The Comptonized X-ray source X1724-308 in the globular cluster Terzan 2

TL;DR

This paper presents BeppoSAX observations of the X-ray source X1724-308 in Terzan 2, revealing its broadband spectrum and suggesting a Comptonization process involving hot plasma, with implications for understanding accretion in globular clusters.

Contribution

First detailed broadband spectral analysis of X1724-308, identifying Comptonization parameters and their correlation with X-ray intensity in a globular cluster source.

Findings

Detection of X1724-308 up to 150 keV with a sharp cutoff at ~90 keV

Spectrum best described by Comptonization of Wien photons by hot plasma

Correlation between plasma temperature and X-ray intensity

Abstract

We report on the BeppoSAX observation of the X-ray source X1724-308 in the globular cluster Terzan 2. The broadband spectrum can be described as the superposition of a power-law with photon spectral index 1.6-1.9 and a thermal component with typical temperature ~1 keV. X1724-308is detected in the PDS up to 150 keV with a S/N ratio >3, notwithstanding a sharp exponential cutoff at \~90 keV. The broadband spectrum can be best interpreted as due to the Comptonization of a Wien photon distribution with kT ~1 keV by a spherically-symmetric hot plasma with electron temperature ~30 keV and optical depth ~3. A comparison with the fainter and steeper state, which was observed by EXOSAT, suggests a correlation between the temperature of the Comptonizing electron distribution (or the plasma optical depth) and the X-ray intensity. The X-ray thermal component could originate from a boundary layer between the accretion disk and the compact object, or from the accretion disk itself.