Phase-resolved XMM-Newton and swift observations of WR 25

TL;DR

This study analyzes 10 years of X-ray and optical data of WR 25, revealing its periodic variability, phase-locked X-ray flux changes, and colliding wind phenomena in a Wolf-Rayet binary system.

Contribution

First long-term multi-wavelength analysis of WR 25 showing its orbital period, phase-locked variability, and detailed plasma characteristics in a colliding wind binary.

Findings

WR 25 has a 208-day orbital period with phase-locked X-ray and optical variability.

X-ray flux varies by a factor of 2, peaking near periastron.

X-ray spectra indicate two plasma temperatures, with plasma properties remaining constant over the orbit.

Abstract

We present an analysis of long-term X-ray and optical observations of the Wolf-Rayet binary WR 25. Using archival data from observations with the XMM-Newton and the Swift observatories spanning over ~10 yr, we show that WR 25 is a periodic variable in X-rays with a period of $208 \pm 3$ days. X-ray light curves in the 0.5-10.0 keV energy band show phase-locked variability, where the flux increased by a factor of ~2 from minimum to maximum, being maximum near periastron passage. The light curve in the soft energy band (0.5-2.0 keV) shows two minima indicating the presence of two eclipses. However, the light curve in the hard energy band (2.0-10.0 keV) shows only one minimum during the apastron passage. The X-ray spectra of WR 25 were explained by a two-temperature plasma model. Both the cool and the hot plasmas were constant at 0.628+/-0.008 and 2.75+/-0.06 keV throughout an orbital cycle, where the cooler plasma could be due to the small scale shocks in a radiation-driven outflow and the high temperature plasma could be due to the collision of winds. The column density varied with the orbital phase and was found to be maximum after the periastron passage, when the WN star is in front of the O star. The abundances of WR 25 were found to be non-solar. Optical V-band data of WR 25 also show the phase-locked variability, being at maximum near periastron passage. The results based on the present analysis indicate that WR 25 is a colliding wind binary where the presence of soft X-rays is attributed to individual components; however, hard X-rays are due to the collision of winds.