Proposal #78

Dear AAVSO team,
my proposed target MT91 213 is a high-mass gamma-ray binary with Be-type visible companion, system is located in Cygnus OB2 association. Estimated (based on light-time effect measurements in X-rays) time of periastron of P=45-50 year, very elliptical, orbit is November 11, 2017. System is expected to have a giant outburst when compact object is colliding with dense regions around active Be-star. Please see reference: https://arxiv.org/pdf/1609.06328.pdf

We have previous photometric observations of this system from VARMAS survey (from 2011-2015, first results in https://arxiv.org/abs/1611.02452) showing that Be star in the system is very active, it has frequent large-amplitude optical outbursts (so called phase changes of disc build-up and break-down) - amplitudes in V and I filters 0.3 and 0.6 magnitudes, respectively. To be able to properly interpret X-ray observations and optical spectroscopy of such rapidly changing system, reasonably dense time-series of optical flux measurements would be very much needed.

We would like to ask monitoring of that system in B,V,R,I filters with cadence ca 2x per week till the end of the season (probably up to Christmas 2017, when object is above airmass 2), with targeted SNR=300...500 (from Poisson statistics!). If it is appropriate from amount of data viewpoint, 3 exposures per filter would be most useful. Magnitude estimates according to Simbad and VSX are B=13.1, V=12.0, R=10.6, I=9.7, it is known that the region is rather reddened by the dust. System is not expected to become dimmer, only brighter up to 1 magnitude.
MT91 213 is located in rather crowded smaller group, good spatial sampling in the sky would be needed. Based of those two requirements, SRO would be the ideal telescope in AAVSOnet to carry out proposed observations.

If it is possible (from the AAVSOnet load point of view) to increase cadence of observations in +-1 week surrounding predicted periastron time to one observation per two days, it would even more useful.
Best wishes,
Tõnis Eenmäe