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

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