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

TL;DR

This paper presents evidence that the compact object in the HD 49798/RX J0648.0--4418 binary is a young, contracting white dwarf, based on its stable spin-up over 20 years, marking the first such direct evidence.

Contribution

The study demonstrates that the stable spin-up of the system is best explained by contraction of a young white dwarf, providing new insights into early white dwarf evolution.

Findings

Stable spin-up over 20 years indicates a contracting white dwarf.

The white dwarf's age is estimated at around 2 million years.

First direct evidence of a young white dwarf contracting in an accreting binary.

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.