In the constellation Monoceros, an X-ray source known as A0620-00 brightened by more than five orders of magnitude (100,000 times) in the winter of 1975. This X-ray nova event is associated with a main sequence K star known to show optical brightness variations and thus designated V616 Mon. The K star orbits an unseen companion once every 7.75 hours with a maximum velocity of 457 km/s. It is believed that the light of the K star varies because the star's shape is distorted by the gravitational pull of its invisible but massive companion. The mass of the compact star must be greater than 3.2 solar masses and may be greater than 7.3 solar masses, making this an excellent candidate for a black hole.
"Stalking the Black Hole in the Star Garden of the Unicorn", McClintock, J., 1987, Mercury Jul-Aug, 108.
"The mass of the black hole in A0620-00" Shahbaz, T., Naylor, T., and Charles, P.A. 1994, Mon. Not. Roy. Astr. Soc. 268, 756.
"A detection of orbital radial velocity variations in the primary component of the black hole binary A0620-00", 1990, Haswell, C.A. and Shafter, A.W. 1990, Astrophys. J.(Lett.), 359, L47.
"On the mass of the compact object in the black hole binary A0620-00", 1990, Haswell et al. 1993, Astrophys. J., 411, 802.
``Evidence for Black Holes in Stellar Binary Systems'', Cowley, A.P., 1992, Annual Reviews of Astronomy and Astrophysics 30, 287 (see discussion of V616 Mon on pp. 292-4; you may also want to look at some of the references cited herein).
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