GX~3+1: the stability of spectral index as a function of mass accretion
  rate
  We present an analysis of the spectral and timing properties observed in
X-rays from neutron star binary GX~3+1. We analyze all observations of this
source obtained with the RXTE and BeppoSAX satellites. We find that the X-ray
broad-band energy spectra during these spectral transitions can be adequately
reproduced by a composition of a low-temperature blackbody component, a
Comptonized component (COMPTB) and Gaussian component. We argue that the
electron temperature kT_e of the Compton cloud monotonically increases from 2.3
keV to 4.5 keV, when GX~3+1 makes a spectral transition. Using a disk seed
photon normalization of COMPTB, which is proportional to mass accretion rate,
we find that the photon power-law index Gamma is almost constant (Gamma=2.00+/-
0.02) when mass accretion rate changes by factor 4. We interpret this
quasi-stability of the index Gamma and a particular form of the spectrum in the
framework of a model in which the energy release in the transition layer
located between the accretion disk and neutron star surface dominates that in
the disk. Moreover, this index stability effect now established for GX~3+1 was
previously found in the atoll source 4U~1728-34 and suggested for a number of
other LMXB NS binaries (see Farinelli & Titarchuk). This intrinsic behavior of
neutron stars, in particular for atoll sources, is fundamentally different from
that seen in black hole binary sources where the index monotonically increases
during spectral transition from the low state to the high state and then
finally saturates at high values of mass accretion rate.
