A Giant Peanut-shaped Ultra-High-Energy Gamma-Ray Emitter Off the Galactic Plane

TL;DR

We report the discovery of a giant off-plane ultra-high-energy γ-ray emitter, nicknamed the Peanut, detected by LHAASO-KM2A with a distinctive strip-plus-two-hotspot morphology extending several degrees and photons up to ~760 TeV. A joint maximum-likelihood, morphology, and spectrum analysis shows the emission is best described by a strip-like diffuse component plus two point sources, with the overall spectrum favoring a PLEC shape and competing hadronic and leptonic interpretations; quantitative fits imply proton or electron energy contents of order 10^{48} erg or 10^{45} erg, respectively, under plausible distance and density assumptions. The most plausible accelerator candidate within Peanut is the energetic millisecond pulsar J0218+4232, suggesting highly anisotropic diffusion in the Galactic halo and challenging standard MSP PeVatron expectations. These results highlight a potential new class of PeV accelerators and provide important constraints on cosmic-ray transport and interstellar magnetic fields in the halo, while underscoring the need for multi-wavelength follow-up to definitively determine the emission mechanism.

Abstract

Ultra-high-energy (UHE), exceeding 100 TeV (10^12 electronvolts), γ-rays manifests extreme particle acceleration in astrophysical sources. Recent observations by γ-ray telescopes, particularly by the Large High Altitude Air Shower Observatory (LHAASO), have revealed a few tens of UHE sources, indicating numerous Galactic sources capable of accelerating particles to PeV (10^15 electronvolts) energies. However, discerning the dominant acceleration mechanisms (leptonic versus hadronic), the relative contributions of specific source classes, and the role of particle transport in shaping their observed emission are central goals of modern UHE astrophysics. Here we report the discovery of a giant UHE γ-ray emitter at -17.5° off the Galactic plane - a region where UHE γ-ray sources are rarely found. The emitter exhibits a distinctive asymmetric shape, resembling a giant "Peanut" spanning 0.45° \times 4.6°, indicative of anisotropic particle distribution over a large area. A highly aged millisecond pulsar (MSP) J0218+4232 is the sole candidate accelerator positionally coincident with the Peanut region. Its association with UHE γ-rays extending to 0.7 PeV, if confirmed, would provide the first evidence of a millisecond pulsar powering PeV particles. Such a finding challenges prevailing models, which posit that millisecond pulsars cannot sustain acceleration to PeV energies. The detection reveals fundamental gaps in understanding particle acceleration, cosmic-ray transport, and interstellar magnetic field effects, potentially revealing new PeV accelerator (PeVatron) classes.

Figures (10)

• Figure 1: Significance maps $\sqrt{ \rm TS}$ ($\sigma$), derived by the Wilks’s theoremWilks:1938dza, of the Peanut in two energy intervals 25 TeV-100 TeV (top-left), above 100 TeV (top-right). The 68% containment radius (represented as the white circles) is estimated to be $0.35^\circ$ and $0.24^\circ$ for these two energy bins, respectively. Contours represent the significance levels of 3$\sigma$, 5$\sigma$ and 7$\sigma$. The green squares mark the TeV sources. The cyan stars are the pulsars. The red squares represent GeV sources. The rectangle shows the template for the strip-like diffuse emission component. The $\gamma$-ray event profiles in the R.A. direction are shown in the two bottom panels. The error bar represents a statistic at the 68% confidence level.
• Figure 2: Fourteen photons at energy above 400 TeV were detected by LHAASO from the Peanut region. The number of background events, dominated by residual cosmic rays, in this region is estimated to be 3.2. The dashed contours indicate the statistical significance of photons above 100 TeV. Additionally, the cyan star indicates the position of PSR J0218+4232.
• Figure 3: Spectral energy distribution of the entire Peanut above 10 TeV observed by LHAASO. Flux points are marked with black dots. The error bar represents a statistic at the 68% confidence level. Two flux upper limits are at 95% confidence level. The spectrum fitted with the PLEC function $dN/dE_{\gamma}=(8.10\pm1.0)\times 10^{-17}~(E_{\gamma}/50 {\rm\ TeV})^{-(1.95\pm0.19)}e^{-E_{\gamma}/(250\pm82{\rm\ TeV})}\rm\ cm^{-2}\ s^{-1}\ TeV^{-1}$ is shown with the gray belt. The proton spectrum is given by a PLEC(cyan dashed line), $(E_{p}/{\rm 1\ TeV})^{-1.45\pm0.55}\exp{[-(E_{p}/0.86\pm0.48{\rm\ PeV}]}$. The total energy in protons above 100 TeV is $W_p = (3.6\pm0.3) \times 10^{48}\rm\ erg$ assuming a distance of 3.15 kpc and target density of $1$ cm${}^{-3}$. The electron spectrum(red solid line) is given by a super-exponential cutoff (PLSuperEC), $(E_{e}/{\rm 1\ TeV})^{-2.48\pm0.27}\exp{[-(E_{e}/0.77\pm0.24{\rm\ PeV})^2]}$. The total energy in electrons above 50 TeV is $W_e = (1.2\pm0.3)\times 10^{45}\rm\ erg$ assuming the same distance.
• Figure S1: The distribution of the ratio $R$ for events within the Peanut source region and background region. The events with energy above 100 TeV are used. The dotted line indicates the cut value -2.36 that used to select $\gamma$-ray-like events.
• Figure S2: The source survey is derived from the TeV catalog, SNR catalog, 4FGL, and pulsars in the ATNF catalog around the Peanut region, respectively. No SNR mark is shown in the figure means no this kind of source in this region. Contours in the Peanut region are displayed at 4$\sigma$, 6$\sigma$, 8$\sigma$, and 10$\sigma$ levels.