Timing properties of the X-ray pulsar EXO 2030+375 during an X-ray outburst

We present Rossi X-ray Timing Explorer (RXTE) observations of the Be/X-ray transient EXO 2030 + 375 during an outburst after a period of quiescence between 1993 August and 1996 April. When active, EXO 2030 + 375 is normally detected at each periastron passage of the neutron star. Our observations correspond to the third periastron passage after the source 'turned on' again. All outbursts after the quiescent period, including the one reported here, have been occurring at a much earlier binary phase than in the past. We discuss the possible mechanisms that may explain this shift in the onset of the outburst. Pulsations in the X-ray radiation are detected throughout the entire run. The neutron star spun up during the outburst at a rate of -1.16 x 10(-8) s s(-1), but no variations in the shape of the pulse profile as a function of intensity were seen. A correlation between the hardness ratio and the intensity is observed at low energies (6-12/2-6 keV). By comparing the magnetospheric and corotation radii we argue that the neutron star spins at a rate close to the equilibrium period. Finally, we perform pulse-phase spectroscopy and comment on changes seen as a function of spin phase.