M Dwarf Flare Continuum Variations on One-Second Timescales: Calibrating and Modeling of ULTRACAM Flare Color Indices

TL;DR

This study uses high-speed ULTRACAM observations and modeling to analyze the spectral evolution of M dwarf flares on one-second timescales, revealing continuum emission characteristics and temperature estimates.

Contribution

It provides the first detailed calibration and modeling of ULTRACAM flare color indices, linking observed flux ratios to physical flare parameters and emission processes.

Findings

NUV/blue flux ratio equals Balmer jump ratio

Blue/red flux ratio estimates optical continuum temperature (~10,000 K)

High time-resolution observations reveal significant continuum color variations

Abstract

We present a large dataset of high cadence dMe flare light curves obtained with custom continuum filters on the triple-beam, high-speed camera system ULTRACAM. The measurements provide constraints for models of the NUV and optical continuum spectral evolution on timescales of ~1 second. We provide a robust interpretation of the flare emission in the ULTRACAM filters using simultaneously-obtained low-resolution spectra during two moderate-sized flares in the dM4.5e star YZ CMi. By avoiding the spectral complexity within the broadband Johnson filters, the ULTRACAM filters are shown to characterize bona-fide continuum emission in the NUV, blue, and red wavelength regimes. The NUV/blue flux ratio in flares is equivalent to a Balmer jump ratio, and the blue/red flux ratio provides an estimate for the color temperature of the optical continuum emission. We present a new "color-color" relationship for these continuum flux ratios at the peaks of the flares. Using the RADYN and RH codes, we interpret the ULTRACAM filter emission using the dominant emission processes from a radiative-hydrodynamic flare model with a high nonthermal electron beam flux, which explains a hot, T~10,000 K, color temperature at blue-to-red optical wavelengths and a small Balmer jump ratio as are observed in moderate-sized and large flares alike. We also discuss the high time-resolution, high signal-to-noise continuum color variations observed in YZ CMi during a giant flare, which increased the NUV flux from this star by over a factor of 100.