Fermi Discovery of Gamma-ray Emission from the Globular Cluster Terzan 5

TL;DR

This paper reports the first detection of gamma-ray emission from the globular cluster Terzan 5 using Fermi data, suggesting millisecond pulsars as the source and providing spectral details.

Contribution

First detection of gamma-ray emission from Terzan 5, expanding knowledge of globular cluster high-energy phenomena and identifying millisecond pulsars as likely sources.

Findings

Terzan 5 is detected at ~27σ in gamma rays.

The spectrum fits an exponential cutoff power-law model.

Gamma-ray emission is likely from millisecond pulsars.

Abstract

We report the discovery of gamma-ray emission from the Galactic globular cluster Terzan 5 using data taken with the Fermi Gamma-ray Space Telescope, from 2008 August 8 to 2010 January 1. Terzan 5 is clearly detected in the 0.5-20 GeV band by Fermi at ~27\sigma level. This makes Terzan 5 as the second gamma-ray emitting globular cluster seen by Fermi after 47 Tuc. The energy spectrum of Terzan 5 is best represented by an exponential cutoff power-law model, with a photon index of ~1.9 and a cutoff energy at ~3.8 GeV. By comparing to 47 Tuc, we suggest that the observed gamma-ray emission is associated with millisecond pulsars, and is either from the magnetospheres or inverse Compton scattering between the relativistic electrons/positrons in the pulsar winds and the background soft photons from the Galactic plane. Furthermore, it is suggestive that the distance to Terzan 5 is less than 10 kpc and > 10 GeV photons can be seen in the future.