Constraints on Stellar Flare Energy Ratios in the NUV and Optical from a Multiwavelength Study of GALEX and Kepler Flare Stars

Abstract

We present a multiwavelength study of stellar flares on primarily G-type stars using overlapping time domain surveys in the near-ultraviolet (NUV) and optical regimes. The NUV (the Galaxy Evolution Explorer; GALEX) and optical (Kepler) wavelength domains are important for understanding energy fractionations in stellar flares, and for constraining the associated incident radiation on a planetary atmosphere. We follow up on the NUV flare detections presented in Brasseur et al., using coincident Kepler long (1557 flares) and short (two flares) cadence light curves. We find no evidence of optical flares at these times, and place limits on the flare energy ratio between the two wave bands. We find that the energy ratio is correlated with GALEX band energy, and extends over a range of about 3 orders of magnitude in the ratio of the upper limit of Kepler band flare energy to NUV flare energy at the same time for each flare. The two flares with Kepler short-cadence data indicate that the true Kepler band energy may be much lower than the long-cadence-based upper limit. A similar trend appears for the bulk flare energy properties of nonsimultaneously observed flares on the same stars. We provide updated models to describe the flare spectral energy distribution from the NUV through the optical including continua and emission lines to improve upon blackbody-only models. The spread of observed energy ratios is much larger than encompassed by these models and suggests new physics is at work. These results call for a better understanding of NUV flare physics and provide a cautionary tale about using only optical flare measurements to infer the UV irradiation of close-in planets.

X-Ray, Near-ultraviolet, and Optical Flares Produced by Colliding Magnetospheres in the Young High-eccentricity Binary DQ Tau

“…Due to the limited number of data points in the UVOT observations (six per individual orbital phase), our data fitting is restricted to a simple blackbody model. However, Brasseur et al (2023) argue that such simplistic models may not adequately fit both the NUV and optical flare data simultaneously, considering the time-and wavelength-dependent optical depths of flare emission in the lower stellar atmosphere. We confirm their assertion by finding that fitting DQ Tau's combined NUV and optical flare data with a one-temperature blackbody model yields a poor fit (not shown here).…”

“…The fitting procedure for the NUV flare is performed similarly, with the initial blackbody temperature parameter upper boundary raised to 36,000 K as considered for GALEX-NUV stellar flares in Brasseur et al (2023). Such a range also includes the temperature value of 25,000 K proposed for the UV component of solar "white-light" flares (Fletcher et al 2007).…”

Observations of Winds and CMEs of Low-Mass Stars

“…Recent work by Brasseur et al (2023) uses a unique dataset of constraints from whitelight flare measurements via the Kepler spacecraft, and near ultraviolet measurements with the GALEX spacecraft, to put constraints on the energy partition between these two bandpasses. From a sample of 12 stars observed to flare in both wavelength regions, but not simultaneously, we see the same conclusion as noted above from Osten & Wolk: the index of the flare frequency distributions of the two bandpasses are within the errors (Figure 4).…”

“…The more tantalizing result in Brasseur et al (2023) lies in the strictly simultaneous flare measurements. For over 1500 flares observed in the near UV, there are measurements obtained with the Kepler spacecraft.…”

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

“…While limited in sample size, these studies have obtained important results regarding the relative contributions of continuum and line emissions in NUV flare spectra. Limited flare studies with the Galaxy Evolution Explorer (GALEX) have been conducted by Welsh et al (2007), who searched for flaring M-dwarfs in early GALEX observations, Fleming et al (2022), who studied flares in the GJ 65 system, and Brasseur et al (2019Brasseur et al ( , 2023, who conducted a large survey of flares in GALEX and Kepler data, albeit focused on G-stars with a limited sample of M-dwarfs. There have also been multiwavelength flare studies in the NUV/optical and NUV/X-ray regimes conducted by Hawley et al (2007); Paudel et al (2021); Jackman et al (2023); and Jackman (2022).…”