The X-Ray Outburst from X0115+634 in 1990 February

An X-ray outburst was observed from X0115 + 634 in February 1990 with a maximum intensity of about 400 mcrab around February 11. Reasonable values for orbital parameters were obtained using a pulse timing analysis for the data when the source intensity was above 80 mcrab, but this solution could not be extended to the data around 40 mcrab or less. This suggests that the pulse profile changed around L(crit). Apsidal motion was detected for the first time in an X-ray pulsar, within a value of 0.030 +/- 0.016 deg/yr. This value puts a constraint on the mass radius relation of the companion star which is consistent with a type B main-sequence star if Omega is greater than two. Slower rotation of the companion is excluded by these observations. The X-ray spectrum up to 60 keV was obtained, and the cyclotron absorption lines around 12 and 23 keV are confirmed. These can be understood as first and second harmonics originating from a magnetic field whose strength is about 10 exp 12 G.