PeV emission of the Crab Nebula: constraints on the proton content in pulsar wind and implications

TL;DR

This paper analyzes LHAASO PeV photon detections from the Crab Nebula to constrain the proton content in pulsar winds, exploring proton escape effects and their implications for cosmic-ray flux and nebula energetics.

Contribution

It provides new constraints on the energy of relativistic protons in pulsar winds using recent PeV observations, considering proton escape effects.

Findings

Up to 50% of pulsar spin-down energy could convert into relativistic protons.

Escaping protons may significantly contribute to 10-100 PeV cosmic rays.

Current data allows for a proton energy loss 10-100 times greater than remaining in the nebula.

Abstract

Recently, two photons with energy of about 1 PeV have been detected by LHAASO from the Crab nebula, opening an ultra-high energy window for studying the pulsar wind nebulae (PWNe). Remarkably, the LHAASO spectrum at the highest-energy end shows a possible hardening, which could indicate the presence of a new component. A two-component scenario with a main electron component and a secondary proton component has been proposed to explain the whole spectrum of the Crab Nebula, requiring a proton energy of $10^{46}-10^{47}{\rm ergs}$ remaining in the present Crab Nebula. In this paper, we study the energy content of relativistic protons in pulsar winds with the LHAASO data of the Crab Nebula, considering the effect of diffusive escape of relativistic protons. Depending on the extent of the escape of relativistic protons, the total energy of protons lost in the pulsar wind could be 10-100 times larger than that remaining in the nebula presently. We find that the current LHAASO data allows up to $(10-50)\%$ of the spindown energy of pulsars being converted into relativistic protons. The escaping protons from PWNe could make a considerable contribution to the cosmic-ray flux of 10-100 PeV. We also discuss the leptonic scenario for the possible spectral hardening at PeV energies.