Study of the Long-Term X-Ray Variability of a Possible Quasar RX J0957.9+6903 with ASCA

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 of photon index 1.58 $\pm$ 0.03, and an absorption column of $\sim$ 1$\times {10}^{21}$ ${\mathrm{c}\mathrm{m}}^{-2}$. The 2–10 keV flux of RX J0957.9+6903 varied by a factor of four over the period of six years, around a mean of $\sim$ 8.8$\times {10}^{-12}$ erg ${\mathrm{s}}^{-1}$ ${\mathrm{c}\mathrm{m}}^{-2}$. 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 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/$f_{\mathrm{b}}$ = $1600_{-1100}^{+\mathrm{\infty }}$ 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.