Suzaku X-Ray Observation of the Dwarf Nova Z Camelopardalis at the Onset of an Optical Outburst

We present the result of a Suzaku X-ray spectroscopic observation of the dwarf nova Z Cam, which was conducted by chance at the onset of an optical outburst. We used the X-ray Imaging Spectrometer (a 38 ks exposure) and the Hard X-ray Detector (34 ks) to obtain a 0.35-40 keV spectrum simultaneously. Spectral characteristics suggest that the source was in the X-ray quiescent state, despite being in the rising phase of an outburst in the optical band. The spectrum shows a clear signature of circumstellar absorption in excess of interstellar absorption and the reprocessed-emission features of Fe fluorescence and Compton scattering. The extra absorption is explained as being due to partial covering by either neutral or ionized matter. We found a spectral change during the observation, which is attributable only to a change in the circumstellar absorption. Such an X-ray spectral variation was reported for the first time in the case of dwarf novae. We speculate that the variation in the circumstellar absorption is interpreted as a time-varying disk wind, or geometrically flaring disk around the white dwarf during the propagation of a heat wave along inward the accretion disk at the beginning of the outburst, in which optical outburst and X-ray quiescent states co-exist.