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 \pm 0.03$ and an absorption column of $ \sim 1 \times 10^{21}~\mathrm{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 $ \sim 8.8 \times 10^{-12}~\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$. Peak-to-peak variability within each observation was less than 25% on $ \sim 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^{-4.3}$ Hz by utilizing the structure function method and a Monte Carlo simulation assuming a broken power-law type PSD. Then, the break frequency $ f_\mathrm{b}$ of the PSD of RX J0957.9+6903 has been constrained as 1/$ f_\mathrm{b} = 1600^{+\infty}_{-1100}$ days, and the logarithmic slope of the high-frequency region of the PSD as $ \alpha =-1.55 \pm 0.2$ . A comparison of the estimated PSDs is made between RX J0957.9+6903 and the M81 nucleus, observed in the same field of view.