On the expected γ-ray emission from nearby flaring stars

Abstract

Stellar flares have been extensively studied in soft X-rays (SXR) by basically every X-ray mission. Hard X-ray (HXR) emission from stellar superflares, however, have only been detected from a handful of objects over the past years. One very extreme event was the superflare from the young M-dwarf DG CVn binary star system, which triggered Swift/BAT as if it was a γ-ray burst (GRB). In this work, we estimate the expected γ-ray emission from DG CVn and the most extreme stellar flares by extrapolating from solar flares based on measured solar energetic particles (SEPs), as well as thermal and non-thermal emission properties. We find that ions are plausibly accelerated in stellar superflares to 100 GeV energies, and possibly up to TeV energies in the associated coronal mass ejections. The corresponding π 0 -decay γ-ray emission could be detectable from stellar superflares with ground-based γ-ray telescopes. On the other hand, the detection of γ-ray emission implies particle densities high enough that ions suffer significant losses due to inelastic proton-proton scattering. The next-generation Cherenkov Telescope Array (CTA) should be able to probe superflares from M-dwarfs in the solar neighbourhood and constrain the energy in interacting cosmic rays and/or their maximum energy. The detection of γ-ray emission from stellar flares would open a new window for the study of stellar physics, the underlying physical processes in flares and their impact on habitability of planetary systems.

Galactic Cosmic Rays: The first detection of TeV gamma-rays from Red Dwarfs

“…The integral fluxes averaged over each year's observation period are shown. M-Dwarf stars are characterized by flaring events which occur frequently with flare duration varying in time but for a total duration of ∼ 10 4 sec [3,4]. Very high energy γ-ray fluxes presented here demonstrate that these objects produce the flares of ∼ 10 33 − 10 35 ergs.…”

“…In the studies of flaring stars it was shown, that the frequency of flaring is uncertain [3,4], so to search for TeV γ-ray emission from these objects we chose objects that we have collected data from during different long-term observations. In accordance with the long-term program studying galactic γ-ray sources, more than ten-year-long observations of Tycho's SNR, Crab Nebula and Cyg X-3, are being carried out in the SHALON experiment [8,9].…”

A study of super-luminous stars with the Fermi Large Area Telescope

“…Even if our models take only the IC halo component into account, this component should be dominant in super-luminous stars. However, the possibility of enhanced 𝛾-ray flux due to massive stellar flares is also appealing (Ohm & Hoischen 2018). For the Sun, some massive flares emit more 𝛾-rays within one hour than the whole solar disk in one year (Abdo et al 2011;Ackermann et al 2014).…”

Red Dwarfs as Sources of Cosmic Rays

“…These stars produce powerful stellar flares, which sometimes occur several times per day, but its flare frequency is uncertain. The total energy release estimates as 10 34 − 10 36 ergs [1,2]. The total energy of cosmic rays produced by the dwarf star flares in the Galaxy is estimated as ≈ 10 51 − 10 53 ergs [3].…”