X-ray counterparts to stellar MeerKAT Galactic-plane compact radio sources

TL;DR

This study combines MeerKAT radio data with X-ray surveys to identify and analyze active stars in the Galactic plane, testing their emission properties against the G"udel-Benz relation and revealing deviations from expected correlations.

Contribution

It provides the first large sample of radio and X-ray detected stars in the Galactic plane, characterizes their emission mechanisms, and challenges the universality of the G"udel-Benz relation.

Findings

137 stars detected in both radio and X-ray.

Most stars are consistent with gyrosynchrotron emission.

Sample lies below the canonical G"udel-Benz relation.

Abstract

Radio emission from magnetically active stars arises mainly from non-thermal processes and complements high-energy X-ray emission. Sensitive, wide-field radio and X-ray surveys now allow identification of larger samples of active stars across the Galaxy. We aim to identify and characterise radio and X-ray-emitting stars in the Galactic plane by combining MeerKAT radio data with soft X-ray observations and assess their consistency with the canonical G\"udel-Benz relation, which links thermal coronal X-rays to non-thermal gyrosynchrotron radio emission. We cross-matched compact sources from the SARAO MeerKAT Galactic Plane Survey with counterparts from the ROSAT All-Sky Survey and the first release of SRG/eROSITA (eRASS1). We computed radio-brightness temperatures and radio-X-ray luminosities to test the relation. We identify 137 stars with both radio and X-ray detections. Their $T_B$ ranges from $10^7$ to $10^{12}$ K, except two outliers: AXJ1600.9-5142 ($4.8 \pm 1.5 \times 10^{12}$ K) and HD~124831 ($8 \pm 1 \times 10^{6}$ K). The remainder are consistent with incoherent gyrosynchrotron emission. The sample lies below the canonical G\"udel-Benz relation, driven by enhanced 1.3 GHz radio luminosities relative to the 5 GHz relation. This suggests the classical relation represents an upper envelope rather than a tight correlation. Additionally, eROSITA detections show early-type stars lie below the typical $\log (L_{\rm X}/L_{\rm bol}) \sim -3$ relation.