The Fourth HAWC Catalog of Very-High-Energy Gamma-Ray Sources

TL;DR

The paper presents the 4HWC catalog, derived from the HAWC Pass 5 data, and introduces an automated and robust multi-source likelihood framework (ALPS) to model the TeV sky with flexible source morphologies (point-like and symmetric Gaussian) and spectra (power-law and log-parabola), while incorporating a detailed model of Galactic diffuse TeV emission. It reports 85 sources detected at $4\sigma$, with extensive treatment of systematic uncertainties, source recovery, and false-positive rates, and demonstrates strong cross-catalog consistency with TeVCat, 1LHAASO, and other surveys. A key outcome is the prevalence of extended sources (68 of 85) and numerous associations with pulsars and TeV halos, alongside a set of unassociated sources offering follow-up Opportunities. The work establishes a rigorous foundation for wide-field TeV sky surveys and provides a rich resource for future facilities (e.g., CTAO, SWGO) to further resolve source morphologies and spectral features across the Milky Way and beyond.

Abstract

We present an updated catalog of TeV gamma-ray sources based on the fifth pass of data from the High-Altitude Water Cherenkov (HAWC) Observatory. This release benefits from improved event reconstruction and nearly three additional years of observations. It also incorporates a systematic multi-source fitting framework, enabling more flexible and accurate modeling of the gamma-ray sky. This fitting procedure was modeled after the manual approach used in HAWC analyses of individual sources and regions, as well as other gamma-ray catalogs, like the 4FGL. In addition to more varied modeling of source morphology and spectral parameters compared to previous HAWC catalogs, this catalog uses a robust modeling of Galactic diffuse TeV emission. The fitting procedure uses both point-like and symmetric Gaussian spatial templates to model the source morphology. The spectral shape of the emission is modeled with either a simple power-law or log-parabola to explore curvature in the spectral energy distribution. We report 85 sources at the 4σ level, including 11 sources not associated with any TeVCat source using a distance-based association criterion. Distance-based association with the 1LHAASO catalog results in 22 4HWC sources without a counterpart. Additionally, there are 12 sources not associated with any physical counterpart in the Low- or High-Mass X-Ray Binary, the ATNF, or Fermi Pulsar, or SNR catalogs of sources. Five of the aforementioned sources have no counterpart in any of the catalogs searched and represent an opportunity for follow-up observations.

Figures (21)

• Figure 1: The full all-sky significance map from the HAWC map used to create the 4HWC catalog. This map is in celestial coordinates, and the significance shown is calculated using a point-source assumption with a fixed $-2.6$ power-law index.
• Figure 2: Flowchart of the ALPS method. Beginning with the dark green "HAWC Data" oval node, the green sections correspond to the Point-Source-Adding phase. The cyan sections show the flow of the Alternate-Hypothesis-Testing phase. The orange sections show the final refitting section that precedes the final output model. Each diamond-shaped purple node shows a branching point in the algorithm.
• Figure 3: Sensitivity Curves for the various spectral assumptions (power-law indices $-2.0$, $-2.5$, and $-3.0$) with the flux of the 4HWC sources plotted for reference.
• Figure 4: HAWC significance map of region $l$ = $-20$° to $l$ = 20°, $\lvert b \rvert <$ 10° with the 4HWC sources overlaid as dashed circles with radius equal to the extension parameter. A small dot represents the center of the source model emission. Map generated using a point-source assumption with a $-2.6$ power-law index. The white or transparent portion of the map represents empty portions of the sky beyond the zenith cut of the data set.
• Figure 5: HAWC significance map of region $l$ = 20° to $l$ = 60°, $\lvert b \rvert <$ 10° with the 4HWC sources overlaid as dashed circles with radii equal to the extension parameter. A small dot represents the center of the source model emission. Map generated using a point-source assumption with a $-2.6$ power-law index.