Periodic photometric variability of the black hole binary V404 CYGNI

Recently, Casares, Charles, & Naylor (1992) reported the discovery of absorption lines characteristic of a late G- or early K-type star with a radial velocity curve of amplitude 211 km s^-1 and a period of 6.473 days in the quiescent spectrum of the X-ray nova V404 Cygni. The mass function implied by these observations is 6.3 ± 0.3 M_⊙ and can be considered as a lower limit to the mass of the compact object, which is then most likely a black hole. We have obtained CCD photometry, in the I band, of V404 Cyg during 1990, 1991, and 1992 and have discovered periodic variations having a period of 6.474 ± 0.005 days with a full amplitude of 0.2 mag, thus confirming the previously reported spectroscopic period. The light curve consists of two maxima and two minima per cycle, and the shape suggests that it is due to ellipsoidal variations of the secondary star which is tidally distorted by the presence of the massive compact object. We find that the hypothesis that the secondary star consists of a 4 M_⊙ K0 III star can be excluded. Instead, we propose that the secondary is a ≃1 M_⊙ K0 IV star with M_v ≃ 2.5 mag at a distance of ≃ 3.5 kpc. Our results suggest that if the secondary star is nearly filling its Roche lobe, then the orbital inclination must be about 60°. If the inclination is as high as 80°, a plausible upper limit since no X-ray eclipses were observed, then the secondary star fills about 90% of the Roche lobe. Combining the spectroscopic and photometric results, we find that these limits imply a compact star mass of 8-12 M_⊙ and thus strengthen the evidence that the compact star is extremely massive and possibly a black hole.