Study of the long-term X-ray variability of a possible quasar RX J0957.9+6903 with ASCA

The long-term variability and spectral properties of a possible quasar, RX J0957.9+6903, were studied utilizing 16 ASCA observations spanning 5.5 years. The average 0.7-10 keV spectrum of RX J0957.9+6903 is well represented by a power-law continuum having a photon index of 1.58 ±0.03 and an absorption column of ∼ 1 × 10²¹ cm^-2. The 2-10 keV flux of RX J0957.9+6903 varied by a factor of four over a period of six years, around a mean of ∼ 8.8 × 10^-12 ergs^-1 cm^-2. Peak-to-peak variability within each observation was less than 25% on ∼ 1 day time scale. These properties support the classification of RX J0957.9+6903 as a quasar. The power spectrum density (PSD) was estimated in a "forward" manner over a frequency range of 10^-8.2-10^-43 Hz by utilizing the structure function method and a Monte Carlo simulation assuming a broken power-law type PSD. Then, the break frequency f_b of the PSD of RX J0957.9+6903 has been constrained as 1/f_b = 1600^+∞_-1100 days, and the logarithmic slope of the high-frequency region of the PSD as α = -1.55 ± 0.2. A comparison of the estimated PSDs is made between RX J0957.9+6903 and the M 81 nucleus, observed in the same field of view.