Accretion mode changes in QS Tel (RE 1938-461): EUVE, ROSAT and optical observations

Pointed EUVE and ROSAT observations of the AM Her type binary QS Tel (RE 1938 — 461) are reported, together with complementary contemporaneous optical measurements. The EUVE data reveal a double-peaked orbital light curve, dramatically different from the ‘bright-faint’ morphology seen during the ROSAT WFC survey discovery observations, indicating that two accretion sites were active. A deep dip is present during one of the EUVE flux maxima and probably arises from occultation of the emission site by the accretion flow rather than via an eclipse by the companion star. This dip, which does not appear to be accompanied by significant spectral hardening, possesses a slow ( ∼ 300 s) ingress but much more rapid (∼ 40 s) egress. Both ingress and a restricted phase interval near mid-dip are affected by strong flare-like activity. Blackbody representation of the EUVE spectra yields a low emission temperature ∼ 15 eV. The inferred estimates of the soft component flux point to a large soft/hard component flux ratio (≿15). However, we also find tentative evidence of an ionization edge at 85 Å and absorption lines at 98 and 116 Å, possibly due to Ne VI, Ne VIII and Ne VII respectively. Contemporaneous optical photometry shows the light curve to vary substantially over 3 d but, where we have simultaneity, there is no significant correlation between the optical and EUV fluxes. During the earlier ROSAT pointed observation, the source was in a single-pole mode but the intensity of the bright interval had declined by a factor ∼ 5 relative to the survey observation. A counterpart to the EUVE dip was seen in both the WFC and PSPC bands during the bright interval, the WFC event being similar in morphology to the EUVE dip and more protracted than the PSPC dip. Quasi-simultaneous V- and I-band photometry shows a broad dip centred on the X-ray event, with greater depth at red wavelengths. A hard spectral component is detected in the ROSAT spectrum. This collection of observations highlights the variable behaviour of QS Tel but also provides a rather clean, simple example of a change of accretion mode in an AM Her system. We consider the implications of this result and also discuss the nature of the accretion flow.