BC UMa: Very rare outburst showing WZ Sge-like features

March - April superoutburst

As reported by P. Schmeer on March 31, the superoutburst of SU UMa-type dwarf nova BC UMa occurred on March 31 at 12.1mag (vsnet-alert 4522, vsnet-alert 4523, vsnet-alert 4524). BC UMa is one of famous large-amplitude SU UMa-type dwarf novae with very long outburst intervals. The latest outburst (according to my record) of BC UMa was observed on 1994 April 28, which was detected by M. Iida (VSOLJ) at mv=12.5, who subsequently detected first-ever superhumps in this star. The observation first established the SU UMa-type nature of BC UMa. The superhump period was 0.0619 d. BC UMa is one of famous dwarf novae which show both bright and faint superoutbursts (other known examples include SW UMa and WX Cet). The 2000 March-April outburst is almost certainly one of "bright superoutburst" after the six years of dormancy (vsnet-alert 4530.) T. Kato and M. Uemura confirmed the outburst on April 1 at the magnitude of 12.2 (vsnet-alert 4532, vsnet-alert 4537, vsnet-alert 4553, vsnet-alert 4556).

Early superhumps

The analysis of the Kyoto observation of BC UMa on April 1 revealed the presence of clear low-amplitude superhumps (0.05-0.07 mag), which are clearly one of demonstration of "early superhumps". The best period has been determined as 0.0600 d, which is about 3 percent shorter than Iida's superhump period (1994) (vsnet-alert 4562, vsnet-alert 4567, vsnet-alert 4577).

Normal superhumps

T. Vanmunster obtained a 8.5 hrs unfiltered CCD photometry on April 6 and the corresponding light curve showed very prominent superhumps, with an average amplitude of 0.33 mag. (vsnet-alert 4584). Using the VSNET collaboration data until April 7, T. Kato reported the remarkable growth of "true" superhumps (see also vsnet-obs 27154, vsnet-alert 4584). The long delay of the superhump growth, the detection of "early superhumps" (vsnet-alert 4562) and their subsequent decay are all characterstic of a WZ Sge-type superoutburst. Combined with N. James' data, the VSNET collaboration data yielded a superhump period of 0.06452 d. On April 8, R. Novak's run also showed about 0.064d periodicity (vsnet-alert 4595). The analysis of the early superhumps has yield a period of 0.0621 d, which is remakably close to the previously determined "superhump" period (0.0619 d) by Iida (1994). If we assume these period reflect common superhump and orbital period, the fractional superhump excess becomes ~3 percent. The periodicity in the earliest data (vsnet-alert 4562) thus can be "negative superhump", whose existence in the early superoutbursting stage has been recently suggested (Gao et al. 1999, ApJ 527, L55). (vsnet-alert 4586, vsnet-alert 4591, vsnet-alert 4601). On April 13, the Kyoto team reported that superhumps still prominently exist, apparently associated with the apperance of secondary superhumps (vsnet-alert 4613).

Fading phase and rebrightening

The observation by G. Comello on April 12 indicated that BC UMa may have started fading more rapidly. (vsnet-alert 4614). On April 22, T. Kinnunen reported a rebrightening at 14.2mag (vsnet-alert 4665), which was confirmed by G. Poyner at 14.3mag (vsnet-alert 4666, 4667, 4668, 4669, 4670, 4671, 4672).

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