DISCOVERY OF OPTICAL PULSATIONS IN V2116-OPHIUCHI ALWAYS EQUALS GX-1+4

We report the detection of pulsations with an approximately 124 s peri od in V2116 Oph, the optical counterpart of the low-mass X-ray binary GX 1 + 4. The pulsations are sinusoidal with modulation amplitude of u p to 4% in blue light and were observed in 10 different observing sess ions during 1996 April-August using a CCD photometer at the 1.6 m and 0.6 m telescopes of Laboratorio Nacional de Astrofisica, in Brazil. Th e pulsations were also observed with the UBVRI fast photometer. With o nly one exception, the observed optical periods are consistent with th ose observed by the BATSE instrument on board the Compton Gamma Ray Ob servatory at the same epoch. There is a definite correlation between t he observability of pulsations and the optical brightness of the syste m: V2116 Oph had R magnitude in the range 15.3-15.5 when the pulsed si gnal was detected, and R = 16.0-17.7 when no pulsations were present. The discovery makes GX 1 + 4 only the third of about 40 accretion-powe red X-ray pulsars to be firmly detected as a pulsating source in the o ptical. The presence of flickering and pulsations in V2116 Oph adds st rong evidence for an accretion disk scenario in this system. The absol ute magnitude of the pulsed component on 1996 May 27 is estimated to b e M-v similar to -1.5. The implied dimensions for the emitting region are 1.1 R., 3.2 R., and 7.0 R., for blackbody spectral distributions w ith T = 10(5) K, 2 x 10(4) K, and 1 x 10(4) K, respectively.