ASCA X-ray observations of the disc wind in the dwarf nova Z Camelopardalis

We present ASCA observations of the dwarf nova Z Camelopardalis during outburst and during a transition from quiescence to another outburst. At the beginning of the transition the X-ray count rate was an order of magnitude higher and the spectrum much harder than during the outburst. As the transition progressed, the spectrum remained hard as the X-ray flux decreased by a factor of 3, with no spectral softening. Spectral modelling reveals an optically thin, high-temperature component kT ~~ 10 keV which dominates the transition observation and is also observed during outburst. This is expected from material accreting on to the white dwarf surface. The outburst spectra require additional emission at lower temperatures, through either an additional discrete temperature component, or a combination of a cooling flow model and an ionized absorber. Fits to both observations show large amounts of absorption N[SUBSRCRIPT H] = 8–9 x 10^21 cm^-2, two orders of magnitude greater than the measured interstellar value, and consistent with UV measurements of the outburst. This suggests that a disc wind is present even in the earliest stages of outburst, possibly before the outburst heating wave has reached the boundary layer.