First observation of ultra-high-energy γ rays from LMC X-4

Abstract

Ultra-high-energy γ rays (>1015 eV) have recently been observed from Cygnus X-3 (refs 1, 2) and from Vela X-1 (ref. 3). The discovery that some neutron star binary X-ray sources are emitting ultra-high-energy (UHE) γ rays has led to considerable interest and speculation as to the nature of the cosmic particle accelerators responsible for the γ rays4–7 and to the notion that much of the galactic cosmic radiation above 1016 eV may originate from such objects7–9. Data from the University of Adelaide air-shower array at Buckland Park taken over a 3-year period have been analysed to search for evidence of UHE γ-ray emission from additional neutron-star binary X-ray sources having known orbital periods. We present here the result of this search and report the detection of UHE γ rays from LMC X-4 above 1016 eV with an integral flux of (4.6±1.7)×10−11 photons m−2 s−1. This is the first extragalactic object to be positively detected at these energies.