A possible GeV-TeV connection in the direction of the Globular Cluster UKS 1

Abstract

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). In addition to the confirmed $\gamma -$ray GCs in the 4FGL catalog, we report a GeV detection at the position of UKS 1 with a post-trial probability of $\sim 8\times 10^{-5}$ of it being a fluctuation. Its spectrum within this energy range is well described by a power-law model with $\Gamma \simeq 2.3\pm 0.5$. Furthermore, this GeV feature appears to extend southeast in a direction towards 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 $\sim 3\times 10^{-4}$. If confirmed, this would be 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 $\sim 270$ km s${}^{-1}$, our findings are consistent with a scenario where the VHE $\gamma -$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.

6 pages, 4 figures, 1 table, A&A Letter in press

References in corpus (7)

• Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST and literature data
• 3HWC: The Third HAWC Catalog of Very-High-Energy Gamma-ray Sources
• The enigmatic globular cluster UKS~1 obscured by the bulge: \textit{H}-band discovery of nitrogen-enhanced stars
• Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
• Constraining the cosmic ray spectrum in the vicinity of the supernova remnant W28: from sub-GeV to multi-TeV energies
• Discovery of gamma-ray pulsations of PSR J1835$-$3259B in the Globular Cluster NGC 6652
• Likely detection of gamma-ray pulsations of PSR~J1717+4308A in NGC~6341 and implication of the gamma-ray millisecond pulsars in globular clusters

Cited by in corpus (1)

• Search for TeV emission from spider millisecond pulsars with HAWC