Timing the X-ray pulsating companion of the hot subdwarf HD 49798 with NICER

Abstract

HD 49798 is a hot subdwarf of O spectral type in a 1.55 d orbit with the X-ray source RX J0648.0−4418, a compact object with a spin period of $13.2\,$s. We use recent data from the Neutron Star Interior Composition Explorer instrument, joined with archival data from XMM–Newton and ROSAT, to obtain a phase-connected timing solution spanning ∼30 yr. Contrary to previous works, which relied on parameters determined through optical observations, the new timing solution could be derived using only X-ray data. We confirm that the compact object is steadily spinning up with $\dot{P} = -2.28(2) \times 10^{-15}\,$s s−1 and obtain a refined measure of the projected semimajor axis of the compact object aXsin i = 9.60(5) light-second. This allows us to determine the inclination and masses of the system as $i=84.5(7)\,$deg, MX = 1.220(8) $\rm {M}_\odot$, and $M_{\rm opt}=1.41(2)\,$$\rm {M}_\odot$. We also study possible long-term (approximately years) and orbital variations of the soft X-ray pulsed flux, without finding evidence for variability. In the light of the new findings, we discuss the nature of the compact object, concluding that the possibility of a neutron star in the subsonic propeller regime is unlikely, while accretion of the subdwarf wind on to a massive white dwarf can explain the observed luminosity and spin-up rate for a wind velocity of ∼800 km s−1.

X-ray observations of isolated neutron stars

“…Possible solutions are either real ages of these objects are much smaller than their characteristic ages due to large initial spin periods, or some mechanism prevents the crustal magnetic field from decaying in time [see 109, and refrences therein]. Associations with white dwarfs instead of neutron stars have been considered and discarded [108], but not because of the spin periods: white dwarfs with 𝑃 ≲ 70 s have been found both in binary systems [110,111] and as isolated sources [112].…”