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

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 a_Xsin i = 9.60(5) light-second. This allows us to determine the inclination and masses of the system as $i=84.5(7)\,$deg, M_X = 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.