1. Introduction
Owing to frequent stellar encounter, compact binaries can be formed dynamically inside a globular cluster (GC) including millisecond pulsars (MSPs; Hui et al. Reference Hui, Cheng and Taam2010). Since the magnetosphere of MSPs can accelerate electrons/positrons to relativistic speeds with Lorentz factor
$\gt10^{6}$
, their collective contributions make GCs as
$\gamma$
-ray emitters. Shortly after Fermi Large Area Telescope (LAT) was launched, a number of GCs were detected in the GeV band, forming a distinct source class (Tam et al. Reference Tam, Hui and Kong2016; Abdo et al. Reference Abdo and Ackermann2010; Ballet et al. Reference Ballet, Bruel, Burnett and Lott2023).
There are two possible origins of the GeV emission from a GC. Since a number of
$\gamma-$
ray GCs have spectra which resemble that of a MSP, namely a power-law with an exponential cutoff at energies of
$\lesssim10$
GeV, some studies argued that the GeV emission from GCs entirely come from the collective pulsed emission originating within the magnetosphere (e.g. Abdo et al. Reference Abdo and Ackermann2010). On the other hand, some spectra of GCs show hard tails at energies larger than few tens of GeV (Song et al. Reference Song, Macias, Horiuchi, Crocker and Nataf2021). We have shown that this is consistent with a scenario where these
$\gamma-$
rays originate from the inverse Compton scattering (ICS) between the relativistic pulsar wind and the ambient soft photons (Cheng et al. Reference Cheng, Chernyshov, Dogiel, Hui and Kong2010; Hui et al. Reference Hui, Cheng, Wang, Tam, Kong, Chernyshov and Dogiel2011).
For a GC moving in the rest frame of interstellar medium (ISM) with a speed of order 100 km s
$^{-1}$
, a bow shock can be formed. The pulsar wind particles can be further accelerated at the shock front. Through the ICS between these re-accelerated leptons and soft photons, very-high-energy (VHE)
$\gamma-$
rays can be produced (Bednarek and Sitarek Reference Bednarek and Sitarek2007; Bednarek and Sobczak Reference Bednarek and Sobczak2014). Possible evidence of this process was first found in the direction of Terzan 5 (H. E. S. S. Collaboration et al. 2011). An asymmetric TeV nebula has been detected around this GC with an offset between
$\gamma-$
ray emission peak and cluster center. In contrast, the GeV emission coincides with Terzan 5 (Kong et al. Reference Kong, Hui and Cheng2010).
The relativistic particles emanating from the bow shock are expected to be strongly oriented along the trailing magnetotail, with anisotropic ICS emission directed along their propagation vector. Self-scattering leads to a gradual broadening of the particle pitch angle distribution, isotropizing their emission and allowing it to become visible some distance away from the acceleration site (Krumholz et al. Reference Krumholz, Crocker, Bahramian and Bordas2024). Under this scenario, TeV nebulae powered by GCs can provide a key to constrain the scattering processes of cosmic rays in the Milky Way.
However, the TeV feature found in the direction of Terzan 5 has remained the only confirmed detection for more than a decade. This has motived our search for possible GeV-TeV GCs in archival data.
2. Discovery of GeV-TeV emission from UKS 1
Using 16 years public data collected by Fermi-LAT and the 1523 days of 3HWC survey data acquired by High Altitude Water Cherenkov (HAWC) observatory (Albert et al. Reference Albert, Alfaro and Alvarez2020), we reported the detection of a GeV
$\gamma$
-ray feature at the location of the GC UKS 1, which is associated with a TeV excess identified in the 3HWC survey data (Shin et al. Reference Shin, Hui, Kim, Oh and Owen2025). For details, refer to Shin et al. (Reference Shin, Hui, Kim, Oh and Owen2025). Here we highlight our major results.
Figure 1 shows the
$\gamma-$
ray excess in the direction of UKS 1 as found in the LAT data (right panel) and the 3HWC data (left panel), respectively. The TeV excess is found from a point-source search by assuming a power-law energy spectrum with an index of 2.5. The contours illustrate significance levels of
$3\sigma$
,
$3.3\sigma$
,
$4\sigma$
and
$4.3\sigma$
. The peak significance of the feature in proximity to UKS 1 is located at RA=
$17^{\rm h}55^{\rm m}54.38^{\rm s}$
, Dec=
$-23^{\circ}57^{'}30.96^{''}$
(J2000), with a significance of
$4.4\sigma$
. It is displaced from the center of UKS 1 by
$\sim0.39^\circ$
in the direction of N60.7
$^{\circ}$
E. Interestingly, we found the displacement almost traces the direction opposite to the proper motion of UKS 1 which is illustrated by the arrow in 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
To compare the
$\gamma$
-ray excesses as observed by Fermi (which is detected at a significance of
$\sim6\sigma$
) and HAWC, we show a close-up view of the
$3^{\circ}\times3^{\circ}$
region centered on UKS 1 as observed by LAT with the 3HWC significance contours overlaid in the right panel of Figure 1. Both excesses are apparently extended in a similar orientation toward the Galactic plane (the red dashed line in the left panel of Figure 1).
The Fermi-LAT spectrum of UKS is best described a single power-law model with a photon index of
$\Gamma=2.3\pm0.5$
. There is a clear excess above
$\sim10$
GeV. Integrating the model between 0.3-100 GeV yields an energy flux of
$f^{\rm LAT}_{\gamma}=(3.24\pm2.74)\times10^{-11}$
erg cm
$^{-2}$
s
$^{-1}$
. At the distance of UKS 1 (
$d=$
15.6 kpc Baumgardt and Vasiliev Reference Baumgardt and Vasiliev2021), this translates to a
$\gamma$
-ray luminosity of
$L^{\rm LAT}_{\gamma}=(9.44\pm7.97)\times10^{35}$
erg s
$^{-1}$
. Based on 3HWC survey data, the differential flux at the peak position of the TeV feature is found to be
$f^{\rm 3HWC}_{\gamma}=(3.27\pm1.61)\times10^{-14}$
cm
$^{-2}$
s
$^{-1}$
TeV
$^{-1}$
.
3. Physical Implications
Assuming an average MSP spin-down power of
$\langle\dot{E}\rangle \sim 2\times10^{34}\,{\rm erg\,s^{-1}}$
with a conversion efficiency
$\eta_\gamma \sim 0.08$
, the
$\gamma-$
ray luminosity
$L^{\rm LAT}_{\gamma}$
of UKS 1 suggests it may host
$\sim 100$
MSPs which can provide a strong collective pulsar wind outflow.
Given its high velocity of
$\sim270$
km s
$^{-1}$
moving around the Galactic disk (Fernández-Trincado et al. Reference Fernández-Trincado and Minniti2020), the GC wind is expected to be confined by a bow shock at
$R_{bs} \sim 0.6$
pc, comparable to the cluster core radius. Relativistic leptons accelerated at this shock primarily cool through ICS. Due to UKS 1’s relatively low stellar luminosity (
$\sim4\times10^4 L_\odot$
), the cooling length for multi-TeV particles reaches
$\sim100$
pc, consistent with the observed offset of
$0.39^\circ$
at 15.6 kpc.
Compared to the other TeV nebula around Terzan 5, UKS 1 exhibits a much larger TeV-to-GeV flux ratio, plausibly due to weaker starlight fields and, hence, less severe radiative losses (H. E. S. S. Collaboration et al. 2011; Kong et al. Reference Kong, Hui and Cheng2010). This highlights UKS 1 as an unusual case among GCs, capable of sustaining extended, displaced TeV emission.
4. Future Prospects
GCs such as UKS 1 that move supersonically through the Galactic disk are natural laboratories for studying bow shock physics and cosmic ray transport on 10–100 pc scales. The supersonic motion through the disk not only confines the GC wind but may also seed turbulence that affects the scattering and diffusion of cosmic rays into the Galactic environment. Future observations with the Cherenkov Telescope Array Observatory (CTAO) can constrain cosmic ray scattering processes in GC magnetotails. Establishing GCs as a TeV emitter would have broader implications for understanding how MSP winds contribute to the Galactic cosmic ray population (Hui Reference Hui2018).
Recently, Owen et al. (Reference Owen, Inoue, Hui, Fujiwara and Kong2025) showed that GCs can be major contributors to the
$\gamma$
-ray emission of massive quiescent galaxies. The GeV-TeV spectrum from a GC is governed by several environmental parameters: luminosities of the stars and MSPs hosted by the cluster, the strength of the local soft photon field, and the ambient ISM density. Since these factors vary with galactocentric radius, the
$\gamma$
-ray properties of a galaxy’s GC system is expected to depend on the spatial distribution of its clusters. The distribution of GCs is closely related to the assembly history of a galaxy. Therefore, once we unveil how different galaxy formation scenarios imprint distinct spectral signatures on the
$\gamma$
-ray emission from GC systems, observations in GeV-TeV regime may be able to trace galactic evolutionary pathways.
6∘×6∘
3σ
3.3σ
4σ
4.3σ
3∘×3∘
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