Probing the physical properties of the NGC 5548 Broad Line Region using   Balmer lines
  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. 
L. C. Popovic, A. I. Shapovalova, V. H. Chavushyan, D. Ilic, A. N.   Burenkov, A. Mercado, N. G. Bochkarev