The ultraviolet variability X-ray pulsars : observations of VELA X-1,A 0535+26 and 4U 1145-619.

Abstract

The IUE satellite has been used in a special trailed mode to search for modulation in the ultraviolet flux from the binary X-ray pulsars Vela X-1, A 0535+26 and 4U 1145-619. The trailing technique allows the UV spectrum from these sources to be sampled during the X-ray pulse cycle, with a resolution ≡1/10th of the pulse period. Modulations of 15 - 20 per cent have been observed from Vela X-1 on two occasions, and of 10 - 15 per cent from A 0535+26 on one occasion. The possible origin of this variation in terms of reprocessed X-rays and/or direct emission from the proximity of the neutron star magnetic poles is discussed. However, it is concluded that the most probable cause in Vela X-1 is an intrinsic variation in the flux from the supergiant B0.5 Ib companion star.