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

TL;DR

This study reports the first detection of variable gamma rays up to $3.7$ PeV from Cygnus X-3 with the LHAASO array, revealing an intrinsic spectrum that hardens toward $\, ext{PeV}$ energies after correcting for absorption by the CMB and ISRF. The emission shows month-scale variability and hints of orbital modulation, implying production near the binary. The authors model the data with photomeson production in the inner jet via $p\gamma$ interactions with UV and X-ray photon fields, requiring protons to be accelerated to multi-PeV energies, while leptonic scenarios are disfavored by cooling and Hillas constraints. Collectively, the results establish Cygnus X-3 as a super‑PeVatron microquasar, with significant implications for jet physics and the capability of compact binaries to accelerate hadrons to extreme energies.

Abstract

We report the discovery of variable $γ$-rays up to petaelectronvolt from Cygnus X-3, an iconic X-ray binary. The $γ$-ray signal was detected with a statistical significance of approximately 10 $σ$ 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. The detected month-scale variability,together with a 3.2$σ$ evidence for orbital modulation, suggests that the PeV $γ$-rays originate within, or in close proximity to, the binary system itself. The observed energy spectrum and temporal modulation can be naturally explained by $γ$-ray production through photomeson processes in the innermost region of the relativistic jet, where protons need to be accelerated to tens of PeV energies.

Figures (4)

• Figure 1: Fluxes and Test Statistic (TS) values of $\geq 0.1 \, \rm PeV$$\gamma$-rays from Cygnus X-3 as a function of time. a, Flux above 0.1 PeV. The arrival times of individual high-energy photons in the 0.4–1 PeV and $\geq 1 \, \rm PeV$ ranges are indicated by blue and red arrows, respectively. Vertical dashed lines mark the commencement of operations for the three-quarter and full KM2A array configurations. b, TS values corresponding to the detected $\geq$0.1 PeV $\gamma$-ray signals. For data with TS$\leq$ 4 (those below the horizontal dashed line), the 95$\%$ confidence-level flux upper limits are shown in the top panel. c, The 0.1–100 GeV light curve observed by Fermi-LAT. The horizontal dotted line indicates the average $\gamma$-ray flux. Time intervals where the sliding-window flux (grey points) exceeds this average define the active states, shaded in light red. The remaining periods are considered quiescent states. The first two flux points above the mean, part of a previous high-flux episode not fully covered by LHAASO, are excluded from the high-flux state classification.
• Figure 2: a, Significance map of $\geq 0.1$ PeV photons toward Cygnus X-3, based on KM2A observations during active states. The cyan cross marks the position of Cygnus X-3. Five $\geq 1$ PeV photons are shown as black dots. Contributions from background sources (indicated by open circles) have been subtracted. The white dashed circle indicates the 68% point-spread function at 0.1 PeV. b, Orbital light curves of Cygnus X-3 measured by LHAASO ($\geq$ 0.1 PeV), Fermi-LAT (0.1--100 GeV) and MAXI (2--20 keV). For clarity, the fluxes are normalized using the following factors: $4.94 \times 10^{-15}$ for KM2A, $8.47 \times 10^{-7}$ for Fermi-LAT, and 1.36 for MAXI, all in units of ${\rm cm}^{-2}~{\rm s}^{-1}$. The red solid line represents a sinusoidal fit to the KM2A data, capturing the orbital modulation. The orbital phases of individual high-energy photons are marked at the top: $\geq 1$ PeV (red circles) and 0.4–1 PeV (blue circles).
• Figure 3: Gamma-ray spectral energy distributions (SEDs). Open black circles represent fluxes measured in the high state, which can be described by a power law (dotted line). Red squares indicate the same fluxes but corrected for absorption by the interstellar radiation field (ISRF) and the 2.7 K cosmic microwave background (CMB). Open black triangles represents the flux upper limits in the quiescent state. For comparison, the best-fit SED of the Crab Nebula, derived from KM2A data, is shown as a grey dashed band.
• Figure 4: Broadband modelling of the absorption-corrected SED of Cygnus X-3, based on $p\gamma_{X}~+~p\gamma_{UV}$ interactions (see Methods C for details). The contributions from individual components are shown with dotted ($p\gamma_{X}$) and dashed ($p\gamma_{UV}$) lines.