Search for high energy gamma-ray emission from tidal disruption events with the Fermi Large Area Telescope

TL;DR

This study searches for high-energy gamma-ray emission from relativistic tidal disruption events using Fermi LAT data, finding no significant emission and setting upper limits that inform jet physics and absorption effects.

Contribution

The paper provides the first upper limits on GeV emission from multiple relativistic TDEs, constraining jet Lorentz factors and emission models.

Findings

No significant GeV emission detected from the TDEs.

Upper limits suggest low flux ratios between GeV and X-ray emissions.

Constraints on jet Lorentz factors, $\\Gamma \la 30$, based on absorption hypotheses.

Abstract

Massive black holes at galaxy center may tear apart a star when the star passes occasionally within the disruption radius, which is the so-called tidal disruption event(TDE). Most TDEs radiate with thermal emission resulted from the accretion disk, but three TDEs have been detected in bright non-thermal X-ray emission, which is interpreted as arising from the relativistic jets. Search for high-energy gamma-ray emission from one relativistic TDE (Swift J164449.3+573451) with the \textsl{Fermi} Large Area Telescope (LAT) has yielded non-detection. In this paper, we report the search for high energy emission from the other two relativistic TDEs (Swift J2058.4+0516 Swift J1112.2-8238) during the flare period. No significant GeV emission is found, with an upper limit fluence in LAT energy range being less than $1\%$ of that in X-rays. Compared with gamma-ray bursts (GRBs) and blazars, these TDEs have the lowest flux ratio between GeV emission and X-ray emission. The non-detection of high-energy emission from relativistic TDEs could be due to that the high-energy emission is absorbed by soft photons in the source. Based on this hypothesis, upper limits on the bulk Lorentz factors, $\Gamma\la 30$, are then obtained for the jets in these TDEs. We also search for high-energy gamma-ray emission from the nearest TDE discovered to date, ASASSN-14li. No significant GeV emission is found and an upper limit of $L(\rm 0.1-10 GeV)\le 4.4\times 10^{42}$ erg s$^{-1}$ (at $95\%$ confidence level) is obtained for the first $10^{7}$ s after the disruption.