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
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
T. Kato and M. Uemura confirmed the outburst on April 1 at the magnitude
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)
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.
Using the VSNET collaboration data until April 7, T. Kato reported
the remarkable growth of "true" superhumps (see also
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
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).
On April 13, the Kyoto team reported that superhumps still prominently
exist, apparently associated with the apperance of secondary superhumps
Fading phase and rebrightening
The observation by G. Comello on April 12 indicated
that BC UMa may have started fading more rapidly.
On April 22, T. Kinnunen reported a rebrightening at 14.2mag
which was confirmed by G. Poyner at 14.3mag