Two decades of X-ray observations of the isolated neutron star RX??J1856.5???3754: detection of thermal and non-thermal hard X-rays and refined spin-down measurement

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(infinity) approximate to 60 eV, but evidence of harder emission above similar to 1 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 (nu)over dot = -6.042(4) x10(-16) 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 x10(13) G. We also detect two spectral components above similar to 1 keV: a blackbodylike one with kT(infinity) = 138 +/- 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)) x10(-15) erg cm(-2) 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.