We investigate the physical characteristics of the Broad Line Region ( BLR ) of NGC 5548 using the Boltzmann plot ( BP ) method , that we applied on the Balmer lines observed from 1996 to 2004 .
We find that variability earlier detected in the lines , is also present in the temperature parameter A , obtained from the BP method .
Using the BP temperature parameter A to calculate temperature , we find that the average temperature for the considered period was T \approx 10000 K , and that varies from 5000 K ( in 2002 ) to 15000 K ( in 1998 ) .
This variation correlates with the AGN-component of the optical continuum flux ( r = 0.85 ) and may indicate the existence of an accretion disc in the BLR of NGC 5548 .