[论文解读] HAWC Study on the Ultra-High-Energy Gamma-Ray Emissions from the Pulsar Wind Nebula G32.64+0.53

R. Alfaro, C. Alvarez+91 more|arXiv (Cornell University)|Mar 20, 2026

Astrophysics and Cosmic Phenomena被引用 0

一句话总结

tldr: HAWC analyzes 2860 days of data to confirm ultra-high-energy gamma-ray emission from PWN G32.64+0.53, modeling it with a time-dependent leptonic scenario and constraining it as a PeV electron accelerator.

ABSTRACT

Multi-TeV gamma-ray emission around eHWC J1850+001 (a source from the first HAWC catalog of gamma-ray sources emitting above 56 TeV) is spatially coincident with the pulsar wind nebula (PWN) G32.64+0.53, powered by PSR J1849-0001. The absence of counterparts in radio, optical, and GeV energy ranges, contrasted with clear detections in X-rays and very-high-energy (VHE) gamma-rays, is indicative of a non-thermal leptonic origin for the nebula. We apply a systematic analysis pipeline, including a sophisticated model for the Galactic diffuse emission, to 2860 days of data from the HAWC Observatory. Our detailed analysis confirms that the ultra-high-energy (UHE) emission originates from G32.64+0.53, and we measure its spectrum up to 270 TeV with significant emission well beyond 100 TeV. We fit the multi-wavelength observations with a time-dependent leptonic model powered by the pulsar's rotational energy, and the results establish the nebula as a leptonic PeV accelerator, capable of accelerating electrons to a maximum energy of $E_{\mathrm{cut}}=1.5_{-0.6}^{+1.7}~\mathrm{PeV}$. The model also constrains the true age of the system to $26.8~\mathrm{kyr}$ and the nebular magnetic field to a low value of $2.5 ~\mathrm{μG}$, supporting a leptonic PWN origin for the observed UHE emission.

研究动机与目标

• Motivate the study of Galactic UHE gamma-ray sources and PWNe as potential PeV accelerators.
• Characterize the gamma-ray emission around G32.64+0.53 with HAWC data and a detailed diffuse emission model.
• Quantify the spectral and morphological properties of the source and assess its leptonic emission scenario.
• Constrain physical parameters of the nebula (age, magnetic field, electron spectrum) through multi-wavelength modeling.

提出的方法

• Use 2860 days of HAWC Pass 5 data with energies above 1 TeV and neural-network–estimated energies.
• Apply a region-of-interest (ROI) likelihood analysis with a sophisticated Galactic diffuse emission (GDE) template generated by HERMES.
• Implement an iterative source-finding pipeline that adds point sources, tests extensions, and probes elliptical morphologies.
• Convolve source morphologies with the PSF and use Bayesian Information Criterion to select spectral/morphological models.
• Fit multi-source models including the GDE and four sources; analyze spectral forms such as PL, COPL, LogP, and spatial templates (Gaussian, Laplace, elliptical).
• Perform forward-folding likelihood analyses to derive flux normalizations and spectral parameters across energy bins up to 270 TeV.

Figure 1: HAWC significance map of the ROI. The map is generated by fitting a test point source with a fixed power-law index ( $\alpha=2.5$ ) at each pixel, optimizing only the flux normalization. The map is inter polated for presentation. The blue label indicates the position of G32.64+0.53 (C. Kim

实验结果

研究问题

• RQ1 Is the ultra-high-energy gamma-ray emission in the region around G32.64+0.53 attributable to a PWN powered by PSR J1849-0001?
• RQ2 What are the spectral shape and spatial morphology (extent, ellipticity) of the gamma-ray emission associated with G32.64+0.53 across TeV to PeV energies?
• RQ3 Can a time-dependent leptonic model reproduce the multi-wavelength emission and constrain the PWN's physical properties (age, magnetic field, electron spectrum)?
• RQ4 How does inclusion of Galactic diffuse emission modeling impact source identification and parameter estimation in this region?

主要发现

• The HAWC analysis confirms that the ultra-high-energy emission originates from G32.64+0.53 with significant emission up to 270 TeV.
• The source HAWC J1849-0000 has best-fit parameters: extension 0.09 deg and TS = 347, with a spectral index alpha = 2.07 and flux normalization Phi0 = 9.37e-15 (TeV cm^2 s)^{-1} (statistical and systematic uncertainties reported).
• The ROI model includes four sources (three extended, one point-like); one extended source shows elliptical morphology with eccentricity e = 0.93 and is best described by a COPL spectrum.
• Two other extended sources are best described by a LogP spectrum with a Laplace spatial distribution, while a fourth source is modeled as a point-like entity aligning with catalog counterparts.
• A time-dependent leptonic model powered by the pulsar’s spin-down energy yields a PeV electron accelerator with E_cut = 1.5_{-0.6}^{+1.7} PeV, and constrains the true age to 26.8 kyr and nebular magnetic field to B ≈ 2.5 μG.
• The SED and multi-wavelength modeling support a leptonic origin for the UHE emission and the PWN interpretation for G32.64+0.53.]
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Figure 4: The spectral energy distribution (SED) of HAWC J1849-0000. The data points represent the flux in each energy bin, with error bars showing $1\sigma$ statistical uncertainties. The dashed line is the best-fit spectral model (Log-Parabola), and the shaded region is the $1\sigma$ confidence in