(cba:news) CR Boo faint... and farewell to AM CVn

Joe Patterson jp42 at columbia.edu
Mon Jun 4 12:25:45 EDT 2018

Hi CBAers,

CR Boo faint!  We've basically never gotten a time series of any decent 
length when CR Boo was faint.  Good time of year, too (long presence in 
night sky, rare for near-equatorial stars). Despite the great and 
continued importance of the new AM CVn star (J1411+48) and black-hole 
transient (1830+07), I'd rate this the prime target of the night!  As 
for AM CVn itself, I'd say it's fine to kiss that one off for the year.  
Sure was a great year, though.

Just for the record...

There were two significant motivations for our long campaign on AM CVn.  
The clearest one was measuring the long-term orbital period change.  We 
have a continuous cycle count since 1992 (and a probable count since 
1978), and normally that would be enough to yield a highly precise dP/dt 
measure for this 17 minute binary.  But the orbital signal at 1028.7322 
s is highly convolved with the 1051.2 s positive superhump and the 
1011.4 s negative superhump.  These contaminating signals are of larger 
amplitude than the orbital signal, and they vary slightly in period on a 
timescale of a couple of weeks - so they're REALLY hard to remove 
(without thereby disturbing the measured phase of the orbital signal).  
The solution is to get a lot of data over a long range of terrestrial 
longitudes, and that's what we got. The preliminary result is that AM 
CVn is slowly decreasing its period -opposite to what is predicted by 
the standard theory (which assumes that both stars are white dwarfs).

The other motivation was to compare the phase drifts (or period drifts) 
of the positive and negative superhumps.  Again in theory, the 
period-displacement from Porb is a proxy for "disk size" (i.e. the size 
of the secondary's perturbation on orbits in the disk.  So the two 
drifts should be oppositely phased.  This measurement has really never 
been made before, but the long baseline of the 2018 data will give us a 
good chance to constrain this.  (No result yet).

Old Maxie (the black-hole transient) is tantalizing.  The star shows 
obvious waves in the light curve... and an obvious period should have 
popped up in the power spectrum.  A likely-but-not-quite-obvious peak in 
the power spectrum has popped up (near 7.8 hours), but surrounded by 
much more noise and aliasing than I'd consider plausible.  So we need to 
keep pressing, especially at "exotic longitudes".  Between Maxie at +7 
degrees and CR Boo at +8, I REALLY hope that AU/NZ observers will 
disable the devices which explode their telescopes if they dare to point 
even marginally north.


-------- Forwarded Message --------
Subject: 	(cba:chat) CR Boo faint
Date: 	Mon, 4 Jun 2018 08:58:49 -0500
From: 	Joe Ulowetz <joe700a at gmail.com>
Reply-To: 	cba-chat at cbastro.org
To: 	cba-chat at cbastro.org

Hi Joe P,

I don't know where CR Boo stands in priority now, but previously you 
said it was important if it dropped down to mag 17. Well, it did that 
last night (mag 16.8 to be precise).  It had been at mag 15 two days ago.

Actually, I didn't even mean to get data on it last night, but I 
accidentally left it in my target list and got 5 images around 3:30 UT 
(June 4) and was surprised to see that it had gone faint when I 
processed data this morning.

I can continue to target it, including taking some time away from 
ASASSN-18ey if desired.

Any preference here?

-Joe U.

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#SOFTWARE=MaxIm DL Version 5.08
CR Boo,2458273.6422569444,16.683,0.091,CV,NO,STD,000-BBT-981,13.370,000-BBT-991,11.750,1.220,NA,6851KN,na
CR Boo,2458273.6431134259,16.725,0.099,CV,NO,STD,000-BBT-981,13.355,000-BBT-991,11.740,1.221,NA,6851KN,na
CR Boo,2458273.6439699074,16.633,0.089,CV,NO,STD,000-BBT-981,13.348,000-BBT-991,11.734,1.222,NA,6851KN,na
CR Boo,2458273.6448263889,16.832,0.104,CV,NO,STD,000-BBT-981,13.386,000-BBT-991,11.749,1.223,NA,6851KN,na
CR Boo,2458273.6456828704,16.802,0.106,CV,NO,STD,000-BBT-981,13.394,000-BBT-991,11.777,1.224,NA,6851KN,na
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