GR Muscae

GR Muscae, also known as 2S 1254-690 is a binary star system in the constellation Musca 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.^[2] A magnitude 19 blue star was pinpointed as the optical counterpart of the X-ray source in 1978.^[3] Its apparent magnitude varies from 18 to 19.1 over a period of 0.16 days.^[4]

GR Muscae

Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Musca
Right ascension	12h 57m 37.153s[1]
Declination	−69° 17′ 18.98″[1]
Apparent magnitude (V)	19.1[1]
Database references
SIMBAD	data

Visual band light curves for GR Muscae, adapted from Cornelisse et al. (2013).^[2]

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.^[5] GR Muscae received its variable star designation in 1985.^[6]

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.^[2]