Peculiar radio$-$X-ray relationship in active stars

TL;DR

This paper reports a surprising finding that some active stars with coherent radio emission still follow the established G"udel-Benz relationship, challenging existing theories about stellar coronal heating and emission mechanisms.

Contribution

It reveals a population of active stars that adhere to the G"udel-Benz relationship despite exhibiting coherent radio emission, prompting a re-evaluation of the underlying physics.

Findings

Coherent radio emission can follow the G"udel-Benz relationship.

Active stars with coherent emission challenge existing emission mechanism models.

Results suggest a closer link between solar and stellar flare processes.

Abstract

The empirical relationship between the non-thermal 5GHz radio luminosity and the soft X-ray luminosity of active stellar coronae, canonically called the G\"udel-Benz relationship (G\"udel & Benz 1993), has been a cornerstone of stellar radio astronomy as it explicitly ties the radio emission to the coronal heating mechanisms. The relationship extends from microflares on the Sun to the coronae of the most active stars suggesting that active coronae are heated by a flare-like process (Benz & G\"udel 1994). The relationship is thought to originate from a consistent partition of the available flare energy into relativistic charges, that emit in the radio-band via the incoherent gyrosynchrotron mechanism, and heating of the bulk coronal plasma, that emits in the X-ray band via the Bremsstrahlung mechanism. Consequently, coherent emission from stellar and sub-stellar objects is not expected to adhere to this empirical relationship, as is observed in ultracool dwarf stars and brown dwarfs. Here we report a population of radio-detected chromospherically active stars that surprisingly follows the G\"udel-Benz relationship despite their radio emission being classified as coherent emission by virtue of its high circularly polarised fraction and high brightness temperature. Our results prompt a re-examination of the physics behind the G\"udel-Benz relationship, its implication for the mechanism of coronal heating and particle acceleration in active stars and the phenomenological connection between solar and stellar flares.