H-ALPHA MONITORING OF THE X-RAY PULSAR GX1-RAY FLARE(4 DURING AN X)

Regular monitoring of the flux in the H alpha region from V2116 Oph, t he optical counterpart of the X-ray pulsar GX 1 + 4, commenced in 1993 . A 1.5-mag flare with slow rise commenced in 1993 July and peaked on September 11 within a few days of the X-ray flare monitored by the BAT SE experiment on CGRO. The H alpha increase preceded the X-ray flare b y similar to 30 d, but the declines began within a few days of each ot her. In this paper we interpret the flaring in terms of an impulsive m ass transfer event from the companion on to the accretion disc. The H alpha emission is generated by interaction of UV and soft X-rays from the disc with circumstellar matter. Hard X-ray flaring commences when the diffusing matter is accreted by the neutron star. The delay before X-ray flaring corresponds to the disc diffusion time. Its magnitude i s near the upper limit of viscous time-scales for Shakura-Sunyaeva alp ha-disc models unless the viscosity parameter alpha<1.