1,307 research outputs found
Characteristics of enzymatic induction provoked by chlordane
The effects of various stresses, such as restraint and lowering or raising of environmental temperature, in mice pretreated with chlordane were investigated. (Chlordane is an inhibitor of protein synthesis.) It was found that restraint or exposure to a cold environment for three hours mobilized the chlordane stored in the adipose tissue of mice
A New Equilibrium for Accretion Disks Around Black Holes
Accretion disks around black holes in which the shear stress is proportional
to the total pressure, the accretion rate is more than a small fraction of
Eddington, and the matter is distributed smoothly are both thermally and
viscously unstable in their inner portions. The nonlinear endstate of these
instabilities is uncertain. Here a new inhomogeneous equilibrium is proposed
which is both thermally and viscously stable. In this equilibrium the majority
of the mass is in dense clumps, while a minority reaches temperatures K. The requirements of dynamical and thermal equilibrium completely
determine the parameters of this system, and these are found to be in good
agreement with the parameters derived from observations of accreting black
holes, both in active galactic nuclei and in stellar binary systems.Comment: AAS LaTeX, accepted to Ap. J. Letter
Big Blue Bump and Transient Active Regions in Seyfert Galaxies
An important feature of the EUV spectrum (known as the Big Blue Bump,
hereafter BBB) in Seyfert Galaxies is the narrow range in its cutoff energy E0
from source to source, even though the luminosity changes by 4 orders of
magnitude. Here we show that if the BBB is due to accretion disk emission, then
in order to account for this ``universality'' in the value of E0, the emission
mechanism is probably optically thin bremsstrahlung. In addition, we
demonstrate that the two-phase model with active regions localized on the
surface of the cold disk is consistent with this constraint if the active
regions are very compact and are highly transient, i.e., they evolve faster
than one dynamical time scale.Comment: Accepted for publication in ApJL, uses aas2pp4.st
Detecting Compton Reflection and a Broad Iron Line in MCG-5-23-16 with the Rossi X-ray Timing Explorer
We report the detection with the Rossi X-ray Timing Explorer of a Compton
reflection signature in the Seyfert galaxy MCG-5-23-16. RXTE also resolves the
Fe K-alpha fluorescence line with FWHM ~48,000 km s^{-1}. This measurement
provides the first independent confirmation of ASCA detections in Seyfert
galaxies of broad Fe K-alpha lines that are thought to be the signature of
emission from the inner regions of an accretion disk orbiting a black hole.
Under the assumption that reflection arises from an isotropic source located
above a neutral accretion disk, and using a theoretical model that accounts for
the dependence of the reflected spectrum on inclination angle, we derive a 90%
confidence range for the disk inclination of i = 50 to 81 degrees. The large
inclination is consistent with that expected from the unified model for
MCG-5-23-16 based on its Seyfert 1.9 classification. If we assume that the
high-energy cutoff in the incident spectrum lies at energies larger than a few
hundred keV, then the equivalent width of the Fe K-alpha line is much larger
than predicted for the amount of reflection. This implies either an enhanced
iron abundance, a covering factor of reflecting material > 0.5, or a cutoff in
the incident spectrum at energies between ~60 and ~200 keV.Comment: Accepted for publication in ApJ, LaTeX. 14 pages including 3 figures,
with 1 table as a separate postscript file. Typo corrected in abstrac
Discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an historical low X-ray flux state
We report the discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an
extremely low X-ray state. A comparison of Swift observations obtained in May
and June/July 2007 with all previous X-ray observations between 1971 to 2006
show the AGN to have diminished in flux by a factor of more than 30, the lowest
X-ray flux Mkn 335 has ever been observed in. The Swift observations show an
extremely hard X-ray spectrum at energies above 2 keV. Possible interpretations
include partial covering absorption or X-ray reflection from the disk. In this
letter we consider the partial covering interpretation. The Swift observations
can be well fit by a strong partial covering absorber with varying absorption
column density N_H= 1-4 x 10^{23} cm-2 and a covering fraction f_c=0.9 - 1.
When corrected for intrinsic absorption, the X-ray flux of Mkn 335 varies by
only factors of 4-6. In the UV Mkn 335 shows variability in the order of 0.2
mag. We discuss the similarity of Mkn 335 with the highly variable NLS1
WPVS007, and speculate about a possible link between NLS1 galaxies and
broad-absorption line quasars.Comment: ApJ Letter accepted; 8 pages, 2 figures; The new version has three
more sentences in the introduction and three references added to the
discussio
Long-Term X-ray Spectral Variability in Seyfert 1 Galaxies
Direct time-resolved spectral fitting has been performed on continuous RXTE
monitoring of seven Seyfert 1 galaxies in order to study their broadband
spectral variability and Fe K alpha variability characteristics on time scales
of days to years. Variability in the Fe K alpha line is not detected in some
objects but is present in others, e.g., in NGC 3516, NGC 4151 and NGC 5548
there are systematic decreases in line flux by factors of ~2-5 over 3-4 years.
The Fe K alpha line varies less strongly than the broadband continuum, but,
like the continuum, exhibits stronger variability towards longer time scales.
Relatively less model-dependent broadband fractional variability amplitude
(Fvar) spectra also show weaker line variability compared to the continuum
variability. Comparable systematic long-term decreases in the line and
continuum are present in NGC 5548. Overall, however, there is no evidence for
correlated variability between the line and continuum, severely challenging
models in which the line tracks continuum variations modified only by a
light-travel time delay. Local effects such as the formation of an ionized skin
at the site of line emission may be relevant. The spectral fitting and Fvar
spectra both support spectral softening as continuum flux increases.Comment: Accepted for publication in ApJ. 29 page
An RXTE Survey of Long-Term X-ray Variability in Seyfert 1 Galaxies
Data from the first three years of RXTE observations have been systematically
analyzed to yield a set of 300 day, 2-10 keV light curves with similarly
uniform, ~5 day sampling, for a total of nine Seyfert 1 galaxies. This is the
first X-ray variability survey to consistently probe time scales longer than a
few days in a large number of AGN. Comparison with ASCA data covering a similar
band but much shorter (~1 day) time scales shows that all the AGN are more
strongly variable on long time scales than on short time scales. This increase
is greatest for the highest-luminosity sources. The well-known anticorrelation
between source luminosity and variability amplitude is both stronger and
shallower in power-law slope when measured on long time scales. This is
consistent with a picture in which the X-ray variability of Seyfert 1s can be
can be described by a single, universal fluctuation power density shape for
which the cutoff moves to longer time scales for higher luminosity sources. All
of the Seyfert 1s exhibit stronger variability in the relatively soft 2-4 keV
band than in the harder 7-10 keV band. This effect is much too pronounced to be
explained by simple models based on either the dilution of the power-law
continuum by the Compton reflection component or on the hard X-rays being
produced in a static, pair-dominated, plane-parallel Comptonizing corona.Comment: 18 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
Quasi-thermal Comptonization and gamma-ray bursts
Quasi-thermal Comptonization in internal shocks formed between relativistic
shells can account for the high energy emission of gamma-ray bursts. This is in
fact the dominant cooling mechanism if the typical energy of the emitting
particles is achieved either through the balance between heating and cooling or
as a result of electron-positron pair production. Both processes yield sub or
mildly relativistic energies. In this case the synchrotron spectrum is
self-absorbed, providing the seed soft photons for the Comptonization process,
whose spectrum is flat [F(v) ~ const], ending either in an exponential cutoff
or a Wien peak, depending on the scattering optical depth of the emitting
particles. Self-consistent particle energy and optical depth are estimated and
found in agreement with the observed spectra.Comment: 10 pages, ApJ Letters, accepted for publicatio
Comparisons of various model fits to the Iron line profile in MCG-6-30-15
The broad Iron line in MCG-6-30-15 is fitted to the Comptonization model
where line broadening occurs due to Compton down-scattering in a highly ionized
optically thick cloud. These results are compared to the disk line model where
the broadening is due to Gravitational/Doppler effects in the vicinity of a
black hole. We find that both models fit the data well and it is not possible
to differentiate between them by fitting only the ASCA data. The best fit
temperature and optical depth of the cloud are found to be kT = 0.54 keV and
from the Comptonization model. This model further suggests that
while the temperature can be assumed to be constant, the optical depth varies
during the observation period. We emphasis an earlier conclusion that
simultaneous broad band data ( keV) can rule out (or confirm) the
Comptonization model.Comment: 4 figures. uses aasms4.sty, accepted by ApJ, email:
[email protected]
Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts
The blast-wave model for gamma-ray bursts (GRBs) has been called into
question by observations of spectra from GRBs that are harder than can be
produced through optically thin synchrotron emission. If GRBs originate from
the collapse of massive stars, then circumstellar clouds near burst sources
will be illuminated by intense gamma radiation, and the electrons in these
clouds will be rapidly scattered to energies as large as several hundred keV.
Low-energy photons that subsequently pass through the hot plasma will be
scattered to higher energies, hardening the intrisic spectrum. This effect
resolves the "line-of-death" objection to the synchrotron shock model.
Illuminated clouds near GRBs will form relativistic plasmas containing large
numbers of electron-positron pairs that can be detected within ~ 1-2 days of
the explosion before expanding and dissipating. Localized regions of pair
annihilation radiation in the Galaxy would reveal past GRB explosions.Comment: 9 pages, 1 figure, submitted to ApJ Letter
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