Kavli Affiliate: Claudio Ricci
| First 5 Authors: Roberto Serafinelli, Alessandra De Rosa, Alessia Tortosa, Luigi Stella, Fausto Vagnetti
| Summary:
Active galactic nuclei (AGN) are extremely variable in the X-ray band down to
very short time scales. However, the driver of the X-ray variability is still
poorly understood. Previous results suggest that the hot corona, which is
responsible for the primary Comptonized emission observed in AGN, should have
an important role in driving the X-ray variability. In this work, we
investigate the connection between the X-ray amplitude variability and the
coronal physical parameters, namely the temperature ($kT$) and optical depth
($tau$). We present the spectral and timing analysis of 46 NuSTAR observations
corresponding to a sample of 20 AGN. For each source we derive the coronal
temperature and optical depth through X-ray spectroscopy, and we compute the
normalized excess variance for different energy bands at a time scale of $10$
ks. We find a strong inverse correlation between $kT$ and $tau$, with
correlation coefficient $r<-0.9$ and negligible null probability. No clear
dependence between the temperature and physical properties such as the black
hole mass and the Eddington ratio is found. We also find that the observed
X-ray variability does not correlate with either the coronal temperature or
optical depth under the thermal equilibrium assumption, while it is
anti-correlated with the black hole mass. These results can be interpreted in a
scenario where one possibility is that the observed X-ray variability could be
mainly driven by variations in coronal physical properties on a time scale of
less than $10$ ks, whereas we assume thermal equilibrium at such time scales in
this paper, given the capability of the currently available hard X-ray
telescopes. However, it is also possible that the X-ray variability is mostly
driven by the absolute size of the corona, which depends on the supermassive
black hole mass, rather than by its physical properties.
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