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.