Detection of a high frequency break in the X - ray power spectrum of Ark 564

We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of ∼ 3.5 decades. We detect a high frequency break at ∼ 2 × 10 −3 Hz. The power spectrum has an rms of ∼ 30% and a slope of ∼ −1 and ∼ −2 below and above the break frequency. When combined with the results from a long RXTE observation (Pounds et al. 2001), the observed power spectra of Ark 564 and Cyg X-1 (in the low/hard state) are almost identical, showing a similar shape and rms amplitude. However, the ratio of the high frequency breaks is very small (∼ 10 3−4), implying that these characteristic frequencies are not indicative of the black hole mass. This result supports the idea of a small black hole mass/high accretion rate in Ark 564 .