2,138 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
A universal average spectral energy distribution for quasars from optical to extreme ultraviolet
The well-known anti-correlation between the optical/ultraviolet (UV) emission
line equivalent widths of active galactic nuclei and the continuum luminosity
(the so-called Baldwin effect) is a long-standing puzzle. One common hypothesis
is that more luminous sources have softer spectral energy distribution (SED) in
the extreme UV (EUV), as revealed by some observational studies. In this work
we revisit this issue through cross-matching SDSS quasars with GALEX
far-UV/near-UV catalogs and correcting the effect of a severe observational
bias of significant UV detection incompleteness, i.e., the more luminous in
observed-frame optical, the more likely detected in observed-frame UV. We find
that, for GALEX detected quasars at 1.8 < z < 2.2, the rest-frame mean UV SED
(~ 500 -- 3000 Angstrom) bewilderingly shows no luminosity dependence at
log(\nu L_\nu(2200 Angstrom)) > 45 (up to 47.3), contrary to the standard thin
disc model predictions and the observed Baldwin effect in this luminosity
range. Probably, the universal mean UV SED is the result of a local
atomic-originated process, and in fainter quasars stronger disk turbulence
launching more clouds is the main origin of the Baldwin effect. After
correcting for the absorption of the intergalactic medium, a rest-frame
intrinsic mean EUV SED is derived from a sub-sample of bright quasars and is
found to be much redder in the EUV than all previous quasar composite spectra,
highlighting the significance of properly accounting for the sample
incompleteness. Interestingly, the global consistence between our extremely red
mean EUV SED and the line-driven wind model again supports an origin of a local
physical process.Comment: 27 pages, 15 figures, author's initial version submitted to Nature
Astronom
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