Photometric Observation and Numerical Simulation of Early Superhumps in BC UMa during the 2003 Superoutburst

We carried out time-resolved photometric observations of an SU UMa type dwarf nova, BC UMa, during its superoutburst in February 2003. We detect early superhumps ( or outburst orbital humps) during the first two days of the outburst. The period of early superhumps is 0.06258(13) d and the amplitude is 0.04 mag. After the early superhump phase, common superhumps with an amplitude of 0.3 mag and a period of 0.064466(16) d developed. The change rate of superhump periods ( ̇ Psh/Psh) was positive through the superoutburst. The superhump period excess (ε≡ Psh/Porb − 1) is 3% and we derive a mass ratio of 0.13. This is twice as large as that of WZ Sge, suggesting that the mechanism of early superhumps in BC UMa is not the 2:1 resonance which was proposed in WZ Sge. We have modelled early superhump light curves including irradiation effects of the accretion disk and secondary star by the white dwarf and accretion disk. The observed early superhumps can be reproduced when two-armed spirals appear on the accretion disk. We have found the long term data taken from AAVSO, VSOLJ and VSNET shows BC UMa has normal outbursts with a maximummagnitude ofmV ∼13 and two types of superoutbursts: one has a short duration (around 10 days) and faint maximum magnitude (mV ∼ 12.5), the other has a long duration (around 20 days) and bright maximum (mV ∼ 11−11.5). BC UMa is the first example of dwarf novae showing the two types of superoutbursts. The supercycle of BC UMa is between 600 days and 1000 days. This is shorter than WZ Sge type dwarf novae and longer than normal SU UMa type stars. This phenomenon suggests that BC UMa is an intermediate dwarf nova between WZ Sge and SU UMa.