On the origin of a possible hard VHE spectrum from M87 discovered by LHAASO

TL;DR

This study investigates the origin of a potential spectral hardening in M87's VHE emission observed by LHAASO, analyzing multiple radiation mechanisms to identify the most plausible explanation.

Contribution

The paper evaluates various models for the VHE spectral feature in M87, concluding that pion decay gamma rays from pp interactions best explain the observations within a one-zone framework.

Findings

Pion decay gamma rays from pp interactions can explain the spectral hardening.

Proton synchrotron emission cannot produce the observed hard spectrum.

Photomeson process requires an incompatible emission zone near the black hole.

Abstract

Recent LHAASO observations hint at potential spectral hardening around 20 TeV in M87's very high energy (VHE) emission, suggesting a possible new radiation component. In this work, we construct averaged multiwavelength SEDs by combining data from Chandra and Swift-UVOT/XRT covering the same period as the LHAASO detection to investigate the origin of this feature. We test several radiation mechanisms, including the pp interaction, proton synchrotron emission, photomeson process and two-zone leptonic model. We find that only the pion decay gamma rays in pp interactions can interpret this feature in the framework of the one-zone model. With analytical analysis, we prove that proton synchrotron emission cannot generate a hard spectrum above 0.17~TeV. For photomeson model, it requires an emission zone compressed near the Schwarzschild radius of the central supermassive black hole, incompatible with broadband optical-GeV spectral constraints. In addition, the two-zone leptonic model also emerges as a viable alternative.