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

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.

Figures (5)

• 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 XrayPWN.
• Figure 2: (a) Residual significance map after modeling the three extended sources, but before adding the final point source. The upper green labels indicate the positions of the sources measured by LHAASO KM2A, published at the first LHAASO catalog 1LHAASO. The other labels are taken from TeVCat TeVCat. A hotspot with a significance of $\sim 4.5\sigma$ is visible at $(l,b) \approx (33.96^{\circ}, 0.36^{\circ})$, which is close to 1LHAASO J1852+0050u. (b) The one-dimensional version of (a), showing a positive significance tail around $4\sigma$.
• Figure 3: Sky maps of the ROI in Galactic coordinates. (a) The initial HAWC significance map. (b) The significance map produced from our final, four-source model. The purple labels indicate the sources found and modeled in this work. (c) The final residual significance map. (d) The one-dimensional version of (c), showing no significant emission remains after subtracting our model from the data. The two pixels around $4\sigma$ are from the edge of the ROI at $l\sim30\degr$.
• 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 interval.
• Figure 5: (a) The posterior distributions obtained by MCMC. The diagonal panels show the marginalized posterior distributions for $\log_{10}P_{0}$, $\log_{10}\eta$, $\alpha$, $\log_{10}E_{\mathrm{cut}}$, and $\log_{10} B(\tau)$. The green lines indicate the median values of the distributions. (b) The best-fit SED from the one-zone, time-dependent leptonic model, overlaid with multi-wavelength data from Chandra, NuSTAR, Fermi-LAT, H.E.S.S., LHAASO, Tibet-AS$\gamma$, and this work (HAWC) XrayPWNFermiPWNHGPS1LHAASOTibetPWN.