GR Muscae, also known as 2S 1254-690, is a binary star system in the constellationMusca composed of a neutron star of between 1.2 and 1.8 times the mass of the Sun and a low-mass star likely to be around the mass of the Sun in close orbit. A magnitude 19 blue star was pinpointed as the optical counterpart of the X-ray source in 1978. Its apparent magnitude varies from 18 to 19.1 over a period of 0.16 days.
While the optical counterpart to the X-ray source was identified in 1978, optical variability was not detected until 1980, when a bright flare was seen that increased the white-light flux by a factor of two in a time interval of about 1.7 seconds. GR Muscae received its variable star designation in 1985.
The neutron star has an accretion disk that takes around 6.74 days to complete a revolution, and is inclined at an angle to the incoming stream of material from the donor star.
↑Mason, K. O.; Middleditch, J.; Nelson, J. E.; White, N. E. (9 October 1980). "An optical burst from the star identified with the X-ray source 2S1254-690". Nature. 287 (5782): 516–518. Bibcode:1980Natur.287..516M. doi:10.1038/287516a0.