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Abstract The astronomical origins of the most energetic galactic cosmic rays and gamma rays are still uncertain. X-ray follow-up of candidate “PeVatrons”—systems producing cosmic rays with energies exceeding 1 PeV—can constrain their spatial origin, identify likely counterparts, and test particle emission models. Using ∼120 ks of XMM-Newton observations, we report the discovery of a candidate pulsar wind nebula, a possible counterpart for the LHAASO PeVatron J0343+5254u. This extended source has a power-law X-ray spectrum with spectral index Γ_X = 1.9—softer at greater distance from the center—and asymmetric spatial extension out to $\approx 2\prime$. We conduct leptonic modeling of the X-ray and gamma-ray radiation from this complex system, showing that a fully leptonic model with elevated IR photon fields can explain the multiwavelength emission from this source, similar to other very high-energy pulsar wind nebulas; excess gamma-ray emissivity not explained by a leptonic model may be due to hadronic interactions in nearby molecular cloud regions, which might also produce detectable astroparticle flux. 

Abstract We report a new CO observation survey of LHAASO J0341+5258, using the Nobeyama Radio Observatory 45-m telescope. LHAASO J0341+5258 is one of the unidentified ultra-high-energy (UHE;E> 100 TeV) gamma-ray sources detected by LHAASO. Our CO observations were conducted in 2024 February and March, with a total observation time of 36 hr, covering the LHAASO source (∼0 $\stackrel{\text{°}}{.}$3–0 $\stackrel{\text{°}}{.}$5 in radius) and its surrounding area (1° × 1 $\stackrel{\text{°}}{.}$5). Within the LHAASO source extent, we identified five compact (<2 pc) molecular clouds at nearby distances (<1–4 kpc). These clouds can serve as proton–proton collision targets, producing hadronic gamma rays via neutral pion decays. Based on the hydrogen densities (700–5000 cm⁻³) estimated from our CO observations and archived Hidata from the Dominion Radio Astrophysical Observatory survey, we derive the total proton energy ofW_p(E> 1 TeV) ∼ 10⁴⁵erg to account for the gamma-ray flux. One of the molecular clouds appears to be likely associated with an asymptotic giant branch (AGB) star with an extended CO tail, which may indicate some particle acceleration activities. However, the estimated maximum particle energy below 100 TeV makes the AGB-like star unlikely to be a PeVatron site. We conclude that the UHE emission observed in LHAASO J0341+5258 could be due to hadronic interactions between the newly discovered molecular clouds and TeV–PeV protons originating from a distant SNR or due to leptonic emission from a pulsar wind nebula candidate, which is reported in our companion X-ray observation paper. 

Abstract Young supernova remnants (SNRs) are believed to be the origin of energetic cosmic rays (CRs) below the “knee” of their spectrum at ∼3 PeV (10¹⁵eV). Nevertheless, the precise location, duration, and operation of CR acceleration in young SNRs are open questions. Here, we report on multiepoch X-ray observations of Cassiopeia A (Cas A), a 350 yr old SNR, in the 15–50 keV band that probes the most energetic CR electrons. The observed X-ray flux decrease (15% ± 1% over 10 yr), contrary to the expected >90% decrease based on previous radio, X-ray, and gamma-ray observations, provides unambiguous evidence for CR electron acceleration operating in Cas A. A temporal model for the radio and X-ray data accounting for electron cooling and continuous injection finds that the freshly injected electron spectrum is significantly harder (exponential cutoff power-law indexq= 2.15), and its cutoff energy is much higher (E_cut = 36 TeV), than the relic electron spectrum (q = 2.44 ± 0.03,E_cut = 4 ± 1 TeV). Both electron spectra are naturally explained by the recently developed modified nonlinear diffusive shock acceleration (mNLDSA) mechanism. The CR protons producing the observed gamma rays are likely accelerated at the same location by the same mechanism as the injected electrons. The Cas A observations and spectral modeling represent the first time radio, X-ray, gamma-ray, and CR spectra have been self-consistently tied to a specific acceleration mechanism—mNLDSA—in a young SNR. 

Geminga is the first pulsar around which a remarkable gamma-ray halo extending over a few degrees was discovered at TeV energies by MILAGRO and HAWC and later by H.E.S.S., and byFermi-LAT in the GeV band. More middle-aged pulsars have exhibited gamma-ray halos, and they are now recognised as an emerging class of Galactic gamma-ray sources. The emission appears in the late evolution stage of pulsars, and is most plausibly explained by inverse Compton scattering of CMB and interstellar photons by relativistic electrons and positrons escaping from the pulsar wind nebulae. These observations pose a number of theoretical challenges, particularly the origin of the inferred, significantly lower effective diffusion coefficients around the pulsar when compared to typical Galactic values. Tackling these questions requires constraining the ambient magnetic field properties, which can be achieved through X-ray observations. If the gamma-ray halos originate from a distribution of highly energetic electrons, synchrotron losses in the ambient magnetic fields of the same particles are expected to produce a diffuse X-ray emission with a similar spatial extension. We present the most comprehensive X-ray study of the Geminga pulsar halo to date, utilising archival data fromXMM-NewtonandNuSTAR. Our X-ray analysis covers a broad bandwidth (0.5 − 79 keV) and large field of view (θ ∼ 4°) for the first time. This was achieved by accurately measuring the background over the entire field of view, and taking into account both focused and stray-light X-ray photons from the pulsar halo withNuSTAR. We find no significant emission and set robust constraints on the X-ray halo flux. These are translated to stringent constraints on the ambient magnetic field strength and the diffusion coefficient by using a physical model considering particle injection, diffusion, and cooling over the pulsar’s lifetime, which is tuned by fitting multi-wavelength data. Our novel methodology for modelling and searching for synchrotron X-ray halos can be applied to other pulsar halo candidates.