| L.I. Mainardi, A. Paizis, R.Farinelli, E.
Kuulkers, J. Rodriguez, D. Hannikainen, P. Savolainen, S. Piraino, A. Bazzano, A. Santangelo |
| The aim of this work is to investigate in a
physical and quantitative way the spectral
evolution of bright Neutron Star Low-Mass X-ray
Binaries (NS LMXBs), with special regard to the
transient hard X-ray tails.
We analyzed INTEGRAL data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1,
GX 9+1) and built broad-band X-ray spectra
from JEM-X1 and IBIS/ISGRI data. For each
source, X-ray spectra from different states were
fitted were fitted with the recently proposed model
compTB.
The spectra have been fit with a two-compTB
model. In all cases the first compTB describes
the dominant part of the spectrum that we
interpret as thermal Comptonization of soft seed
photons (<1 keV), likely from the accretion disk,
by a 3-5 keV corona. In all cases, this component
does not evolve much in terms of Comptonization
efficiency, with the system converging to thermal
equilibrium for increasing accretion rate. The
second compTB varies more dramatically spanning from bulk plus thermal Comptonization of blackbody
seed photons to the blackbody emission alone.
These seed photons (R<12 km, kTs>1 keV), likely
from the neutron star and the innermost part of
the system, the Transition Layer, are Comptonized
by matter in a converging flow. The presence and
nature of this second compTB component (be it a
pure blackbody or Comptonized) are related to the
inner local accretion rate which can influence
the transient behaviour of the hard tail: high
values of accretion rates correspond to an
efficient Bulk Comptonization process (bulk
parameter delta not null) while even higher values
of accretion rates suppress the Comptonization,
resulting in simple blackbody emission
(delta=0).
The spectral evolution of the sources has been
successfully studied in terms of thermal and Bulk
Comptonization efficiency, in relation to the
physical conditions in the Transition Layer. |
