Probing the Physical Properties of the NGC 5548 Broad Line Region Using Balmer Lines

We investigated the physical characteristics of the broad line region (BLR) of NGC 5548 using the Boltzmann plot (BP) method, which we applied to the Balmer lines observed from 1996 to 2004. We found 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 found that the average temperature for the considered period was T 10000 K, and that it 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 disk in the BLR of NGC 5548.