A Census of Near-UV M-dwarf Flares Using Archival GALEX Data and the gPHOTON2 Pipeline

Abstract

M-dwarfs are common stellar hosts of habitable-zone exoplanets. Near-UV (NUV) radiation can severely impact the atmospheric and surface conditions of such planets, making the characterization of NUV flaring activity a key aspect in determining habitability. We use archival data from the Galaxy Evolution Explorer (GALEX) and XMM-Newton telescopes to study the flaring activity of M-dwarfs in the NUV. The GALEX observations form the most extensive data set of M-dwarfs in the NUV to date, with the exploitation of this data possible due to the new gphoton2 pipeline. We run a dedicated algorithm to detect flares in the pipeline-produced lightcurves and find some of the most energetic flares observed to date within the NUV bandpass, with energies of ∼1034 erg. Using GALEX data, we constrain flare frequency distributions for stars from M0-M6 in the NUV up to 105 s in equivalent duration and 1034 erg in energy, orders of magnitude above any previous study in the UV. We estimate the combined effect of NUV luminosities and flare rates of stars later than M2 to be sufficient for abiogenesis on habitable-zone exoplanets orbiting them. As a counterpoint, we speculate the high frequencies of energetic UV flares and associated coronal mass ejections would inhibit the formation of an ozone layer, possibly preventing the genesis of complex Earth-like life-forms due to sterilizing levels of surface UV radiation. We also provide a framework for future observations of M-dwarfs with ULTRASAT, a wide field-of-view NUV telescope to be launched in 2026.

ULTRASAT: A Wide-field Time-domain UV Space Telescope

“…For the latter in quiescence, we expect to detect only O(1-10) stars for coadds of three images, and O(10 3 ) stars for coadds reaching an estimated confusion limit of 24 mag in the ULTRASAT wave band. Assuming the NUV flare frequency distribution (FFD) derived by Rekhi et al (2023) using archival GALEX data, each 300 s ULTRASAT image will capture ∼150 flaring dM stars, with the equivalent duration distribution shown in Figure 9, for which ≈50% of the flares are in the poorly constrained high-energy regime. The distributions shown in Figures 8 and 9 demonstrate how the long-baseline observations of ULTRASAT will allow us to monitor the high-energy tail of the FFD (i.e., equivalent duration 10 4 s), which is poorly constrained to date.…”

“…The highenergy tail of the distribution, with equivalent duration 10 4 s, is poorly constrained. A Monte Carlo simulation in which the NUV FFD was empirically derived from GALEX observations suggests each ULTRASAT image will capture ∼150 flares from dM stars(Rekhi et al 2023).…”

A Multiwavelength Survey of Nearby M Dwarfs: Optical and Near-ultraviolet Flares and Activity with Contemporaneous TESS, Kepler/K2, Swift, and HST Observations

“…We list the values of fitted parameters of flare frequency distributions (FFDs) in Table 8 for NUV flares and optical flares. Rekhi et al (2023) also studied NUV M dwarf flares using archival GALEX data. However, they estimated that the slopes of the NUV FFDs were slightly less steep than the values we estimated in this study.…”

“…One of the factors that might lead to different slope values could be the different energy ranges considered in the two studies. The range of NUV flare energies is approximately 10 29 -10 35 erg in Rekhi et al (2023).…”