The ISO-SWS Based on observations made with ISO , an ESA project with instruments funded by ESA member states ( especially the PI countries : France , Germany , The Netherlands and the United Kingdom ) and with the participation of ISAS and NASA .
The SWS is a joint project of SRON and MPE . infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151 , which are described in a companion paper , are used together with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus .
We present a new method to determine the best fitting photoionizing continuum and emission line cloud model from a heterogeneous set of emission line data .

For NGC 4151 , we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average , and which is consistent with the observed geometry of the optical Narrow Line Region ( NLR ) .
Our model consists of a clumpy , optically thick ( ionization bounded ) gas distribution , with a hydrogen gas density of \sim 1000 cm ^ { -3 } and a volume filling factor of 6.5 10 ^ { -4 } .

Our best fitting spectral energy distribution ( SED ) falls sharply beyond the Lyman limit and then rises sharply again towards 100 eV .
In particular , it does not display a ‘ Big Blue Bump ’ signature of a hot accretion disk .
We find that this SED , which best reproduces the NLR line emission on the 100–500 pc scale , does not have enough UV photons to produce the observed BLR recombination emission from the inner 1 pc .
This suggests that the BLR is photoionized by the intrinsic continuum source , which does have a strong UV component ( perhaps a Big Blue Bump ) , but that this UV component is absorbed by material located between the NLR and BLR .
Our analysis suggests that the absorber consists of \sim 5 10 ^ { 19 } cm ^ { -2 } of neutral hydrogen .
Such an absorber was independently detected by UV absorption lines ( Kriss et al . [ 1992 , 1995 ] ) .

Using our new method , we confirm our previous conclusion that a Big Blue Bump is present in the SED of the Seyfert 2 galaxy Circinus .