MCG-6-30-15: long time-scale X-ray variability, black hole mass and active galactic nuclei high states

We present a detailed study of the long time-scale X-ray variability of the Seyfert 1 Galaxy MCG-6-30-15, based on eight years of frequent monitoring observations with the Rossi X-ray Timing Explorer. When combined with the published short-time-scale XMM-Newton observations, we derive the power-spectral density (PSD) covering six decades of frequency from ~10^-8 to ~10^-2 Hz. As with NGC 4051, another narrow-line Seyfert 1 galaxy (NLS1), we find that the PSD of MCG-6-30-15 is a close analogue of the PSD of a galactic black hole X-ray binary system (GBH) in a `high' rather than a `low' state. As with NGC 4051 and the GBH Cygnus X-1 in its high state, a smoothly bending model is a better fit to the PSD of MCG-6-30-15, giving a derived break frequency of 7.6⁺¹⁰_-3× 10^-5 Hz. Assuming linear scaling of the break frequency with black hole mass, we estimate the black hole mass in MCG-6-30-15 to be ~2.9^+1.8_-1.6× 10⁶M_solar.

Although, in the X-ray band, it is one of the best observed Seyfert galaxies, there has as yet been no accurate determination of the mass of the black hole in MCG-6-30-15. Here we present a mass determination using the velocity dispersion (M_BH-σ_*) technique and compare it with estimates based on the width of the Hα line. Depending on the calibration relationship assumed for the M_BH-σ_* relationship, we derive a mass of between 3.6 and 6 × 10⁶M_solar, consistent with the mass derived from the PSD.

Using the newly derived mass and break time-scale, and revised reverberation masses for other active galactic nuclei (AGN) from Peterson et al., we update the black hole mass-break-time-scale diagram. The observations are still generally consistent with narrow-line Seyfert 1 galaxies having shorter break time-scales, for a given mass, than broad-line AGN, probably reflecting a higher accretion rate. However, the revised, generally higher, masses (but unchanged break time-scales) are also consistent with perhaps all of the X-ray bright AGN studied so far being high-state objects. This result may simply be a selection effect, based on their selection from high-flux X-ray all-sky catalogues, and their consequent typically high X-ray/radio ratios, which indicate high-state systems.