Unequivocal
evidence for a stellar-size black hole in the Galaxy has been sought for
over two decades. CygX-1 is a strong candidate, but the massive companion
star makes it difficult to set a lower limit of better than 3 solar masses
to the mass of the compact object. The soft-X-ray transients have yielded
potentially more important candidates because of the low mass of the companion
(or secondary) star and its faintness after the X-ray outburst has subsided
into quiescence. The 1975 transient A0620—00 is currently considered to
be the strongest black-hole candidate, with a mass function of 2.9 solar
masses, which represents the lower limit to the mass of the compact object.
Although X-ray and optical light curves similar to those of A0620—00 are
considered sufficient to label an object a 'black hole candidate', the
only certain way of identifying the compact object is to study the radial
velocity of the secondary star and measure the mass function.
In order to get
a good coverage of the radial velocity curve of V404 Cyg, 73 spectra were
obtained using the William Herschel Telescope and the Isaac Newton Telescope.
Radial velocities were then derived by cross-correlating the spectra. These
clearly demostrated a large-amplitude modulation with a period of 6.47
days and a large velocity amplitude of 210.6 km/s. From the mass function
of the system, it's possible to set lower limits to the mass of the primary
star. For a zero secondary star mass and a system inclination of 90 degrees,
we get 6.26 solar masses which substantially exceeds the 3 solar masses
maximum allowed mass of a neutron star. So astronomers concluded that the
compact object had to be a black hole.
The most straightforward
model for V404 Cyg is a binary system consisting of a black hole and a
K0 III secondary. A main-sequence star is ruled out because its radius
would only be 0.14 of its Roche-lobe radius, and it would thus be unable
to transfer mass to the compact object. It must be close to the Roche-lobe
radius to account for the X-ray outbursts. Alternatively, V404 Cyg may
be a triple system, in which the G/K star observed in the spectra is a
main sequence star orbiting a black hole/late-type-dwarf binary. The late-type
dwarf must be less than 0.5 solar masses for its luminosity to be much
lower than that of the G/K star, and thus not be detected in the spectra.
This inner binary would then be similar to other X-ray transients, but
the third star allows to place a limit on its total mass of 6.26 solar
masses. Because the late-type star cannot be more than 0.5 solar masses,
the compact object must still be a black hole, as discussed above.
