RIGOSELLI, MichelaMichelaRIGOSELLIDE GRANDIS, DavideDavideDE GRANDISMEREGHETTI, SandroSandroMEREGHETTIMALACARIA, ChristianChristianMALACARIA2024-12-172024-12-1720230035-8711http://hdl.handle.net/20.500.12386/35525HD 49798 is a hot subdwarf of O spectral type in a 1.55 day orbit with the X-ray source RX J0648.0-4418, a compact object with spin period of 13.2 s. We use recent data from the NICER instrument, joined with archival data from XMM-Newton and ROSAT, to obtain a phase-connected timing solution spanning ~30 years. Contrary to previous works, that 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 Pdot = -2.28(2)x10^-15 s/s and obtain a refined measure of the projected semi-major axis of the compact object aX sini = 9.60(5) lightsec. This allows us to determine the inclination and masses of the system as i = 84.5(7) deg, MX = 1.220(8) Msun and Mopt = 1.41(2) Msun. We also study possible long term (~year) 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 onto a massive white dwarf can explain the observed luminosity and spin-up rate for a wind velocity of ~800 km/s.STAMPAenArticle10.1093/mnras/stad16112-s2.0-85162110668https://academic.oup.com/mnras/article/523/2/3043/7190642http://arxiv.org/abs/2305.15845v1FIS/05 - ASTRONOMIA E ASTROFISICA