Hα monitoring of the X-ray pulsar GX 1+4 during an X-ray flare

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Abstract
Regular monitoring of the flux in the Hα 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 Hα increase preceded the X-ray flare by ∼30d, 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 Hα 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-Sunyaev α-disc models unless the viscosity parameter α<1.

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