be substituted for . We compare the predictions of this model to
numerical simulations previously reported by Brandenburg et al. (1995). In a
radiation pressure dominated environment this estimate for viscosity should be
reduced by a factor of due to magnetic buoyancy.Comment: 23 pages, uses aaste
A Sequence of Declining Outbursts from GX339-4
The flux and spectrum of the black hole candidate GX339-4 has been monitored
by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-ray
Observatory (CGRO) since the observatory became operational in May 1991.
Between the summer of 1991 and the fall of 1996, eight outbursts from GX339-4
were observed. The history of these outbursts is one of declining fluence or
total energy release, as well as a shortening of the time between outbursts. A
rough linear correlation exists between the fluence emitted during an outburst
and the time elapsed between the end of the previous outburst and the beginning
of the current one. The peak flux is also roughly linearly correlated with
outburst fluence. The lightcurves of the earlier, more intense, outbursts
(except for the second one) can be modeled by a fast exponential (time constant
~ 10 days) followed by a slower exponential (~ 100 days) on the rise and a fast
exponential decay (~ 5 days) on the fall. The later, weaker, outbursts are
modeled with a single rising time constant (~ 20 days) and a longer decay on
the fall (~ 50 days). An exponential model gives a marginally better fit than a
power law to the rise/decay profiles. GX339-4 is a unique source in having more
frequent outbursts than other low mass x-ray binary black hole candidates.
These observations can be used to constrain models of the behavior of the
accretion disk surrounding the compact object.Comment: Accepted for Publication in the Astrophysical Journal Letters, AASTE
X-Ray Properties of Narrow-Line Seyfert 1 Galaxies with Very Small Broad-Line Widths
Narrow-line Seyfert\,1 galaxies (NLS1s) with very small broad-line widths
(say, FWHM(\hb) \la 1200\,\kms) represent the extreme type of Seyfert\,1
galaxies that have small black hole masses (\mbh) and/or high Eddington ratios
(\redd). Here we study the X-ray properties of a homogeneously and optically
selected sample of 13 such objects, termed as very narrow line Seyfert\,1
galaxies (VNLS1s), using archival \xmm\ data. It is found that the Fe K
emission line is at most weak in these objects. A soft X-ray excess is
ubiquitous, with the thermal temperatures falling within a strict range of
0.1--0.2\,keV. Our result highlights the puzzling independence of the thermal
temperature by extending the relations to even smaller FWHM(\hb), i.e., smaller
\mbh\ ( \msun) and/or higher \redd. The excess emission can be
modeled by a range of viable models, though the disk reflection and
Comptonization models generally give somewhat better fits over the smeared
absorption and the -free models. At the Eddington ratios around unity and
above, the X-ray spectral slopes in the 2--10\,keV band are systematically
flatter than the Risaliti et al.'s predictions of the relationship with \redd\
suggested previously. Short timescale (1--2 hours) X-ray variability is common,
which, together with the variability amplitude computed for some of the
objects, are supportive of the scenario that NLS1s are indeed AGN with
relatively small \mbh.Comment: 11 figures and 4 table. Accepted for publication in the Astrophysical
Journa
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