First detection of ultra-high energy emission from gamma-ray binary LS I +61 303

TL;DR

This work reports the first definite detection of gamma-ray emission from the gamma-ray binary LS I +61°303 at ultra-high energies ($>100$ TeV) using LHAASO's WCDA and KM2A detectors. The source is detected with high significance (9.2σ in WCDA for 1.4–30.5 TeV and 6.2σ in KM2A for 25–267 TeV), with 16 photon-like events above 100 TeV and a background of 5.1, including a highest-energy photon at $267\pm75$ TeV; the spectrum is well described by a power law with $N_0=(2.18\pm0.20)\times10^{-15}$ TeV$^{-1}$ cm$^{-2}$ s$^{-1}$ and $\Gamma=3.00\pm0.05$, and $F_{>1\,\mathrm{TeV}}=1.10\times10^{-12}$ cm$^{-2}$ s$^{-1}$ (≈3.8% Crab). The analysis reveals orbital modulation in the 25–100 TeV band at $\sim4.0\sigma$ and hints of energy-dependent modulation, suggesting a composite leptonic and hadronic origin with possible inner/outer accelerators. These findings extend the gamma-ray energy frontier for binaries, constraining acceleration and loss processes and motivating future multi-wavelength campaigns to unravel the emission mechanisms in such compact systems.

Abstract

We report the first detection of gamma-ray emission up to ultra-high-energy (UHE; $>$100 TeV) emission from the prototypical gamma-ray binary system LS I +61 303 using data from the Large High Altitude Air Shower Observatory (LHAASO). It is detected with significances of 9.2$σ$ in WCDA (1.4--30.5 TeV) and 6.2$σ$ in KM2A (25--267 TeV); in KM2A alone we identify 16 photon-like events above 100 TeV against an estimated 5.1 background events, corresponding to a 3.8$σ$ detection. These results provide compelling evidence of extreme particle acceleration in LS I +61 303. Furthermore, we observe orbital modulation at 4.0$σ$ confidence between 25 and 100 TeV, and a hint of energy-dependent orbital modulation. These features can be understood in a composite scenario in which leptonic and hadronic processes jointly contribute.

Figures (5)

• Figure 1: Test-statistic maps after subtraction of the contributions from nearby sources and Galactic diffuse emission around LS I +61$^{\circ}$ 303, observed with WCDA (A) and KM2A (B), for the energy ranges 1.5--30.5 TeV and $>$25 TeV, respectively. Red crosses indicate the position of LS I +61$^{\circ}$ 303 at optical band 2023AA...674A...1G, while the green 'x' represent the position of LS I +61$^{\circ}$ 303 as detected by LHAASO. The black dashed line indicates the positional uncertainty of LS I +61$^{\circ}$ 303 at the 95% level, which is consistent with the position measurement at X-ray band. PSF at each energy band is marked by double-headed arrow.
• Figure 2: Spectral energy distribution of LS I +61$^{\circ}$ 303 observed by LHAASO. The meausrements from WCDA and KM2A are marked with black dots and red squares, respectively. The solid line represents the best-fit result assuming a power-law distribution, while the butterfly plot shows the 1$\sigma$ statistical uncertainties. For comparison, measurements from MAGIC (marked with gray points; 2006Sci...312.1771A) and VERITAS (marked with purple circles for orbital phases from 0.8 to 0.5, and blue points for orbital phases from 0.5 to 0.8; 2017ICRC...35..712K) are shown.
• Figure 3: ( A,B) Flux measured by WCDA (1.4--30.5 TeV) and KM2A (25--100 TeV) versus orbital phase. ( C) Energy of photons $>100$ TeV measured by KM2A versus orbital phase. Two orbital cycles are shown for clarity.
• Figure 4: Observed maps within ROI from WCDA (A) and KM2A (B).
• Figure 5: 2D residual map from WCDA (A) and KM2A (B).