Quasi-periodic oscillations during a giant outburst of A0535+262

A major outburst of A0535+262, a 103 s transient X-ray binary pulsar with a Be star companion, was observed in 1994 February and March by the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory. Power spectra of the hard X-ray flux contained a broad quasi-periodic oscillation (QPO) component during the outburst. This QPO was detectable for 33 days during the outburst. During this time the center frequency of the QPO rose from 27 to 72 mHz and then fell again to 25 mHz in a manner that was highly correlated with both the observed hard X-ray flux and the neutron-star spin-up rate inferred from pulse timing. The QPO and the observed spin-up rates, which reached 1.2 x 10(-11) Hz s(-1), strongly suggest the presence of an accretion disk. We compare these observations with the predictions of the beat frequency and Keplerian frequency QPO models and conclude that either model can provide a reasonable explanation of the data.