Characterization of the NUV and optical emission and temperature of flares from Ross 733 with Swift and TESS

We present the results of a coordinated campaign to simultaneously observe the M star binary Ross 733 in the optical and near-ultraviolet (NUV) with TESS and Swift, respectively. We observed two flares in the Swift NUV light curve. One of these was decay phase of a flare that was also detected with TESS and the other was only detected in the NUV. We used the TESS light curve to measure the white-light flare rate of Ross 733, and calculate that the system flares with an energy of 10³³ erg once every 1.5 d. We used our simultaneous observations to measure a pseudo-continuum temperature of $7340^{+810}_{-900}$ K during the flare decay. We also used our observations to test the NUV predictions of the 9000 K blackbody flare model, and find that it underestimates number of flares we detect in our Swift NUV light curve. We discuss the reasons for this and attribute it to the unaccounted contributions from emission lines and continuum temperatures above 9000 K. We discuss how additional observations are required to break the degeneracy between the two in future multiwavelength flare campaigns.