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 $kT\infty \approx {60}\, {\rm eV}$, 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.