We analyze the X-ray spectra and their timing properties of the compact X-ray binary 4U 1820-30 .
We establish spectral transitions in this source seen with Beppo SAX and the Rossi X-ray Timing Explorer ( RXTE ) .
During the RXTE observations ( 1996 – 2009 ) , the source were approximately \sim 75 \% of its time in the soft state making the lower banana and upper banana transitions combined with long-term low-high state transitions .
We reveal that all of the X-ray spectra of 4U 1820-30 are fit by a composition of a thermal ( blackbody ) component , a Comptonization component ( COMPTB ) and a Gaussian- line component .
Thus using this spectral analysis we find that the photon power-law index \Gamma of the Comptonization component is almost unchangeable ( \Gamma \sim 2 ) while the electron temperature kT _ { e } changes from 2.9 to 21 keV during these spectral events .
We also establish that for these spectral events the normalization of COMPTB component ( which is proportional to mass accretion rate \dot { M } ) increases by factor 8 when kT _ { e } decreases from 21 keV to 2.9 keV .
Before this index stability effect was also found analyzing X-ray data for Z- source GX 340+0 and for atolls , 4U 1728-34 , GX 3+1 .
Thus , we can suggest that this spectral stability property is a spectral signature of an accreting neutron star source .
On the other hand in a black hole binary \Gamma monotonically increases with \dot { M } and ultimately its value saturates at large \dot { M } .