First Millimeter Flares Detected From Epsilon Eridani With ALMA

Stellar flares in the millimeter regime have been largely unexplored. The recent detection of these flares from M dwarfs Proxima Centauri and AU Mic with ALMA has revealed interesting flare characteristics such as short durations, negative spectral indices, and linear polarization at the flare peak. There is a clear need for further investigation of millimeter flaring emission from stars of various spectral types to obtain a better physical and statistical understanding of their characteristics. We present the first detection of millimeter flares from Epsilon Eridani, found using archival ALMA observations at 1.34 millimeters. This is the first time that a flare has been detected from a Sun-like star at millimeter wavelengths. Our results indicate that millimeter emission is likely a common feature of stellar flares from stars of all spectral types. We analyzed data taken from 17 January 2015 through 18 January 2015, with a rms noise of 17 micro Janskys beam^-1. The largest flare we detected occurred on 17 January 2015 from 20:09:10.4-21:02:49.3 (UTC) with a peak flux density of 28 ± 7 mJy and corresponding luminosity of 3.4×10¹⁴ ergs s^-1 Hz^-1. We find changes in the spectral index at the flare peak with a value alpha = 1.81 ± 1.94 and a lower limit on the fractional linear polarization of 0.08 ± 0.12. This project was supported in part by the NSF REU grant AST-1757321 and by the Nantucket Maria Mitchell Association.