Observations of an outburst of the transient X-ray pulsar A 0535+26 in 1980.

X-ray observations of the pulsar A0535+26 by the Hakucho satellite are analyzed for evidence of a pulse change period. The scanning was carried out in the energy ranges 1-9 keV and 9-22 keV. A pulse arrival time analysis was carried out, resulting in an estimate of a 40 day orbital period, within the 90% confidence interval. The daily average pulse period decreased gradually from 103.67 to 103.61 sec during the 12-day observation period. An upper limit of nearly 120 days was set on the orbital period after consideration of the mass function. It is suggested that the pulse period change observed can be accounted for by a steady decrease in the intrinsic period at a rate proportional to a diminishing luminosity. Furthermore, the outbursts may occur during periastron passage in a highly eccentric orbit, thus explaining the transience of the X-ray source A0535+26.