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 li ght curve, dramatically different from the 'bright-faint' morphology s een during the ROSAT WFC survey discovery observations, indicating tha t two accretion sites were active. A deep dip is present during one of the EUVE flux maxima and probably arises from occultation of the emis sion site by the accretion flow rather than via an eclipse by the comp anion star. This dip, which does not appear to be accompanied by signi ficant spectral hardening, possesses a slow ( similar to 300 s) ingres s but much more rapid ( similar to 40 s) egress. Both ingress and a re stricted phase interval near mid-dip are affected by strong flare-like activity. Blackbody representation of the EUVE spectra yields a low e mission temperature similar to 15 eV. The inferred estimates of the so ft component flux point to a large soft/hard component flux ratio (gre ater than or similar to 15). However, we also find tentative evidence of an ionization edge at 85 Angstrom and absorption lines at 98 and 11 6 Angstrom, possibly due to Ne VI, Ne VIII and Ne VII respectively. Co ntemporaneous optical photometry shows the light curve to vary substan tially over 3 d but, where we have simultaneity, there is no significa nt correlation between the optical and EUV fluxes. During the earlier ROSAT pointed observation, the source was in a single-pole mode but th e intensity of the bright interval had declined by a factor similar to 5 relative to the survey observation. A counterpart to the EUVE dip w as 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 protra cted than the PSPC dip. Quasi-simultaneous V- and I-band photometry sh ows a broad dip centred on the X-ray event, with greater depth at red wavelengths. A hard spectral component is detected in the ROSAT spectr um. 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 o f this result and also discuss the nature of the accretion flow.