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

¹ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
² South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
³ Department of Astrophysics/IMAPP, Radboud University, PO 9010, 6500 GL Nijmegen, The Netherlands
⁴ Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
⁵ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁸ Institut für Astronomie Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 August 2025
Accepted: 21 January 2026

Abstract

Context. Radio emission from magnetically active stars arises primarily from non-thermal processes and provides information complementary to their high-energy X-ray emission. Recent advances in sensitive, wide-field radio and X-ray surveys have enabled the identification of larger samples of active stars across the Galaxy.

Aims. 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 to assess their consistency with the canonical Güdel-Benz relation, which states that the thermal coronal emission observed in soft X-rays is correlated with the non-thermal gyrosynchrotron emission detected in the radio.

Methods. We cross-matched compact radio sources from the SARAO MeerKAT Galactic Plane Survey with soft X-ray counterparts from the ROSAT All-Sky Survey and the first release of the SRG/eROSITA all-sky survey, the eRASS1. We investigated their radiobrightness temperatures and computed radio and X-ray luminosities to test for consistency with the Güdel-Benz relation.

Results. We identify 137 stellar sources with both radio and X-ray detections. Their radio-brightness temperatures, T_B, range from 10⁷ to 10¹² K, with the exception of two outliers: AXJ1600.9-5142 (T_B = 4.8 ± 1.5 × 10¹² K) and HD 124831 (T_B = 8 ± 1 × 10⁶ K). The remaining sources are consistent with emission from incoherent gyrosynchrotron processes. The sample generally lies below the canonical Güdel-Benz relation, with the offset predominantly driven by enhanced radio luminosities at 1.3 GHz relative to the canonical relation derived at 5 GHz.

Conclusions. Our results suggest that the classical Güdel-Benz relation represents an upper envelope rather than a tight correlation for active stars detected at 1.3 GHz. In addition, the eROSITA detections show that early-type stars are systematically below the log(L_X/L_bol)~ −3 relation, which is usually seen for stars of a later spectral type.