V455 And and CD Ind...

From: Joe Patterson <*jop_at_astro.columbia.edu*> Date: Wed, 19 Sep 2007 08:12:23 -0400 · jop_at_astro.columbia.edu

Dear ,

Pardon me for the brevity.  The start of the school year has set me 
back!  Still, I have been analyzing all the data coming in... and here's 
my report/suggestions.

V455 And = HS2331, as you all know, is having an outburst for the ages. 
  The coverage is almost as good as that of WZ Sge in 2001, and vastly 
better than any other dwarf nova in history.  It's great to have bright 
eruptions from stars transiting near local midnight, and in this case 
with many periodic clocks to study the effects of outburst.

A few of you are obtaining multicolor data; except possibly for the very 
beginning of outburst, there appear to be no significant color effects 
in the star.  It's basically a big blue light bulb, and the main reasons 
for observing it filtered are these:

(1) Archival.  With V data, it's possible to say meaningfully "how
     bright it is".

(2) (Differential) extinction.  Through a very wide or no filter, a CV's 

     extinction always exceeds that of the comparison star, introducing
     unwanted waves in long light curves.

(2) is an item of some consequence, and worth doing if you can afford
the roughly 6x loss of photons (in which case V is probably the best
choice).  But I tend to be skeptical about (1).  CVs are forever 
variable anyway, and there is likely no significance to their exact 
brightness.  *Someone* needs to do V measurements; and everyone - even
unfiltered diehards - needs to make an estimate of the brightness level.
But the sensitivity of time series to periodic signals is greatly 
improved by a healthy count rate.  So for our purposes, unfiltered 
generally wins.  Exceptions are a very bright star (like V455 And the 
last 2 weeks!) and an interest in timescales natural to extinction (say 
6-24 hours).

There's another very specialized reason that applies to this star.  It 
has strong 68 and 34 second signals at quiescence.  Their origin is 
still unclear, but we FERVENTLY would like to measure how they are 
affected by the outburst.  So we need quite short integrations.  To 
resolve both, you need a cycle time (integration + dead time) of <17 s, 
or let's say 13 s to be more comfortable.  Can you do that?  I think for 
most of you the answer is no; if that's true, try for ~20 s, which will 
fail to resolve the harmonic properly, but do a decent job on the 
fundamental.  Avoid, however, cycle times very close to 17 s and 34 s; 
when the period is an exact multiple of the sampling time, terrible 
things happen!  (Like trying to measure the Earth's rotation period with 
brightness measures at noon and midnight every day!)

For the moment, V455 And is certainly the best northern object.  To say 
the least.

In the south, it's time to quit on V1432 Aql.  We've covered 1.2 cycles 
of its 50-day supercycle, done very well (primarily due to Berto), and 
the data clarify the ephemerides and timescale for synchronization. 
It's also time to quit on EC2117 ("Ind").  This is a great eclipsing 
binary, with hardly anything yet published on it; we have excellent 
data, and one or more of you southerners might want to write something 
up - I'll be happy to send you all the data and a preliminary analysis.

Apart from its faintness, CD Ind is a great, great target this month and 
next.  This too is an "asynchronous polar", and our data (mainly Bob 
Rea's) show many effects cycling on the six-day beat between Porb and 
Prot.  They also specify the long-term ephemeris, which has never yet 
been published.  This star may well be the most revealing of this small 
class of CVs (only five known) - revealing of the magnetic torques 
present in this "asynchronous" geometry.  Try hard for CD Ind!

Not so brief... but in a few days, I hope to make up more detailed 
target suggestions.

Please keep up the cba-chat, it's working very well I think!

joe