Photometric analysis of the optical counterpart of the black hole HMXB M 33 X-7 (Research Note)

Aims. Study the high-mass X-ray binary X-7 in M 33 using broad-band optical data. Methods. We used recently published CFHT r′ and i′ data for variable stars in M 33 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 UBVRr′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 33 000 < T_eff < 47 000 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.