Delayed teraelectronvolt emission from GRB 980425/SN 1998bw and the origin of ultra-high-energy cosmic rays

Abstract

The origin of ultra-high-energy cosmic rays (UHECRs, E >10¹⁸ eV) is one of the great mysteries of modern astrophysics. It has been suggested that UHECRs could be accelerated in gamma-ray bursts (GRBs) and engine-driven supernovae (SNe). Here we report the discovery of a 1.4 teraelectronvolt (TeV) photon offset 0.97∘ from the site of the nearby (36.9 megaparsecs) GRB 980425/SN 1998bw explosion. The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected the TeV emission on 17 November 2018, more than 20 years after the original GRB 980425/SN 1998bw trigger. TeV detections at high Galactic latitudes by the LAT are extremely rare, with an average of 6 events per year. We propose that the delayed TeV emission is consistent with ultra-high-energy cosmic rays and/or electron-positron pairs from GRB 980425/SN 1998bw being deflected by the intergalactic magnetic field (IGMF) and subsequently cascading into secondary gamma rays. Based on the arrival time delay of the TeV emission, we estimate an IGMF strength of order B ≃ 10⁻¹² -10⁻¹³ Gauss. This result supports the possibility of UHECR acceleration in GRB 980425/SN 1998bw and suggests that most detected UHECRs are produced in local GRB/SNe within 200 Mpc. In addition, secondary photons from UHECRs out to 0.9-31 Gpc may also offer an explanation for extragalactic background photons with energies ≥1 TeV detected by the Fermi LAT.