For refcode 2009A&A...494..905P: Retrieve 15 NED objects in this reference. Please click here for ADS abstract
NED Abstract
Copyright by European Southern Observatory (ESO).
Reproduced by permission
2009A&A...494..905P
A correlation between the spectral and timing properties of AGN
Papadakis, I. E.; Sobolewska, M.; Arevalo, P.; Markowitz, A.; McHardy, I.
M.; Miller, L.; Reeves, J. N.; Turner, T. J.
Abstract. Context: We present the results from a combined study of the
average Xray spectral and timing properties of 14 nearby AGN. Aims: We
investigate whether a "spectraltiming" AGN correlation exists, similar
to the one observed in Cyg X1, compare the two correlations, and
constrain possible physical mechanisms responsible for the Xray
emission in compact, accreting objects. Methods: For 11 of the sources
in the sample, we used all the available data from the RXTE archive,
which were taken until the end of 2006. There are 7795 RXTE
observations i n total for these AGN, obtained over a period of ~ 711
years. We extracted their 320 keV spectra and fitted them with a
simple powerlaw model, modified by the presence of a Gaussian line (at
6.4 keV) and cold absorption, when necessary. We used the bestfit
slopes to construct their sample distribution function, and we used the
median of the distribution, and the mean of the bestfit slopes, which
are above the 80th percentile of the distributions, to estimate the
mean spectral slope of the objects. The latter estimate is more
appropriate in the case when the energy spectra of the sources are
significantly affected by absorption and/or reflection effects. We also
used results from the literature to estimate the average spectral slope
of the three remaining objects. Results: The AGN average spectral
slopes are not correlated either with the black hole mass or the
characteristic frequencies that were detected in the power spectra.
They are positively correlated, though, with the characteristic
frequency when normalised to the sources black hole mass. This
correlation is similar to the spectraltiming correlation that has been
observed in Cyg X1, but not the same. Conclusions: The AGN
spectraltiming correlation can be explained if we assume that the
accretion rate determines both the average spectral slope and the
characteristic time scales in these systems. The spectrum should
steepen and the characteristic frequency should increase,
proportionally, with increasing accretion rate. We also provide a
quantitative expression between spectral slope and accretion rate.
Thermal Comptonisation models are broadly consistent with our result,
and can also explain the difference between the spectraltiming
correlations in Cyg X1 and AGN, but only if the ratio of the soft
photons' luminosity to the power injected to the hot corona is
proportionally related to the accretion rate.
Key words: galaxies: active, galaxies: Seyfert, Xrays: galaxies
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