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

Abstract We carried out time-resolved photometric observations of an SU UMa-type dwarf nova, BC UMa, during its superoutburst in 2003 February. 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 the superhump periods ($\dot{P}_{\rm SH}/P_{\rm SH}$) ($P_{\rm SH}$: superhump period) was positive throughout the superoutburst. The superhump period excess ($\varepsilon \equiv P_{\rm SH}P_{\rm orb}^{-1}-1$) is 3% and we derived a mass ratio of 0.13. We have modeled early superhump light curves including irradiation effects of the accretion disk and secondary star. The observed early superhumps can be reproduced when two-armed spirals appear on the accretion disk. The long-term data taken from AAVSO, VSOLJ, and VSNET indicate that BC UMa shows normal outbursts and two types of superoutbursts (short-faint and long-bright). BC UMa is the first example to show such two types of superoutbursts. The supercycle of BC UMa is between 600 d and 1000d. This suggests that BC UMa is an intermediate type of dwarf novae between WZ Sge and SU UMa.