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

¹ Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany e-mail: peter.eger@physik.uni-erlangen.de
² Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany

Received: 24 November 2009
Accepted: 19 January 2010

Abstract

Aims. 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.

Methods. 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.

Results. 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 Γ = 0.9 ± 0.5 and a surface flux of F_x = (1.17±0.16) × 10^-7 erg cm^-2 s^-1 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.