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

Regular monitoring of the flux in the H alpha region from V2116 Oph, the 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 BATSE experiment on CGRO. The H alpha increase preceded the X-ray flare by similar to 30 d, but the declines began within a few days of each other. In this paper we interpret the flaring in terms of an impulsive mass 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 is near the upper limit of viscous time-scales for Shakura-Sunyaeva alpha-disc models unless the viscosity parameter alpha<1.