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[vsnet-alert 128] Re: AL Com "dips"



AL Com "dips"

  It would be worth pointing out that there are two types of so-called
"dips" in the late stage of superoutbursts of WZ Sge stars or TOADs.
One is what Dr. Howell has explained.  In this case, the following
re-brightening has all the characteristics of normal outbursts.  The
duration of this intervening faint state tends to be longer than the
other type of "dips".  I interpret the short (normal) outbursts observed
in GRO J0422+32 (USXT = ultrasoft X-ray transient) as similar ones; this
similarity has led me to thinking of USXTs as black hole analogs of WZ Sge
stars or TOADs in the scheme of the disk instability theory. (cf. my paper
presented in Padova-Abano CV Conference in 1994).  We have a good set
of records showing that post-outburst brightenings are not confined to the
TOADs, but are more widely distributed in SU UMa-type stars.  While we
should wait more statistical analysis, the very low frequency of normal
outbursts in the TOADs seems to make these post-outburst brightenings
more conspicuous than in usual SU UMa-type stars.

  The other kind of phenomenon can be properly called true "dips".
Such "dips" have typical duration of one to a few days, and afther the
"dips" an apparent superoutburst continues.  The best observed case can
be seen in the 1978 outburst of WZ Sge.  I have not yet read proper
theoretical explanation of this kind of "dips".  Are post-"dip" long
outbursts real superoutburst?  Are they also triggered by normal
outbursts? ... etc.  This is the reason why I strongly recommend the
observations of AL Com at this stage, which may provide a unique
chance in observationally discriminating several possibilities causing
such post-"dip" brightenings.

Regards,
Taichi Kato (Kyoto University)

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