On the X-ray pulsar HD 49798: a contracting white dwarf with debris disk?

TL;DR

This paper proposes that the X-ray emission from the binary system HD 49798 is due to accretion from a debris disk around the white dwarf, providing a new explanation for observed luminosity and spin evolution.

Contribution

It introduces a debris disk accretion model to explain the X-ray luminosity and spin behavior of the white dwarf in HD 49798, challenging previous wind accretion assumptions.

Findings

Debris disk mass estimated at 3.9×10⁻⁶ M☉.

Magnetic field of the white dwarf constrained to (0.7-7)×10⁴ G.

Polar cap radius larger than observed, indicating complex magnetic field structure.

Abstract

HD49798/RX J0648.0-4418 is a peculiar binary including a hot subdwarf of O spectral type and a compact companion in a 1.55 day orbit. According to the steady spin period derivative $\dot{P}=(-2.17\pm0.01)\times10^{-15} ~\rm s\,s^{-1}$ , the compact object was thought to be a contracting young white dwarf (WD). However, the X-ray luminosity producing by the wind accretion of massive WD is one order of magnitude smaller than the observed value. In this work, we propose an alternative model to account for the observed X-ray luminosity. If the WD was surrounded by a debris disk, the accretion from the debris disk can produce the observed X-ray luminosity and X-ray pulses. Based on the time-varying accretion rate model, the current mass of the debris disk is constrained to be $3.9\times10^{-6}~\rm M_{\odot}$. Comparing with the contraction of the WD, the accretion torque exerting by such a debris disk can only influence the spin evolution of the WD in the early stage. According to the accretion theory, the magnetic field of the WD is constrained to be $\sim (0.7-7)\times10^{4}$ G. The calculated conventional polar cap radius of the WD is larger than the observed emitting-zone radius, which probably originate from the existence of strong and small-scale local magnetic field in the polar cap surface. We expect that further multiband observations on this source can help us to confirm or rule out the existence of a debris disk.