X-ray studies of three binary millisecond pulsars

Abstract

It is thought that millisecond pulsars with white dwarf companions are born from X‐ray binaries. The majority of known systems have been studied uniquely in the radio domain, which limits our understanding of such systems. We present here the X‐ray observations of the millisecond pulsar PSR J0218+4232 and the two faint millisecond pulsars PSR J0751+1807 and PSR J1012+5307, which we discuss in conjunction with radio observations. We confirm the previously detected X‐ray pulsations of PSR J0218+4232 and we show that its folded lightcurve is strongly dependent on energy. We present evidence to suggest that the broad band X‐ray spectrum for this pulsar may not be a simple power law, but that there is some evidence for an excess of soft thermal emission over the power law spectrum, in particular from the strongest pulse, in support of a heated polar cap model for this pulsar. We also present the X‐ray spectra of the two faint millisecond pulsars as well as some evidence to suggest that both of these millisecond pulsars show pulsations in the X‐ray band. We then discuss the implied nature of the magnetic field configuration as a means of discriminating between competing magnetic field evolution theories in millisecond pulsars.