Discovery of spin-up in the X-ray pulsar companion of the hot subdwarf HD 49798

Abstract

The hot subdwarf HD 49798 has an X-ray emitting compact companion with a spin-period of 13.2 s and a dynamically measured mass of 1.28+/-0.05 M_sun, consistent with either a neutron star or a white dwarf. Using all the available XMM-Newton and Swift observations of this source, we could perform a phase-connected timing analysis extending back to the ROSAT data obtained in 1992. We found that the pulsar is spinning up at a rate of (2.15+/-0.05)x10^{-15} s/s. This result is best interpreted in terms of a neutron star accreting from the wind of its subdwarf companion, although the remarkably steady period derivative over more than 20 years is unusual in wind-accreting neutron stars. The possibility that the compact object is a massive white dwarf accreting through a disk cannot be excluded, but it requires a larger distance and/or properties of the stellar wind of HD 49798 different from those derived from the modelling of its optical/UV spectra.

arXiv commentAccepted for publication in MNRAS

References in corpus (7)

• Magnetic White Dwarfs
• An ultra-massive fast-spinning white dwarf in a peculiar binary system
• The discovery of 13s X-ray pulsations from the hydrogen depleted subdwarf O6 star binary HD49798
• X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418
• Magnetic fields of neutron stars in X-ray binaries
• X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion
• Discovery of a compact companion to the hot subdwarf star BD+37 442

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