VX For in outburst – target of opportunity

Below is a digest of information and requests for observations of VS For, latest first.

Joe Patterson, Tue 2009-10-13 22:49 [email protected]

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!

Joe Patterson, Sun 2009-10-11 22:15 [email protected]

Meanwhile, VX For continues to tantalize in the southern sky.  It’s reeling off some echo outbursts, and we’re now getting good coverage from AU (mainly Arto and Chris Stockdale), NZ (Bob Rea), and ZA (Berto).

As for superhumps… whew, I think the answer is no, but the analysis is tricky when the nightly light curves show these strong ramps.  That’s the general expectation: after the main outburst is over, superhumps in dwarf novae tend to die, get weak, get sloppy… or some undecipherable combination.  The long-lived echoes support the idea that this is a very old dwarf nova, with a puny secondary; but only the actual mass ratio, still unknown, will severely test this idea.  Anyway, the star is worth following as long as it keep flashing the echoes (3 so far I think).

Joe Patterson, Wed 2009-09-16 20:37 [email protected]

Rod’s great discovery a few days ago was followed by Berto’s first night of photometry last night… which showed obvious humps, quite likely to be superhumps (meaning with a period slightly longer than the precise Porb).  I’m attaching Berto’s announcement.  So it’s an SU UMa, case closed. That’s pretty significant too; in the star’s only historical outburst (1990) the star was first suspected to be a supernova, mostly because of its presence in Fornax, where galaxies are numerous and eruptive variables are rare.  The spectrum showing zero-redshift features proved it was galactic, but my failure to find periodic humps with quite good data over 1 week was puzzling.

And it’s still puzzling, unless Rod caught it well after the outburst started.  If any of you have information or an educated opinion – or even an uneducated opinion – as to when the present outburst really began, I’d love to hear it.

I think it’s very likely to be a full-bore, certifiable, card-carrying WZ Sge star.  Every piece of evidence suggests that, except this latest (so far weak) indication that superhumps appeared practically at the start of outburst.  (That’s why estimating the true start is quite important.)  Anyway, this should be a great, great target for time-series photometry as long as it stays bright – which I expect to be two weeks or more.

joe

 

——– Original Message ——–

Subject: [vsnet-alert 11476] VX For

Date: Wed, 16 Sep 2009 11:23:20 +0200

From: Berto Monard <[email protected]>

To: [email protected] <[email protected]>

CC: [email protected]

<[email protected]>

 

Timeseries photometry with unfiltered CCD of VX For at CBA Pretoria on the night 15-16 Sep 2009 show modulations with amplitude around 0.2 and period around 1.5h. The mean magnitude was 12.9CR.

Astrometry vs UCAC2 determines VX For at position 03 26 45.72 -34 26 24.6 (2000)  This is exactly the same value as in the 2006 D&S catalogue.

For photometric comparison I have used a star at position 03 26 57.8049 -34 28 00.090  with a derived R magnitude 13.2 .

This star is an easy target for southern observers.

Regards,

Berto Monard / CBA Pretoria

Joe Patterson, Tue 2009-09-15 02:52  [email protected]

Wow, I’ve lived to see the day!

This star zoomed to 12.6 from about 20.5 in 1990, and stayed bright for many weeks (declining at ~0.1 mag/day).  The spectra made it pretty clear it was an erupting dwarf nova, and even in 1990 it seemed very, very likely that the star would flash superhumps.  But I studied it for 7 straight nights at Cerro Tololo, and found only small and apparently aperiodic wiggles in the light curve.  Basically a very high-quality nothing.  Then my run ended… and I’ve always wondered what this star is.

Now it’s 2009, and we know a lot more about the WZ Sge syndrome among dwarf novae.  The most extreme of these stars generally take quite a long time to sprout actual superhumps; WZ Sge itself takes 10 days. So that’s a pretty good conjecture – that it’s quite extreme even among the WZ Sge class… and that had I taken over that 0.9 m telescope and refused to leave, I would eventually have seen those telltale superhumps.

We don’t yet know much about this eruption, but if it’s a super (odds are decent), then this is the glamor object of the year for dwarf novae.

We also don’t yet know when it erupted.  This one’s only for southerners… but I’ll definitely be thrilled to see any data you can get on it!

The basic info is all in the Downes catalog.

Rod Stubbings

Date: Tue, 15 Sep 2009 02:13:48 +1000

From: Rod Stubbings <[email protected]>

To: <[email protected]>

CC: [email protected]

 

The probable dwarf nova VX For appears to be in outburst.  I have never seen this star in outburst before and the only information on it is from IAUC 5127, 1990 on the discovery.

FORVX          090914.636   130  Stu.RASNZ

Regards,

Rod Stubbings.

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