  Observationally, an anti-correlation between the X-ray photon index $\Gamma$
(obtained by fitting the X-ray spectrum between 0.5 and 10 keV with a single
power law) and the X-ray luminosity $L_{\rm 0.5-10keV}$, is found in neutron
star low-mass X-ray binaries (NS-LMXBs) in the range of $L_{\rm 0.5-10keV}\sim
10^{34}-10^{36}\ \rm erg\ s^{-1}$. In this paper, we explain the observed
anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$ within the framework
of the self-similar solution of the advection-dominated accretion flow (ADAF)
around a weakly magnetized NS. The ADAF model intrinsically predicts an
anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. In the ADAF model,
there is a key parameter, $f_{\rm th}$, which describes the fraction of the
ADAF energy released at the surface of the NS as thermal emission to be
scattered in the ADAF. We test the effect of $f_{\rm th}$ on the
anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. It is found that the
value of $f_{\rm th}$ can significantly affect the anti-correlation between
$\Gamma$ and $L_{\rm 0.5-10keV}$. Specifically, the anti-correlation between
$\Gamma$ and $L_{\rm 0.5-10keV}$ becomes flatter with decreasing $f_{\rm th}$
as taking $f_{\rm th}=0.1, 0.03, 0.01, 0.005$, $0.003$ and $0$ respectively. By
comparing with a sample of non-pulsating NS-LMXBs with well measured $\Gamma$
and $L_{\rm 0.5-10keV}$, we find that indeed only a small value of
$0.003\lesssim f_{\rm th}\lesssim 0.1$ is needed to match the observed
anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. Finally, we argue
that the small value of $f_{\rm th}\lesssim 0.1$ derived in this paper further
confirms our previous conclusion that the radiative efficiency of NSs with an
ADAF accretion may not be as high as $\epsilon \sim {\dot M GM\over
R_{*}}/{\dot M c^2}\sim 0.2$.

High Energy Astrophysical Phenomena