Search for high frequency optical variations in Vela XR-1

Abstract

THE binary system associated with the 6.87-mag B0.5Ib star, HD 77581, which is identified1,2 with the Vela X-ray source, 2U0900-40, is a good candidate for optical studies with high time resolution because of its relative brightness. A 28-min data set consisting of consecutive integrations of 1 ms was taken on the night of February 25–26, 1973 with the Cerro Tololo 152-cm telescope using an S-20 photomultiplier with no filter. Analyses using power-spectral and cepstrum techniques3 show no significant activity at a level greater than 0.25% in the range from 0.06 Hz to 500 Hz (except for instrumental artefacts at 60, 180, 300 and 500 Hz). No aperiodic variations were detected to a weaker limit (1%–2%); at low frequencies (<10 Hz) increased variance ascribable to atmospheric scintillation was present. This result was confirmed by a separate analysis of about 15 min of data from an observation of 1 h on the night of January 13–14, 1973. That the X-ray emission is actually associated with a degenerate, collapsed (and perhaps invisible) companion to the brilliant supergiant, however, makes a stronger limit very desirable.