The comptonized X-ray source X 1724-308 in the globular cluster Terzan 2

Abstract

We report on the BeppoSAX observation of the X-ray source X 1724-308 in the globular cluster Terzan 2. The broadband spectrum can be described as the superposition of a power-law with photon spectral index Gamma =~ 1.6-1.9 and a thermal component with typical temperature kT ~ 1 keV. X 1724-308 is detected in the PDS up to 150 keV with a S/N ratio >3, notwithstanding a sharp exponential cutoff with E_{cutoff =~ 90} keV. The broadband spectrum can be best interpreted as due to the Comptonization of a Wien photon distribution with kT_W ~ 1 keV by a spherically-symmetric hot plasma with electron temperature kT_e ~ 30 keV and optical depth tau =~ 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.