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(cba:news) echo outbursts, and missed outbursts (Joe Patterson) [2009-10-13T11:48:55Z]


Dear CBAers,

Just a few musings/speculations this time.

Some of you know that EG Cnc jumped into outburst a few days ago, and is now (or was yesterday) declining rapidly. Definitely one of our favorite stars! It's pretty early in the Cancer observing season, so we have only a weak constraint on when that outburst started. It could be the very beginning of a super, or the end (one of the infamous "echo outbursts"). Or it could be merely a normal outburst; I believe two such events have been seen in EG Cnc, and they are generally documented in WZ Sge stars, other than grand old WZ itself. The next week will probably tell us which of these is true, or perhaps yet another possibility. The most interesting would be the beginning of a super... although the poor seasonal timing (Cancer in mid-October) probably means that we can't expect to get as much coverage as we did in the 1996 outburst. The latter was one of the best-observed DN eruptions in history.

VX For is likely a similar binary, a very old guy with a puny secondary. Accordingly the star is busily executing echo outbursts now - a poorly understood phenomenon but one that appears to characterize the WZ Sge stars. Superhumps have become very weak and hard to follow, maybe even gone altogether. The smart money says they're not likely to come back. But the echoes might go on for a while, and documenting those is mighty important, since this is essentially the first observed outburst in history (that of 1990 really didn't produce anything, aside from many puzzled conversations in Chile). The era of handsome light curves is probably over, but the star's behavior over the next two weeks is likely to be scientifically rewarding. In case it's not obvious, I'm really excited by this return of a mysterious old friend!

A big and very uncertain question for all these normally faint stars which rarely erupt is: just how common are non-supers? (Often called "normal" outbursts, but it's an odd term, since they are presumed to happen very rarely or even never.) Of course we can simply count the ones in the documentary record... but how many do we miss? I don't have any suggestions about this, but thought I'd raise it in case anyone else does.

V368 Peg is probably an "ordinary" SU UMa star, with supers roughly once a year. As Arto remarked and as his light curve proved, the star has declined from plateau, yet kept its superhumps going strong. This could be interesting to track; we know that generally speaking, superhumps tend to outlast the superoutbursts which create them... but there has never been a star (or at least not a hydrogen-rich star) which illustrated this in sufficient detail to yield some nice numerical estimates of this.

Olde Whiteface is finally falling out of the sky, so you might be able to get some good data on these stars, now pretty faint.

joe