On the Size of the Non-Thermal Component in the Radio Emission from Cyg OB2 #5

TL;DR

This study analyzes radio observations of Cyg OB2 #5 to estimate the size of its non-thermal emission component, providing insights into its origin and contributing to understanding stellar wind interactions.

Contribution

The paper presents the first angular size measurement of the non-thermal radio component in Cyg OB2 #5, aiding in distinguishing between proposed emission mechanisms.

Findings

Angular size of 0.02 arcseconds for the non-thermal component

Supports models involving a star with non-thermal emission or wind-collision region

Analysis of archival VLA data at 8.46 GHz

Abstract

Cyg OB2 #5 is a contact binary system with variable radio continuum emission. This emission has a low-flux state where it is dominated by thermal emission from the ionized stellar wind and a high-flux state where an additional non-thermal component appears. The variations are now known to have a period of 6.7 +/- 0.2 yr. The non-thermal component has been attributed to different agents: an expanding envelope ejected periodically from the binary, emission from a wind-collision region, or a star with non-thermal emission in an eccentric orbit around the binary. The determination of the angular size of the non-thermal component is crucial to discriminate between these alternatives. We present the analysis of VLA archive observations made at 8.46 GHz in 1994 (low state) and 1996 (high state), that allow us to subtract the effect of the persistent thermal emission and to estimate an angular size of 0.02 arcseconds for the non-thermal component. This compact size favors the explanation in terms of a star with non-thermal emission or of a wind-collision region.