During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing detailed observation of the companion star during this period. Source: Internet
Each pair of vertical lines delineate the eclipse of the compact object behind its companion star. Source: Internet
In this class of system, the companion star is of relatively low mass allowing for more accurate estimates of the black hole mass. Source: Internet
Spectroscopic examination of the energy states in the outflow of matter from the companion star suggests that it is about 224 AU beyond the primary, giving a combined separation of about 574 AU. Source: Internet
By studying the companion star it is often possible to obtain the orbital parameters of the system and to obtain an estimate for the mass of the compact object. Source: Internet
It is said that the companion star to Sirius, white in colour, is made of such dense material that a cubic inch of it would weigh a tonne here. Source: Internet