312,379 research outputs found
Line digraphs and coreflexive vertex sets
The concept of coreflexive set is introduced to study the structure of
digraphs. New characterizations of line digraphs and nth-order line digraphs
are given. Coreflexive sets also lead to another natural way of forming an
intersection digraph from a given digraph.Comment: 8 pages, 3 figure
The long-term optical behavior of MRK421
All data available in B band for the BL Lac object MRK421 from 22
publications are used to construct a historical light curve, dating back to
1900. It is found that the light curve is very complicated and consists of a
set of outbursts with very large duration. The brightness of MRK421 varies from
11.6 magnitude to more than 16 magnitude. Analyses with Jurkevich method of
computing period of cyclic phenomena reveal in the light curve two kinds of
behaviors. The first one is non-periodic with rapid, violent variations in
intensity on time scales of hours to days. The second one is periodic with a
possible period of years. Another possible period of years is not very significant. We have tested the robustness of the
Jurkevich method. The period of about one year found in the light curves of
MRK421 and of other objects is a spurious period due to the method and the
observing window. We try to explain the period of years under the
thermal instability of a slim accretion disk around a massive black hole of
mass of .Comment: Tex, 14 pages, 5 Postscript figures. Accepted for publication in A&A
Supplement Serie
The Spectral Features of Disk and Corona with Mass Evaporation in the Low/Hard State
We investigate the spectral features of accretion flows composed of an outer
cool, optically thick disk and inner hot, optically thin, advection dominated
accretion flows (ADAF) within the framework of disk and corona with mass
evaporation (Liu et al. 2002a). In this work, both the magnetic field and
Compton scattering of soft photons from the disk by electrons in the corona are
included to calculate the evaporation rates at different distances. The disk is
truncated at the distance where the evaporation rate equals to the accretion
rate (). For a series of accretion rates,
the corresponding truncation radii are calculated out, with which we are able
to calculate the emergent spectra from the inner ADAF + outer disk + corona. At
very low accretion rates, the spectra are similar to that of a pure ADAF
because the disk is truncated at large distances. The disk component becomes
important at high accretion rates since the truncation occurs at small
distances. An anti-correlation between the Eddington ratio and the hard X-ray photon index at low/hard states is predicted by the model. Comparing the theoretical
results with observations, we find that our model can reproduce the
anti-correlation between the Eddington ratio and the hard X-ray photon
index observed for the X-ray binary XTE J1118+480.Comment: 11 pages and 7 figures, published by PAS
The Dependence of Spectral State Transition and Disk Truncation on Viscosity Parameter $\alpha
A wealth of Galactic accreting X-ray binaries have been observed both in
low/hard state and high/soft state. The transition between these two states was
often detected. Observation shows that the transition luminosity between these
two states is different for different sources, ranging from 1% to 4% of the
Eddington luminosity. Even for the same source the transition luminosity at
different outbursts is also different. The transition can occur from 0.0069 to
0.15 Eddington luminosity. To investigate the underlying physics, we study the
influence of viscosity parameter on the transition luminosity on the
basis of the disk-corona model for black holes. We calculate the mass
evaporation rate for a wide range of viscosity parameter, . By fitting the numerical results, we obtain fitting formulae for both the
transition accretion rate and the corresponding radius as a function of
. We find that the transition luminosity is very sensitive to the value
of , . For ,
the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2
Eddington luminosity. Comparing with observations we find that the transition
luminosity can be fitted by adjusting the value of , and the model
determined values of are mostly in the range of observationally
inferred value. Meanwhile we investigate the truncation of the disk in the
low/hard state for some luminous sources. Our results are roughly in agreement
with the observations.Comment: 8 pages, 3 figures,accepted by PAS
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