[vsnet-alert 15669] Next HST observations: 1RXSJ105010.3-140431 (and a sneak-peak of SDSS1538+5123)

[Boris Gaensicke]

Dear All,

yesterday, it happened: we had to cancel the HST observation
of SDSS1544+2553 ... your monitoring of this star was as usual
vital for keeping the detectors on HST safe (read on below
for more technical details). While this is frustrating for all of us,
it had to happen sooner or later as  CV are unpredictable beasts.

But, let me stress here again that it is thanks to your support that
we managed to observe the first 25 targets safely!

Fingers crossed that the next targets will behave again:

* 1RXSJ105010.3-140431 May 10, 2013 07:51:01to 12:01:57

This is now top priority, V~17, a WZ Sge candidate with
no recorded outburst. It is quite unlikely that anything will
happen next week, but your monitoring is essential to
prove this!

Plus, if you have a bit of spare energy and time: please
start to monitor SDSS1538+5123, which will likely
get a firm date for the HST observations in a few days.
It is a short-period CV that should have outbursts, but
none were recorded, as far as know. But it does
exhibit some long-term change in its quiescent brightness
(see e.g. the CRTS light
curve<http://nesssi.cacr.caltech.edu/catalina/20010321/103211520534100553p.html>).
It would be really helpful
to get a good feeling for the current average brightness,
a few weeks ahead of the HST observations.

Best wishes,

Boris

NB - some more details on yesterdays problem.  SDSS1544+2553 is a
long-period (6h) eclipsing CV (Skinner et al.
2011PASP..123..259S<http://adsabs.harvard.edu/abs/2011PASP..123..259S>)

and the M1.5V companion shows nicely up in quiescence (g~16.9).
Our expectation was to get a spectrum of the WD with HST, just as
we did for HS0218+3229 last December. Given that, in quiescence,
the star is quite faint in the UV, we had to use an imaging
strategy to acquire the target into the COS aperture ... which
leaves little headroom for brightening. However, the last few
days saw the system brightening up to V~15.9, and also the
eclipse depth increased, unambiguously showing that the accretion
disk got brighter.  Without a reliable prediction of the temperature
of the disk, the UV flux that HST would have seen in the
acquisition image was uncertain, and there was a real probability
that it would be too bright - which called for the decision to
abort the observations.

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For more information on this HST program, see

http://deneb.astro.warwick.ac.uk/phsdaj/public_html/12870/
http://deneb.astro.warwick.ac.uk/phsdaj/public_html/12870/Targets.html