Letter to the Editor

A possible GeV-TeV connection in the direction of the globular cluster UKS 1

¹ Department of Earth, Environmental & Space Sciences, Chungnam National University, Daejeon 34134, Republic of Korea
² Department of Astronomy and Space Science, Chungnam National University, Daejeon 34134, Republic of Korea
³ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
⁴ Astrophysical Big Bang Laboratory (ABBL), RIKEN Cluster for Pioneering Research, Wakō, Saitama 351-0198, Japan
⁵ Theoretical Astrophysics, Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

^⋆ Corresponding author; cyhui@cnu.ac.kr, huichungyue@gmail.com

Received: 24 February 2025
Accepted: 24 March 2025

Abstract

Aims. Using public data collected by the Fermi Large Area Telescope (LAT) over 16 years and the 1523 days of survey data (3HWC) from the High Altitude Water Cherenkov (HAWC) observatory, we searched for possible GeV-TeV connections in globular clusters (GCs).

Methods. Excluding 44 confirmed γ-ray detections of GCs in the latest Fourth Fermi-LAT point source catalog (4FGL-DR4), we searched for possible GeV emission from the other 113 known GCs based on 16 years of LAT data. We performed a systematic binned likelihood analysis in the energy range of 0.3–100 GeV toward these targets. We also searched for possible TeV excesses in the directions of 27 GeV-detected GCs covered by the 3HWC survey area, assuming a point-source morphology and a power-law spectrum of E^−2.5.

Results. In addition to the confirmed γ-ray GCs in the 4FGL catalog, we report a GeV detection at the position of UKS 1 with a post-trial probability of ∼8 × 10⁻⁵ of it proving to be a fluctuation. Its spectrum within this energy range is well described by a power-law model with Γ ≃ 2.3 ± 0.5. Furthermore, this GeV feature appears to extend southeast in a direction toward the Galactic plane. From the 3HWC survey data, we have also identified a TeV feature in the direction of UKS 1. It is well resolved from any known very-high-energy (VHE) source. The post-trial probability that this feature is a fluctuation is ∼3 × 10⁻⁴. If confirmed, this would mark the second detection of a TeV feature in the proximity of a GC. While the GeV emission mostly coincides with the center of UKS 1, the TeV peak is displaced from the cluster center by several tidal radii in the trailing direction of the GC’s proper motion. Given the supersonic speed of UKS 1 at ∼270 km s⁻¹, our findings are consistent with a scenario where the VHE γ-rays are produced by inverse Compton scattering between relativistic particles and ambient soft photon fields during the course of their propagation away from the head of the bow shock.