(cba:news) ASASSN-18ey = MAXI j1820-070

Joe Patterson jop at astro.columbia.edu
Thu May 10 06:00:57 EDT 2018

Hi CBAers,

Some great data on this star rolling in.  It's declining VERY slowly - 
still about V-12.8 - and showing some waves suggestive of a period near 
3.5 or 7 hours.  Promises to be a great target for the next month or more.

While a few runs showed some quasi-periodic oscillations with a 
timescale near 12 s, this is a REALLY awkward timescale for us to 
investigate.  The readout ("dead") time is a mighty big price to pay, 
and I suggest we abandon this (high-speed) observing style.  We'll get 
much better sensitivity if we adopt integration times appropriate for 
the brightness - maybe 20 sec in clear light, or ~40 s in V (still 
bright enough for V, if you like your V filter).

The equatorial location and short northern nights are also somewhat 
problematic, since we thrive on LONG nightly time series. We can 
overcome this with an even-more-than-usually-extensive spread in 
terrestrial longitude.  And let's do it!  This ia the brightest 
black-hole transient in many years, and we already have a nice supply of 
X-ray and Hubble data to accompany the optical time series.

The most prolific observers so far are Michael Richmond in Rochester, 
NY, John Rock in England, Tonny Vanmunster in Belgium, and Stephen 
Brincat in Malta.  The first three are in famously cloudy and certainly 
northern locations, so their luck can't continue long.  There's not much 
data yet from the western USA, so that would help a lot.  Also AU/NZ, 
Africa, and southern Europe (except Brincat) have been quiet.

Oops, I forgot Josch Hambsch from Chile.  His data, nearly every night 
and well-calibrated, will eventually have a big impact.  But the lower 
time resolution makes it less suitable for *defining* the features 
(eclipses?) in the light curve.  So I hope that the USA observers will 
keep a close watch on this fascinating star - which seems ready to 
surrender a few secrets, but is making us work hard for them!

joe p

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