Radio and Optical Flares on the dMe Flare Star EV Lac

TL;DR

This study investigates the relationship between radio and optical flares on the M dwarf star EV Lac, revealing that only some optical flares produce radio responses, with delays indicating complex magnetic loop interactions.

Contribution

It provides the first coordinated radio and optical observations of EV Lac, analyzing flare correlations and electron distributions with detailed timing and spectral data.

Findings

Optical flares do not reliably predict radio responses.

Radio responses are associated with higher low-energy electron cutoffs.

Optical and radio flare timings suggest multiple magnetic loops involved.

Abstract

We present the results of a coordinated campaign to observe radio and optical stellar flares from the nearby M dwarf flare star EV~Lac. From a total of 27 hours of radio and 29 hours of optical observations, we examine the correspondence of the action of accelerated electrons of different energies in two distinct regions of the stellar atmosphere. We find that out of 9 optical flares with suitable radio coverage, only four have plausible evidence for a radio response. Optical photometric properties cannot predict which flares will have a radio response. From flares with time-resolved optical spectroscopy available, optical-only flares have similar implied electron distributions, while those with radio responses better correlate with higher low-energy cutoffs. The optical flares with a radio response all exhibit a delay between the optical and radio peaks of $\approx$1-7 minutes, with the optical flare peaking earlier in all cases. This likely indicates multiple loops are involved in the event, and/or the different impacts on electrons trapped in a magnetic loop (producing radio emission), versus those directly precipitating from the loop (producing the optical flare). We also remark on the radio spectral index behavior at early times for the largest radio flare observed in this study, which we interpret as evidence for increased opacity from a chromospheric evaporation front.