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

Abstract

HD 49798/RX J0648.0C4418 is a peculiar binary including a hot subdwarf of O spectral type and a compact companion in a 1.55 day orbit. Based on the steady spin period derivative Ṗ = (−2.17 ± 0.01) × 10−15 s s−1 , the compact object was thought to be a contracting young white dwarf (WD). However, the X-ray luminosity produced by the wind accretion of the 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 were surrounded by a debris disk, the accretion from the debris disk could 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 3.9 × 10−6 M⊙. The accretion torque exerted by such a debris disk can only influence the spin evolution of the WD in the early stages, and its affect is obviously smaller than that caused by the contraction of the WD in the current stage. According to accretion theory, the magnetic field of the WD is constrained to ∼(0.7−7) × 104 G. The calculated conventional polar cap radius of the WD is larger than the observed emitting-zone radius, which probably originates from the existence of strong and small-scale local magnetic field in the polar cap surface. We expect that further multiband observations of this source could help us to confirm or rule out the existence of a debris disk.