Short-term variability and PSD analysis of the radio-loud AGN 3C 390.3
  We investigate the short-term variability properties and the power spectral
density (PSD) of the Broad-Line Radio Galaxy (BLRG) 3C 390.3 using observations
made by XMM, RXTE, and Suzaku on several occasions between October 2004 and
December 2006. The main aim of this work is to derive model-independent
constraints on the origin of the X-ray emission and on the nature of the
central engine in 3C 390.3. On timescales of the order of few hours, probed by
uninterrupted XMM light curves, the flux of 3C 390.3 is consistent with being
constant in all energy bands. On longer timescales, probed by the 2-day RXTE
and Suzaku observations, the flux variability becomes significant. The latter
observation confirms that the spectral variability behavior of 3C 390.3 is
consistent with the spectral evolution observed in radio-quiet Seyfert
galaxies: the spectrum softens as the source brightens. The correlated
variability between soft and hard X-rays, observed during the Suzaku exposure
and between the 2 XMM pointings, taken 1 week apart, argues against scenarios
characterized by the presence of two distinct variable components in the 0.5-10
keV X-ray band. A detailed PSD analysis carried out over five decades in
frequency suggests the presence of a break at T_br~43 days at a 92% confidence
level. This is the second tentative detection of a PSD break in a radio-loud,
non-jet dominated AGN, after the BLRG 3C120, and appears to be in general
agreement with the relation between T_br, M_BH, and L_bol, followed by Seyfert
galaxies. Our results indicate that the X-ray variability properties of 3C
390.3 are broadly consistent with those of radio-quiet AGN, suggesting that the
X-ray emission mechanism in 3C 390.3 is similar to that of nearby Seyfert
galaxies without any significant contribution from a jet component.
