Title: High Energy Tails in Luminous Globular Cluster X-ray Sources


Proposal ID: 0120188
Subject category: Compact Object
Principal investigator: Sidoli
Institute:


Abstract

The nature of the X-ray sources in globular clusters has provoked intense observational and theoretical interest, especially because of the implied formation mechanism, that is about a factor of 200 more efficient inside globular clusters than in the Galaxy itself. Galactic globular clusters host 12 luminous LMXRBs containing neutron stars. The discovery of a 35-200 keV power-law tail from the X-ray burster X1724-308 (located in the globular cluster Terzan2) demonstrated that also LMXRBs containing neutron stars can be sources of soft-gamma-ray emission. Using BeppoSAX, we performed a highly succesful CP survey of luminous globular cluster sources, demonstrating that the ultracompact binaries display a different 0.1-50 keV spectrum with respect to all other globular cluster sources. We now propose to observe 6 globular cluster sources, the hardest persistent ones, in order to better constrain the parameters of the Comptonized emission (this especially for the ADC source in NGC7078, which does not show evidence for a high energy cut-off up to 100 keV), to better understand the "spectral signature" of the ultracompact binaries that we found with BeppoSAX, to compare the soft-gamma-ray properties of LMXRBs containing neutron stars with accreting black-holes, and eventually to observe the second high energy tail observed so far with BeppoSAX in a limited number of accreting neutron stars.