Long-term X-ray variation of the colliding-wind Wolf-Rayet binary WR 125

WR 125 is considered to be a colliding-wind Wolf-Rayet binary (CWWB), from which the most recent infrared flux increase was reported between 1990 and 1993. We observed the object four times from 2016 November to 2017 May with Swift and XMM-Newton, and carried out a precise X-ray spectral study for the first time. There were hardly any changes in the fluxes and spectral shapes for half a year, and the absorption-corrected luminosity was 3.0 × 10³³ erg s^-1 in the range 0.5-10.0 keV at a distance of 4.1 kpc. The hydrogen column density was higher than that expected from the interstellar absorption, and thus the X-ray spectra were probably absorbed by the Wolf-Rayet (WR) wind. The energy spectrum was successfully modelled by a collisional equilibrium plasma emission, in which both the plasma and the absorbing wind have unusual elemental abundances particular to WR stars. In 1981, the Einstein satellite clearly detected X-rays from WR 125, whereas the ROSAT satellite hardly detected X-rays in 1991, when the binary was probably around the periastron passage. We discuss possible causes for the unexpectedly low soft X-ray flux near the periastron.