14,611 research outputs found
A toy model for X-ray spectral variability of active galactic nuclei
The long term X-ray spectral variability of ten active galactic nuclei (AGN)
shows a positive spectral index-flux correlation for each object (Sobolewska &
Papadakis 2009). An inner advection dominated accretion flow (ADAF) may connect
to a thin disc/corona at a certain transition radius, which are responsible for
hard X-ray emission in AGN. The ADAF is hot and its X-ray spectrum is hard,
while the corona above the disc is relatively cold and its X-ray spectrum is
therefore soft. The radiation efficiency of the ADAF is usually much lower than
that of the thin disc. The increase of the transition radius may lead to
decreases of the spectral index (i.e., a hard spectrum) and the X-ray
luminosity even if the accretion rate is fixed, and vice versa. We propose that
such X-ray variability is caused by the change of the transition radius. Our
model calculations can reproduce the observed index-flux correlations, if the
transition radius fluctuates around an equilibrium position, and the radiation
efficiency of ADAFs is {\guillemotright} 5 per cent of that for a thin disc.
The average spectral index-Eddington ratio correlation in the AGN sample can
also be reproduced by our model calculations, if the equilibrium transition
radius increases with decreasing mass accretion rate.Comment: 5 pages, accepted by MNRAS Letter
Topological Strings and Quantum Spectral Problems
We consider certain quantum spectral problems appearing in the study of local
Calabi-Yau geometries. The quantum spectrum can be computed by the
Bohr-Sommerfeld quantization condition for a period integral. For the case of
small Planck constant, the periods are computed perturbatively by deformation
of the Omega background parameters in the Nekrasov-Shatashvili limit. We
compare the calculations with the results from the standard perturbation theory
for the quantum Hamiltonian. There have been proposals in the literature for
the non-perturbative contributions based on singularity cancellation with the
perturbative contributions. We compute the quantum spectrum numerically with
some high precisions for many cases of Planck constant. We find that there are
also some higher order non-singular non-perturbative contributions, which are
not captured by the singularity cancellation mechanism. We fix the first few
orders formulas of such corrections for some well known local Calabi-Yau
models.Comment: 47 pages, 3 figures. v2: journal version, typos correcte
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