Chandra detection of diffuse X-ray emission from the globular cluster Terzan 5

TL;DR

This study used archival Chandra data to detect and analyze diffuse X-ray emission from the globular cluster Terzan 5, finding evidence for non-thermal processes likely responsible for the emission.

Contribution

First detection and characterization of diffuse X-ray emission around Terzan 5 using Chandra, with spectral analysis suggesting a non-thermal origin.

Findings

Diffuse X-ray emission peaks at the cluster center and decreases outward.

The emission spectrum is best described by a power-law with photon index ~0.9.

Unresolved point sources contribute negligibly to the observed excess.

Abstract

Terzan 5, a globular cluster (GC) prominent in mass and population of compact objects, is searched for diffuse X-ray emission, as proposed by several models. We analyzed the data of an archival Chandra observation of Terzan 5 to search for extended diffuse X-ray emission outside the half-mass radius of the GC. We removed detected point sources from the data to extract spectra from diffuse regions around Terzan 5. The Galactic background emission was modeled by a 2-temperature thermal component, which is typical for Galactic diffuse emission. We detected significant diffuse excess emission above the particle background level from the whole field-of-view. The surface brightness appears to be peaked at the GC center and decreases smoothly outwards. After the subtraction of particle and Galactic background, the excess spectrum of the diffuse emission between the half-mass radius and 3' can be described by a power-law model with photon index $\Gamma$ = 0.9$\pm$0.5 and a surface flux of F$_X$ = (1.17$\pm$0.16) 10$^{-7}$ erg s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in the 1--7 keV band. We estimated the contribution from unresolved point sources to the observed excess to be negligible. The observations suggest that a purely thermal origin of the emission is less likely than a non-thermal scenario. However, from simple modeling we cannot identify a clearly preferred scenario.