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, RXJ0957.9+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 apower-law continuum of photon index 1.58 $\pm$ 0.03, and an absorption columnof $\sim$ 1$\times{\rm 10}^{21}$ ${\rm cm}^{-2}$. The 2--10 keV flux of RXJ0957.9+6903 varied by a factor of four over the period of six years, around amean of $\sim$ 8.8$\times {\rm 10}^{-12}$ erg ${\rm s}^{-1}$ ${\rm cm}^{-2}$.Peak to peak variability within each observation was less than 25% on $\sim$ 1day time scale. These properties support the classification of RX J0957.9+6903as 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 thestructure function method and Monte-Carlo simulation assuming a brokenpower-law type PSD. Then, the break frequency $f_{\rm b}$ of the PSD of RXJ0957.9+6903 has been constrained as 1/$f_{\rm b}$ = $1600^{+\infty}_{-1100}$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 madebetween RX J0957.9+6903 and the M81 nucleus, observed in the same field ofview.Comment: 14 pages, including 3 tables and 19 figures, accepted by Publications of the Astronomical Society of Japa