A Stacking Search for Gamma-ray Emission from Nearby Flare Stars and the Periodic Source TVLM 513-46546

Abstract: So far, the Sun is the only isolated main sequence star detected in gamma-rays, particularly during powerful flares. Young ultracool dwarfs are far more active so they are also plausible gamma-ray sources. We performed a spatial stack of 97 of the nearest X-ray and radio flare stars to search for GeV emission using nearly 12 years of data from the Fermi Gamma-ray Space Telescope. The stacked residual maps showed no significant signal. Modeling the upper limits indicates a peak stellar flux at least a factor of 7 below the noise level. We also analyzed the phase-folded light curve of the rapidly rotating radio star TVLM 513-46546, report a tentative (TS = 30) pulsed signal, and refine its period. We examine the possibility of a false positive signal by analyzing nearby Fermi catalog sources and test fields, and by repeating the analysis using different periods. No other periodic signals are found, despite clear detections of the catalog sources, and the TS value for TVLM 513 increases systematically to the optimal period. The putative gamma-ray signal is nearly in phase with the optical peak, and out of phase with the radio pulses by 0.4 +/- 0.05 rotations. These results argue for emission from relativistic protons streaming down flux tubes towards the photospheric active regions. The protons colliding with the atmosphere create neutral pions that decay into gamma-ray photons. This would be the first detection of a normal, isolated star in gamma-rays, and the strongest evidence yet for proton acceleration in stellar magnetospheres.