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 ± 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 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.
