Study of maximum electron energy of sub-PeV pulsar wind nebulae by multiwavelength modelling

TL;DR

This study models multiwavelength emissions from pulsar wind nebulae associated with ultrahigh-energy gamma-ray sources, revealing electrons can reach energies close to 1 PeV, explaining observed gamma-ray emissions.

Contribution

It introduces a leptonic emission model considering radiative and adiabatic cooling, including reverberation effects, to interpret UHE gamma-ray sources as PWNe with electrons up to 1 PeV.

Findings

Electrons can reach energies above 0.1 PeV, close to 1 PeV.

UHE gamma-ray emissions are consistent with electrons of 1 PeV.

Multiwavelength data can be explained before reverberation phase onset.

Abstract

Recently, the Large High Altitude Air Shower Observatory (LHAASO) reported the discovery of 12 ultrahigh-energy (UHE; $\mathrm{\varepsilon} \ge 100$ TeV) gamma-ray sources located in the Galactic plane. A few of these UHE gamma-ray emitting regions are in spatial coincidence with pulsar wind nebulae (PWNe). We consider a sample of five sources; two of them are LHAASO sources (LHAASO J1908+0621 and LHAASO J2226+6057) and the remaining three are GeV-TeV gamma-ray emitters. In addition, X-rays, radio observations, or upper limits are also available for these objects. We study multiwavelength radiation from these sources by considering a PWN origin, where the emission is powered by the spin-down luminosity of the associated pulsars. In this leptonic emission model, the electron population is calculated at different times under the radiative (synchrotron and inverse-Compton) and adiabatic cooling. We also include the onset of the reverberation phase for the PWN, by assuming radially symmetric expansion. However, in this work, we find that multiwavelength emission can be interpreted before the onset of this phase. The maximum energy of the electrons based on the spectral fit is found to be above 0.1 PeV and close to 1 PeV. For LHAASO J2226+6057, using its observations in radio to UHE gamma-rays, we find that UHE gamma-rays can be interpreted using electrons with maximum energy of 1 PeV. We estimate the upper limits on the minimum Lorentz factor of the electrons and it also infers the minimum value of the pair-multiplicity of charged pairs.