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

McHardy, I.M., Gunn, K.F., Uttley, P. and Goad, M.R. (2005) MCG-6-30-15: long time-scale X-ray variability, black hole mass and active galactic nuclei high states. Monthly Notices of the Royal Astronomical Society, 359 (4), 1469-1480. (doi:10.1111/j.1365-2966.2005.08992.x).

Abstract

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 ~10⁸ to ~10² 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⊗ .
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 (MBH–σ*) technique and compare it with estimates based on the width of the Hα line. Depending on the calibration relationship assumed for the MBH–σ* relationship, we derive a mass of between 3.6 and 6 × 10⁶M⊗ , 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.

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