Quest for the upcoming periastron passage of an episodic dust maker and particle accelerating colliding wind binary: WR 125

TL;DR

This study presents a long-term multi-wavelength investigation of WR 125, revealing episodic dust formation, X-ray variability, and suggesting a 28-29 year orbital period in a colliding wind binary system.

Contribution

It provides new observational evidence linking dust formation episodes with X-ray variability, proposing a long orbital period for WR 125 based on multi-wavelength data.

Findings

Episodic dust formation observed in 2017-2021 similar to 1990s event

X-ray emission shows long-term variability with a low state in June 2020

Orbital period estimated at approximately 28-29 years

Abstract

We have carried out a long term infrared and X-ray investigation of the colliding wind binary WR 125 (WC7+O9III). The source was monitored using AstroSat-Soft X-ray Telescope and TIRCAM2 mounted at the back of 3.6 m Devasthal Optical Telescope. WR 125 appeared brighter in near infra-red K-band during the years 2017-2021 which is attributed to another episode of dust formation similar to the one reported during the likely periastron passage at the beginning of the 1990s. This is further supported by enhanced emission observed in W1 and W2 bands of WISE from 2018-19. By combining archival X-ray data sets with our new measurements, long-term variations have been noticed. The source reaches a lower emission state in June 2020 (close to the recent infrared maximum) which could be due to enhanced absorption of X-rays produced in the colliding wind region by the WC stellar wind close to the periastron in an eccentric orbit. The time interval between the previous and latest X-ray low states may indicate an orbital period of 28-29 years, in fair agreement with the recurrence time of episodic dust production. We also discuss published radio measurements in the context of a common picture based on a long period binary scenario. These results allow us to draw relevant guidelines for future multi-wavelength observations of WR 125.