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∞≈60eV⁠, but evidence of harder emission above ∼1keV 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 ν˙=−6.042(4)×10−16Hzs−1⁠, reducing by more than one order magnitude the uncertainty of the previous measurement, and yielding a characteristic spin-down field of 1.47×1013G⁠. We also detect two spectral components above ∼1keV⁠: a blackbody-like one with kT∞=138±13eV and emitting radius 31+8−16m, and a power law with photon index Γ=1.4+0.5−0.4⁠. The power-law 2–8 keV flux, (2.5+0.7−0.6)×10−15ergcm−2s−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