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Second Discovery of GeV-TeV Connection from the Globular Cluster UKS 1

Jiwon Shin, C. Y. Hui, Sangin Kim, Kwangmin Oh, Ellis R. Owen

TL;DR

The paper addresses whether globular clusters can produce GeV gamma rays from MSP magnetospheres and extended TeV gamma rays via ICS by bow-shock accelerated leptons, focusing on UKS 1. By combining 16 years of Fermi-LAT data with 1523 days of 3HWC observations, it detects a GeV excess and a spatially offset TeV excess, consistent with a GeV-TeV connection and a bow-shock–driven VHE nebula. The interpretation requires a sizeable MSP population (~100) and a bow-shock geometry with $R_{bs} \sim 0.6$ pc and a cooling length of ~$100$ pc, explaining the observed offset at 15.6 kpc. This work positions globular clusters as potential TeV emitters and provides a natural laboratory for studying cosmic-ray transport and anisotropic gamma-ray production, with future CTAO observations expected to tighten constraints on GC wind physics and MSP-driven cosmic-ray winds.

Abstract

Using 16 years of data collected by Fermi Large Area Telescope and 1523 days of survey data from High Altitude Water Cherenkov (HAWC) Observatory, we discovered the long-sought second GeV-TeV connection towards the globular cluster (GC) UKS 1 (Shin et al. 2025). Gamma-ray spectroscopy suggests that the GeV emission can be attributed to both the pulsar magnetosphere and inverse Compton scattering (ICS) by the pulsar wind. In particular, the TeV peak is displaced from the cluster center by several tidal radii in the trailing direction of the proper motion of UKS 1. This alignment supports a scenario in which relativistic leptons, likely driven by a millisecond pulsar population, produce very-high-energy (VHE) gamma-rays via ICS within a bow shock tail. Our findings not only highlights GCs as potential sources of VHE gamma-rays, but also offers a rare opportunity to probe cosmic ray transport in the Milky Way by studying particle propagation and anisotropic gamma-ray production associated with the extended, offset TeV feature of UKS 1.

Second Discovery of GeV-TeV Connection from the Globular Cluster UKS 1

TL;DR

The paper addresses whether globular clusters can produce GeV gamma rays from MSP magnetospheres and extended TeV gamma rays via ICS by bow-shock accelerated leptons, focusing on UKS 1. By combining 16 years of Fermi-LAT data with 1523 days of 3HWC observations, it detects a GeV excess and a spatially offset TeV excess, consistent with a GeV-TeV connection and a bow-shock–driven VHE nebula. The interpretation requires a sizeable MSP population (~100) and a bow-shock geometry with pc and a cooling length of ~ pc, explaining the observed offset at 15.6 kpc. This work positions globular clusters as potential TeV emitters and provides a natural laboratory for studying cosmic-ray transport and anisotropic gamma-ray production, with future CTAO observations expected to tighten constraints on GC wind physics and MSP-driven cosmic-ray winds.

Abstract

Using 16 years of data collected by Fermi Large Area Telescope and 1523 days of survey data from High Altitude Water Cherenkov (HAWC) Observatory, we discovered the long-sought second GeV-TeV connection towards the globular cluster (GC) UKS 1 (Shin et al. 2025). Gamma-ray spectroscopy suggests that the GeV emission can be attributed to both the pulsar magnetosphere and inverse Compton scattering (ICS) by the pulsar wind. In particular, the TeV peak is displaced from the cluster center by several tidal radii in the trailing direction of the proper motion of UKS 1. This alignment supports a scenario in which relativistic leptons, likely driven by a millisecond pulsar population, produce very-high-energy (VHE) gamma-rays via ICS within a bow shock tail. Our findings not only highlights GCs as potential sources of VHE gamma-rays, but also offers a rare opportunity to probe cosmic ray transport in the Milky Way by studying particle propagation and anisotropic gamma-ray production associated with the extended, offset TeV feature of UKS 1.

Paper Structure

This paper contains 4 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Discovery of GeV-TeV emission from UKS 1
  3. Physical Implications
  4. Future Prospects

Figures (1)

  • Figure 1: ( Left panel:) $6^{\circ}\times6^{\circ}$ 3HWC significance map around the center of UKS 1 from a point-source search with contours illustrating significance levels of $3\sigma$, $3.3\sigma$, $4\sigma$, and $4.3\sigma$. The location and extent of nearby TeV sources are indicated by yellow circles. The red-dashed lines shows the orientation of Galactic plane. ( Right panel:) Close-up view within $3^{\circ}\times3^{\circ}$ of the Fermi-LAT Test Statistic (TS) map, with 3HWC contours overlaid. The black crosses and white arrows in both panels illustrate the center and the direction of the proper motion of UKS 1, respectively. Top is north and left is east. A scale bar of $1^{\circ}$ is given in both panels