Long‐Term Continuous Monitoring of the Broad‐Line Radio Galaxies 3C 390.3 and 3C 120 with theRossi X‐Ray Timing Explorer

Abstract

We present a study of the flux and spectral variability of the two broad-line radio galaxies (BLRGs) 3C 390.3 and 3C 120, observed almost daily with the Rossi X-Ray Timing Explorer for nearly 2 months each in 1996 and 1997, respectively. Our original motivation for this study was to search for systematic differences between BLRGs and their radio-quiet counterparts, the Seyfert galaxies, whose temporal and spectral behavior is better studied. We find that both 3C 390.3 and 3C 120 are highly variable but in different ways, and we quantify this difference by means of a structure function analysis. 3C 390.3 is significantly more variable than 3C 120, despite its jet's larger inclination angle, implying either that the X-ray variability is not dominated by the jet or that two different variability processes are simultaneously at work in 3C 390.3. We perform an energy-selected and time-resolved analysis based on the fractional variability amplitude and find that the variability amplitude of both objects is strongly anticorrelated with the energy. This last result, along with the correlated change of the photon index with the X-ray continuum flux, can be qualitatively explained within the scenario of thermal Comptonization, generally invoked for radio-quiet active galaxies. Moreover, the time-resolved and energy-selected fractional variability analysis shows a trend opposite to that observed in jet-dominated active galactic nuclei (blazars), suggesting only a minor contribution of the jet to the X-ray properties of BLRGs. Time-averaged spectral analysis indicates the presence of a strong resolved Fe Kα line with a centroid at 6.4 keV and a weak (Ω/2π 0.1-0.4) reflection component in both objects. The overall PCA+HEXTE spectra are best fitted with the constant density ionization model of Ross & Fabian, but with a modest ionization parameter. We perform a time-resolved spectral analysis of 3C 390.3 with the aim of constraining the delay between Fe Kα line and continuum variability; however, the limited signal-to-noise ratio of the line flux hampers a thorough study of the line variability.