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

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.

💡 Research Summary

The Fourth HAWC Catalog (4HWC) presents an updated inventory of very‑high‑energy (VHE) gamma‑ray sources derived from the fifth data pass (Pass 5) of the High‑Altitude Water Cherenkov (HAWC) Observatory. Pass 5 incorporates nearly three additional years of exposure (total livetime ≈ 2 726 days) and a suite of reconstruction improvements that boost effective area by up to a factor of five at low energies (≈ 0.1 TeV) and bring the high‑zenith‑angle performance close to the physical detector size. Background estimation combines the classic direct‑integration method for well‑populated low‑energy bins with a Monte‑Carlo‑based all‑sky random‑sampling technique for the sparsely populated highest‑energy bins, ensuring a robust cosmic‑ray background model across the full energy range.

Source detection is performed with the Automated Likelihood Pipeline for Sources (ALPS), an end‑to‑end, fully automated, multi‑source fitting framework. The sky is tiled into many Regions of Interest (ROIs): for the Galactic plane (|b| ≤ 10°) 6° × 20° sliding windows shifted by 3° are used, while higher‑latitude regions employ 10° × 10° tiles. Within each ROI a maximum‑likelihood fit is carried out, and nested model comparisons are quantified via a test statistic TS = −2 Δlog L, which follows a χ² distribution under Wilks’ theorem. A detection threshold of TS ≥ 16 (≈ 4σ) is adopted.

Spatial modeling uses either a point‑source template or a symmetric Gaussian (σ ≤ 2°). Spectral shapes are fit with a simple power‑law (Φ ∝ E^Γ) or a log‑parabola (Φ ∝ (E/E₀)^{−α−β log(E/E₀)}) to capture curvature. The pipeline iteratively adds sources, increasing the parameter space by 4–6 degrees of freedom per source, and refits the entire ROI to account for source overlap and diffuse Galactic emission.

The final catalog lists 85 sources above the 4σ significance level. Of these, 11 have no positional counterpart in the TeVCat database, and 22 lack a match in the 1LHAASO catalog. Cross‑matching with catalogs of low‑ and high‑mass X‑ray binaries, ATNF pulsars, Fermi‑LAT pulsars, and supernova remnants yields 12 sources without any known physical counterpart; five of these are completely uncatalogued across all surveyed wavebands, representing prime targets for follow‑up observations with CTA, LHAASO, and multi‑wavelength facilities. For each source the catalog provides coordinates, TS, preferred spatial template, spectral parameters (including curvature when significant), and association flags.

Compared with the previous 3HWC catalog (based on Pass 4), the 4HWC catalog expands the detectable energy range down to ~0.11 TeV and up to ~232 TeV (depending on source flux and declination), improves angular resolution by a factor of three at large zenith angles, and enhances gamma/hadron separation by roughly four times. The systematic multi‑source fitting and explicit modeling of the Galactic diffuse TeV emission reduce biases inherent in earlier analyses that relied on manual source addition.

In summary, the 4HWC catalog delivers the most sensitive, all‑sky TeV gamma‑ray survey to date from a ground‑based water‑Cherenkov detector, introduces a reproducible automated analysis pipeline, and uncovers a set of new VHE emitters that will guide future pointed observations and theoretical studies of particle acceleration in our Galaxy and beyond.