RX J0640-24: an SU UMa-type candidate
P. Schmeer reported an outburst of the ROSAT selected cataclysmic variable
RX J0640-24 on January 6. R. Fried (Braeside Observatory) confirms the
outburst and reports an instrumental unfiltered magnitude of 11.8 for
January 7.185 UT. A minor brightening was observed with the same instrument
on 1999 December 14. The only previously known outburst was recorded
on an ESO B plate (mag about 11)
(vsnet-alert 3956).
M. Uemura and T. Kato performed a time-series observation on January 7 and
, in 1.3-hour continuous run, detected a modulation with an amplitude of
about 0.3mag, which might be attributed to a superhump
(vsnet-alert 3959,
vsnet-alert 3968).
In the next night they found it rapidly faded
(vsnet-alert 3970).
The possible superhump-like feature observed in the January 7
data seems to have decayed on January 8 data. However, there looks like
some evidence of modulations with a likely period less than an hour.
During the 3.4-hour run, the object showed little trend of the continuing
rapid fading. This precludes the possible explanation of the January 7
variation as the effect of extinction. The rapid rate of decline (~1 mag/day
between Jan. 6 and 8) suggests that the object can be a short-period system
(e.g. SU UMa stars)
(vsnet-alert 3980).
On February 22,
RX J0640-24 is again reported to undergo an outburst at 12.0mag and 11.5mag
by R. Stubbings and A. Pearce, respectively
(vsnet-alert 4255,
vsnet-alert 4257).
The outburst again terminated rapidly
(vsnet-alert 4270,
vsnet-obs 26500).
As reported by R. Stubbings and B. Monard in vsnet-outburst
messages, RX J0640-24 experienced the other outburst (11.1mag) on May 13
(vsnet-campaign 49).
W. Walker reported the possible
structure in his two hours light curve on 19 May, but the amplitude
is not more than 0.15 magnitudes
(vsnet-campaign 97).
This presumable superoutburst of RX J0640-24 entered the rapid
decline stage on 24 May
(vsnet-campaign 131,
104). The rate of decline,
the duration of the plateau phase, the rate of rapid decline are all
characteristic to those of superoutbursts.