A young contracting white dwarf in the peculiar binary HD 49798/RX J0648.0--4418?

Abstract

HD 49798/RX J0648.0--4418 is a peculiar X-ray binary with a hot subdwarf (sdO) mass donor. The nature of the accreting compact object is not known, but its spin period $P=13.2$~s and $\dot{P}=-2.15\times 10^{-15}$s~s${}^{-1}$, prove that it can be only either a white dwarf or a neutron star. The spin-up has been very stable for more than 20 years. We demonstrate that the continuous stable spin-up of the compact companion of HD 49798 can be best explained by contraction of a young white dwarf with an age $\sim 2$~Myrs. This allows us to interpret all the basic parameters of the system in the framework of an accreting white dwarf. We present examples of binary evolution which result in such systems. If correct, this is the first direct evidence for a white dwarf contraction on early evolutionary stages.

arXiv comment9 pages, accepted to MNRAS

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• Evolution of Helium Star - White Dwarf Binaries Leading up to Thermonuclear Supernovae
• Surface and core detonations in rotating white dwarfs
• New X-ray observations of the hot subdwarf binary HD49798 / RXJ0648.0-4418
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• Discovery of a ~30-Year-Duration Post-Nova Pulsating Supersoft Source in the Large Magellanic Cloud
• Prospects for the Observation of Continuous Gravitational Waves from Deformed Fast-spinning White Dwarfs
• Timing the X-ray pulsating companion of the hot-subdwarf HD 49798 with NICER
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