Now you see it, now you don't - the circumstellar disc in the GRO J1008-57 system

Multiwavelength observations are reported here of the Be/X-ray binary pulsar system GRO J1008-57. Over ten years worth of data are gathered together to show that the periodic X-ray outbursts are dependant on both the binary motion and the size of the circumstellar disc. In the first instance an accurate orbital solution is determined from pulse periods, and in the second case the strength and shape of the Hα emission line is shown to be a valuable indicator of disc size and its behaviour. Furthermore, the shape of the emission line permits a direct determination of the disc size which is in good agreement with theoretical estimates. A detailed study of the pulse period variations during outbursts determined the binary period to be 247.8 +/- 0.4 d, in good agreement with the period determined from the recurrence of the outbursts.