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

TL;DR

We address how magnetically active stars in the Galactic plane emit in radio and soft X-rays and whether their emission obeys the Güdel-Benz relation linking $L_X$ and $L_{ u,R}$. We cross-match MeerKAT SMGP S 1.3 GHz sources with ROSAT 2RXS and eRASS1, compute radio brightness temperatures and luminosities, and test the GBR. We find 137 stars with both radio and X-ray detections; most have $T_B \lesssim 10^{12}$ K and are consistent with incoherent gyrosynchrotron emission. The data show the GBR acts as an upper envelope, with 1.3 GHz radio luminosities often enhanced relative to the canonical relation, and eROSITA reveals a broader, more distant coronal population, including early-type stars that lie below $\log (L_X/L_{bol}) \sim -3$. These results underscore the importance of survey depth and frequency in interpreting the radio–X-ray activity relation and pave the way for next-generation radio and X-ray surveys.

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 Güdel-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 Güdel-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.

Figures (10)

• Figure 1: SMGPS and X-ray counterpart positions in Galactic coordinates. The circles, triangles, and squares represent eRASS1, RASS, and combined RASS and eRASS1 detections positions respectively. The rectangular dashed lines are SMGPS footprints.
• Figure 2: Positional offsets between SMGPS radio detections and RASS/eRASS1 X-ray counterparts.
• Figure 3: Comparison of X-ray fluxes from the 41 RASS (0.1 -- 2.4 keV band) and eRASS1 (0.2 -- 2.3 keV band) common detections. The solid line shows the log--log fit to the matched sources, while the dashed line indicates the 1:1 (unity) relation. RASS-only and eRASS1-only detections are plotted along the respective axes. Factor-of-two dotted lines ($y = 2x$ and $y = x/2$) illustrate typical potential variability between the surveys.
• Figure 4: Gaia colour-magnitude diagram for all 137 RASS and eRASS1 radio- and X-ray-emitting stars. The markers represent stellar classifications reported in ( SIMBAD). The interacting binaries include the RS CVn, eclipsing, and spectroscopic binaries. Evolved stars comprise classical cepheids and long-period variables. The G-type, K-type, and M-type classifications are based on the Gaia astrophysical parameter catalogue GaiaCollaboration:2023. The grey circles all represent Gaia DR3 sources within 100 pc of Earth, binned in uniform colour and magnitude. Large open circles represent the six detections in RASS that are not present in eRASS1, whereas the open squares show the 56 detections in eRASS1 that are absent in RASS and within $1 \, \rm kpc$; the open hexagon markers show the 34 eRASS1 stars farther than $1\, \rm kpc$ away.
• Figure 5: Gaia ($BP-RP$) colour versus brightness temperature, $T_{\rm B}$, for SMGPS--X-ray matches with RASS and eRASS1 detections. Markers represent stellar classifications reported in SIMBAD. The interacting binaries include the RS CVn, eclipsing, and spectroscopic binaries. Evolved stars comprise classical cepheids and long-period variables. The G-type, K-type, and M-type classifications are based on the Gaia astrophysical parameter catalogue GaiaCollaboration:2023. The vertical error bars show the propagated uncertainties from the flux measurements and distances.