Cygnus X-3: A variable petaelectronvolt gamma-ray source

TL;DR

This paper reports the discovery of variable petaelectronvolt gamma-ray emission from Cygnus X-3, detected by LHAASO, with spectral and temporal features indicating origin near the binary system's relativistic jet.

Contribution

First detection of variable PeV gamma-rays from Cygnus X-3, revealing insights into high-energy processes in X-ray binaries.

Findings

Gamma-ray signal detected with ~10 sigma significance.

Spectral energy distribution extends from 0.06 to 3.7 PeV, rising toward 1 PeV.

Variability observed on month-long timescales with 8.6 sigma significance.

Abstract

We report the discovery of variable $\gamma$-rays up to petaelectronvolt from Cygnus X-3, an iconic X-ray binary. The $\gamma$-ray signal was detected with a statistical significance of approximately 10 $\sigma$ by the Large High Altitude Air Shower Observatory (LHAASO). Its intrinsic spectral energy distribution (SED), extending from 0.06 to 3.7 PeV, shows a pronounced rise toward 1 PeV after accounting for absorption by the cosmic microwave background radiation. We find variability on month-long timescales at a significance of $8.6 \sigma$, coinciding with a high state of the GeV gamma-ray flux detected by the Fermi-LAT. This,together with a 3.2$\sigma$ evidence for orbital modulation, suggests that the PeV $\gamma$-rays originate within, or in close proximity to, the binary system itself. The observed energy spectrum and temporal modulation can be naturally explained by $\gamma$-ray production through photomeson processes in the innermost region of the relativistic jet, where protons need to be accelerated to tens of PeV energies.