First Millimeter Flares Detected from Epsilon Eridani with ALMA

Abstract

We report the detection of three large millimeter flaring events from the nearby Sun-like, $\epsilon$ Eridani, found in archival ALMA 12m and ACA observations at 1.33 mm taken from 2015 January 17-18 and 2016 October 24-November 23, respectively. This is the first time that flares have been detected from a Sun-like star at millimeter wavelengths. The largest flare among our data was detected in the ALMA observations on 2015 January 17 from 20:09:10.4-21:02:49.3 (UTC) with a peak flux density of 28 $\pm$ 7 mJy and a duration of 9 sec. The peak brightness of the largest flare is $3.4\pm 0.9\times 10^{14}$ erg s${}^{-1}$Hz${}^{-1}$, a factor of $>50\times$ times brighter than the star's quiescent luminosity and $>10\times$ brighter than solar flares observed at comparable wavelengths. We find changes in the spectral index (F${}_{\nu }\propto {\nu }^{\alpha }$) at the flare peak, with $\alpha$ = 1.81 $\pm$ 1.94 and a lower limit on the fractional linear polarization $|Q/I|=$ 0.08 $\pm$ 0.12. This positive spectral index is more similar to millimeter solar flares, differing from M dwarf flares also detected at millimeter wavelengths that exhibit steeply negative spectral indices.

arXiv comment9 pages, 4 figures, 1 table, accepted for publication in ApJL

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