V0332+53 in the outburst of 2004-2005: luminosity dependence of the cyclotron line and pulse profile

Abstract

ABSTRACT We present results of observations of the transient X-ray pulsar V0332+53 performedduring a very powerful outburst in Dec, 2004 – Feb, 2005 with the INTEGRAL andRXTE observatories in a wide (3 − 100 keV) energy band. A cyclotron resonancescattering line at an energy of ∼ 26 keV has been detected in the source spectrumtogether with its two higher harmonics at ∼ 50 and ∼ 73 keV, respectively. We showthat the energy of the line is not constant but linearly changes with the source lumi-nosity. Strong pulse profile variations, especially near the cyclotron line, are revealedfor different levels of the source intensity. We discuss the obtained results in terms ofthe theoretical models of X-ray pulsars.Key words: X-ray:binaries – (stars:)pulsars:individual – V0332+53 1 INTRODUCTIONThe transient X-ray pulsar V0332+53 was discovered bythe Vela 5B observatory in 1973 (Terrel & Predhorsky 1973)during an outburst when its intensity reached ∼ 1.4 Crabin the 3 − 12 keV energy band. The outburst lasted aboutthree months before the source became undetectable again.During observations in Nov 1983 – Jan 1984 with theEXOSAT observatory the pulsar’s parameters and orbitalones were determined: the pulse period ∼ 4.375 s, the or-bital period – 34.25 days, the eccentricity – 0.31, and theprojected semimajor axis of the neutron star – a