The Proxima Centauri Campaign—First Constraints on Millimeter Flare Rates from ALMA

• Published in: The Astrophysical journal Vol. 982; no. 1; pp. 43 - 51
• Main Authors: Burton, Kiana, MacGregor, Meredith A., Osten, Rachel A., Howard, Ward S., Weinberger, Alycia J., Shkolnik, Evgenya, Wilner, David J., Forbrich, Jan, Barclay, Thomas
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20.03.2025; IOP Publishing
• Subjects: Extrasolar planets; Flare stars; Flares; Flux density; Frequency distribution; M dwarf stars; Observational astronomy; Power law; Radio astronomy; Radio telescopes; Red dwarf stars; Stellar activity; Submillimeter astronomy; Wavelengths
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/ada5f2

Proxima Centauri (Cen) has been the subject of many flaring studies due to its proximity and potential to host habitable planets. The discovery of millimeter flares from this M dwarf with Atacama Large Millimeter/submillimeter Array (ALMA) has opened a new window into the flaring process and the space-weather environments of exoplanets like Proxima b. Using a total of ~50 hr of ALMA observations of Proxima Cen at 1.3 mm (233 GHz), we add a new piece to the stellar flaring picture and report the first cumulative flare frequency distribution (FFD) at millimeter wavelengths of any M dwarf. We detect 463 flares ranging from energies 10 24 to 10 27 erg. The brightest and most energetic flare in our sample reached a flux density of 119 ± 7 mJy, increasing by a factor of 1000× the quiescent flux, and reaching an energy of 10 27 erg in the ALMA bandpass, with t 1/2 ≈ 16 s. From a log–log linear regression fit to the FFD, we obtain a power-law index of α FFD = 2.92 ± 0.02, much steeper than α FFD values (~2) observed at X-ray to optical wavelengths. If millimeter flare rates are predictive of flare rates at extreme-UV wavelengths, the contribution of small flares to the radiation environment of Proxima b may be much higher than expected based on the shallower power-law slopes observed at optical wavelengths.