Detection of superhumps and quasi-periodic oscillations in the light curve of the dwarf nova SW Ursae Majoris

Abstract

The dwarf nova SW UMa erupted in 1986 March. The eruption lasted more than 3 weeks and had an amplitude greater than 7 mag. We obtained high-speed photometry of SW UMa on 10 nights during the eruption. Superhumps with a period of 84.0 ± 0.1 minutes and peak-to-peak amplitudes of up to 11 % appeared in the light curve for at least 7 days of the eruption, proving conclusively that the eruption was a supermaximum and, therefore, that SW UMa is a member of the SU UMa subclass of dwarf novae. The superhumps in SW UMa have a period 2.7% longer than the 81.8 minute orbital period of the system, a difference typical of the superhumps of SU UMa stars. Quasi-periodic oscillations with a period near 5 minutes, a mean semiamplitude of 0.4%, and a coherence time of rougly 2 cycles appeared in the light curve on at least two nights of the eruption. Low-amplitude quasi-periodic oscillations with a mean period of 22.3 s were also present on several nights. This is the shortest period found so far for any bona fide quasi-periodic oscillation in a dwarf nova. The addition of SW UMa to the list of short-period dwarf novae with supermaxima lends further support to the hypothesis that all dwarf novae with periods less than 3 hr are SU UMa stars and that all SU UMa stars have orbital periods less than 3 hr. SW UMa is probably an intermediate polar. If so, it is the only one with unambiguous dwarf nova eruptions and now the first to show supermaxima and superhumps. If the 15.9 minute rotation period deduced for the white dwarf by Shafter et al. is correct, the intermediate polar model for superhumps cannot be correct.