(cba:news) AM CVn and Maxie

[Joe Patterson]

Hi (mostly northern) CBAers,

The AM CVn data rolling in is a thing of beauty.  On most mornings, my 
first move is to check the overnight mail for new data... and then do a 
preliminary analysis before the first sip of coffee.  A nice preview of 
my retired-professor life!  (A few years off.)

I just wanted to check a few things.

1. Except for Jim Seargeant who specifies HJD, I assume that everyone is 
using JD, as the labels say.  Correct me if I'm wrong!  But Jim, you 
don't need to change, at least for me; your data is so good that I 
wouldn't change a thing.  (I'm not sure whether AAVSO cares, as long as 
the label is correct.)

2. Calibration.  Most people are covering AM CVn unfiltered, which is 
fine... and arguably better since the signal is weak (0.01-0.02 mag). 
However, it does mean that the almost-constant reported magnitudes are 
offset from each other by as much as 0.35 mag - reflecting different 
bandpasses of detectors and choices of comparison stars... and possibly, 
at a lower level, sky conditions.  It matters little to me since I 
routinely zero out those offsets - very easy to do when there's a lot of 
data.  But for those of you who take a lot of data, I'm interested in 
your opinion on this: do you see nightly variability exceeding ~0.05 mag 
in your data?   I've never noticed, but am so busy with the zeroing-out 
that I've practically paralyzed  my ability to see such things.  Lemme know!

AM CVn is one of the most famous and archetypal CVs in the sky... yet 
has no C and barely any V, except in its name. What a fascinating star!

Re Maxie = ASASSN-18ey.  Having rebrightened on its one-year 
anniversary, the star is hanging in there at 14.0... with some huge 
(~0.5 mag?) very fast variability.  For this star, generally a V filter 
is best, because the flux will get compared to filtered measurements at 
many other wavelengths.  HOWEVER, you might be interested in studying 
that ultrafast variability.  If you can manage, say, 5-10 s monitoring, 
you could possibly study it.  And for that I'd recommend no filter, 
because you need all the photons.

Finally, I'm inclined to say: time to pull the plug (end coverage) on BH 
Lyn.  Results later...

joe p

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