Photometric Analysis of the Optical Counterpart of the Black Hole HMXB M 33 X - 7

Aims. Study the high-mass X-ray binary X-7 in M33 using broad-band optical data. Methods. We used recently published CFHT r ′ and i ′ data for variable stars in M33 to extract the light curve of the optical counterpart of X-7. We combined these data with DIRECT B and V measurements in order to search for an independent optical modulation with the X-ray periodicity. The periodic modulation is modelled with the ellipsoidal effect. We used U BVRr ′ i ′ magnitudes of the system to constrain the temperature and radius of the optical component. Results. The optical data revealed a periodicity of 3.4530 ± 0.0014 days , which is consistent with the known X-ray period. Double modulation, which we attributed to ellipsoidal modulation, is clearly seen in four different optical bands. The absolute magnitude in six optical bands is most consistent with a stellar counterpart with 33000 < T e f f < 47000 K and 15 < R < 20 R ⊙. We modelled the optical periodic modulation and derived the masses of the two components as a function of the orbital inclination and the radius of the stellar component. The resulting mass range for the compact object is 1.3 < M < 23 M ⊙. Conclusions. The system is probably a black hole HMXB, similar to Cyg X-1, LMC X-1 and LMC X-3.