We present the first robust evidence of an anti-correlation between the X-ray photon index , \Gamma , and the X-ray luminosity in a single low luminosity active galactic nuclei ( LLAGN ) , NGC 7213 .
Up to now , such anti-correlation trends have been seen only in large samples of LLAGN that span a wide range of X-ray fluxes , although the opposite behaviour ( i.e .
a positive correlation between \Gamma and X-ray luminosity ) has been extensively studied for individual X-ray bright active galactic nuclei .
For NGC 7213 , we use the long-term X-ray monitoring data of Rossi X-ray Timing Explorer ( RXTE ) , regularly obtained on average every two days from March 2006 to December 2009 .
Based on our X-ray data , we derive the \Gamma versus flux and the hardness ratio versus flux relations , indicating clearly that NGC 7213 follows a ‘ harder when brighter ’ spectral behaviour .
Additionally , by analysing radio and optical data , and combining data from the literature , we form the most complete spectral energy distribution ( SED ) of the source across the electromagnetic spectrum yielding a bolometric luminosity of 1.7 \times 10 ^ { 43 } erg s ^ { -1 } .
Phenomenologically , the SED of NGC 7213 is similar to that of low-ionization nuclear emission-line region .
The robust anti-correlation trend that we find between \Gamma and X-ray luminosity together with the low accretion rate of the source , 0.14 per cent that of Eddington limit , make NGC 7213 the first LLAGN exhibiting a similar spectral behaviour with that of black hole X-ray binaries in ‘ hard state ’ .