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 less than or similar to 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 10(6) 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(-alpha) with alpha greater than or similar to I. A further steepening to alpha approximate to 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-l, albeit with break frequencies that differ by a factor of 10(-2) and 10(4), respectively. If the break frequencies are indicative of the central black hole mass, then this mass may be as low as 10(6) M.. An upper limit of similar to 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-l, this limit is consistent with a relatively low central black hole mass.