Recovery of the Orbital Parameters and Pulse Evolution of V0332+53 during a Huge Outburst

Abstract

The high-mass X-ray binary V0332+53 became active at the end of 2004, and the outburst was observed in hard X-rays by RXTE and INTEGRAL. Based on these hard X-ray observations, we measure the orbital parameters by fitting the Doppler-shifted spin periods. The derived orbital period and eccentricity are consistent with those obtained by Stella et al. from EXOSAT observations, whereas the projected semimajor axis and the periastron longitude are found to have changed from 48+/-4 to 86⁺⁶_-10 lt-s and from 313°+/-10° to 283°+/-14°, respectively. This would indicate an angular speed of >=1.5d+/-0.8d yr^-1 for the rotation of the orbit over the past 21 years. The periastron passage time of MJD 53,367+/-1 is just around the time when the intensity reached maximum, and an orbital period earlier is the time when the outburst started. This correlation resembles the behavior of a type I outburst. During outburst, the source spun up at a rate of 8.01^+1.00_-1.14×10^-6 s day^-1. The evolution of the pulse profile is highly intensity dependent. The separation of double pulses remained almost constant (~0.47) when the source was bright and dropped to 0.37 within 3 days as the source became weaker. The pulse evolution of V0332+53 may be correlated with a change in dominance of the emission between fan-beam and pencil-beam mechanisms.