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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. [ABSTRACT FROM AUTHOR]

The soft X-ray pulsar RX J1856.5 − 3754 is the brightest member of a small class of thermally emitting, radio-silent, isolated neutron stars. Its X-ray spectrum is almost indistinguishable from a blackbody with |$kT^\infty \approx {60}\, {\rm eV}$|⁠ , but evidence of harder emission above |$\sim {1}\, {\rm keV}$| has been recently found. We report on a spectral and timing analysis of RX J1856.5 − 3754 based on the large amount of data collected by XMM-Newton in 2002–2022, complemented by a dense monitoring campaign carried out by NICER in 2019. Through a phase-coherent timing analysis we obtained an improved value of the spin-down rate |$\dot{\nu }=-6.042(4)\times 10^{-16}\, {\rm Hz\, s}^{-1}$|⁠ , reducing by more than one order magnitude the uncertainty of the previous measurement, and yielding a characteristic spin-down field of |$1.47\times 10^{13}\, {\rm G}$|⁠. We also detect two spectral components above |$\sim 1\, {\rm keV}$|⁠ : a blackbody-like one with |$kT^\infty =138\pm 13\,$| eV and emitting radius |$31_{-16}^{+8}\,$| m, and a power law with photon index |$\Gamma =1.4_{-0.4}^{+0.5}$|⁠. The power-law 2–8 keV flux, |$(2.5_{-0.6}^{+0.7})\times 10^{-15}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$|⁠ , corresponds to an efficiency of 10−3, in line with that seen in other pulsars. We also reveal a small difference between the 0.1–0.3 keV and 0.3–1.2 keV pulse profiles, as well as some evidence for a modulation above 1.2 keV. These results show that, notwithstanding its simple spectrum, RX J1856.5 − 3754 still has a non-trivial thermal surface distribution and features non-thermal emission as seen in other pulsars with higher spin-down power. [ABSTRACT FROM AUTHOR]