Clues from the photonuclear time scale on the nature of particle accelerators in Cygnus X-3 and Vela X-1

Abstract

Ultrahigh-energy γ rays (≳ 10¹⁵eV) have recently been observed from the binary X-ray sources Cygnus X-3 and Vela X-1. The source of these γ rays remains unknown but it seems likely that protons or nuclei are accelerated, the γ rays resulting from nuclear interactions with matter or radiation. Objects of this type could then contribute significantly to the observed cosmic radiation. The maximum proton energy that can be achieved (this may determine the maximum γ-ray energy) occurs when the acceleration time scale becomes greater than the energy loss time scale, that is, the attenuation time. Here we have calculated the attenuation times of protons and heavier nuclei in the radiation fields of the stellar companions to the Cyg X-3 and Vela X-1 compact objects. The maximum observed γ-ray energy enables us to put an upper limit on the acceleration time scale of protons at 3×10¹⁶ eV of 10⁴−3 × 10⁵ s for Cyg X-3 and 10⁵ s for Vela X-1. This result can be used to put some constraints on the nature of the particle accelerator. If binary X-ray sources are contributing significantly to the cosmic rays, the cosmic-ray abundances will be affected by photonuclear interactions.