Implications of the X-ray variability for the mass of MCG -6-30-15

The bright Seyfert 1 galaxy MCG -6-30-15 shows large variability on a variety of timescales. We study the ≲3 day timescale variability using a set of simultaneous archival observations that were obtained from the Rossi X-Ray Timing Explorer (RXTE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA). The RXTE observations span nearly 10⁶ s and indicate that the X-ray Fourier power spectral density has an rms variability of 16%, is flat from approximately 10^-6 to 10^-5 Hz, and then steepens into a power law proportional to f^-α with α ≳ 1. A further steepening to α ≈ 2 occurs between 10^-4 and 10^-3 Hz. The shape and rms amplitude are comparable to what has been observed in NGC 5548 and Cyg X-1, albeit with break frequencies that differ by a factor of 10^-2 and 10⁴, respectively. If the break frequencies are indicative of the central black hole mass, then this mass may be as low as 10⁶ M_⊙. An upper limit of ∼2 ks for the relative lag between the 0.5-2 keV ASCA band compared to the 8-15 keV RXTE band was also found. Again by analogy with NGC 5548 and Cyg X-1, this limit is consistent with a relatively low central black hole mass.