(cba:news) ASASSN-18ey = MAXI J1820+070 (Joe Patterson) [2018-08-03T11:42:36Z]

Hi CBAers,

More on the Great Star of 2018.

Despite what I wrote last time, it appears now that the X-ray transient 
has entered a new phase.  The superhump (amplitude) has either faded 
rapidly, or is accompanied now by some other signal of comparable size. 
If the latter, it will likely be a challenge to figure out, because its 
period would be *close* to the 0.69 day period of the superhump.  So 
this is NOT a time to decrease vigilance.  In particular, the nightly 
light curves over the last week have been largely missing contributions 
from the Americas - so I hope we can improve on that!

Another comment of lower weight.  Over the ~100 day campaign so far, 
about half the observers have used V filters, and half unfiltered. 
Until recently, it has been straightforward to convert between these 
scales with additive constants (constant for a given observer, 
reflecting mainly the passband of the unfiltered or lightly-filtered 
CCD).  But the conversions of a few weeks/months ago don't seem to apply 
now.  Possibly because the star's color has changed a lot, or the 
instrumentation?  Hard to say when the coverage is getting more sketchy.

It also seems that a strong UV/blue component may have turned on, 
because the time-series at airmasses beyond 2.0 are pretty corrupted by 
the extra extinction.  So I advise cutting off the time series around 2 
airmasses.

These issues can be greatly minimized by using a V (or Sloan g) filter, 
and it doesn't really matter whose V filter you use - it merely nudges 
the magnitude a few hundredths, which is easy for me to measure (and 
apply that measured constant).  Since the star is still 13th magnitude 
and likely to stay near that level for another month or more, you might 
consider switching to a V filter.  (For the rest of the year; switching 
back and forth is not a good idea.)

On the other hand, don't abandon the star!  With enough coverage  I can 
still measure these additive constants - even if they're slowly changing.

Sorry to be so much "in the weeds" on this issue.  It's an exciting 
star!  Maybe I'll go have a cup of herbal tea now....

joe p

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