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

TL;DR

This study uses Suzaku X-ray observations to analyze the spectral characteristics of Z Camelopardalis during the onset of an optical outburst, revealing unique circumstellar absorption features and spectral variations.

Contribution

It reports the first observation of X-ray spectral variation in dwarf novae linked to circumstellar absorption changes during outburst onset.

Findings

Detected circumstellar absorption exceeding interstellar levels.

Observed spectral change due to circumstellar absorption variation.

First report of X-ray spectral variation in dwarf novae during outburst onset.

Abstract

We present the result of a Suzaku X-ray spectroscopic observation of the dwarf nova Z Camelopardalis, 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 due to partial coverage by either neutral or ionized matter. We found a spectral change during the observation, which is attributable only to the change in the circumstellar absorption. Such an X-ray spectral variation is reported for the first time in 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 inward along the accretion disk at the beginning of the outburst, in which optical outburst and X-ray quiescent states co-exist.