The nature of the broad-line region in the radio-loud active galactic nucleus 3C 390.3

We present the results of an analysis of the ultraviolet and X-ray variability of the broad-line radio (BLR) galaxy 3C 390.3 over 15 years. The ultraviolet (UV) continuum of 3C 390.3 shows large variations with amplitudes of up to a factor of 10. We find the following results. (1) The variations of C IV and Ly alpha are highly correlated with the UV continuum, and are delayed with respect to the continuum variations by 50-110 d with the red wing of both C IV and Ly alpha leading the blue wing. (2) The C IV/Ly alpha ratio is positively correlated with both the continuum flux and UV line strength, a behaviour different from other AGNs studied so far. (3) The blue sides of the Ly alpha and C IV profiles are similar to the blue side of the Balmer lines, while the red sides are different, suggesting a different origin for the red peak in the Balmer lines. The X-ray spectra of 3C 390.3 observed with ROSAT can be well fitted by a single power law at Galactic absorption with a spectral slope of alpha = 0.9. The overall optical, UV to X-ray spectrum can also be described by a single power law with alpha(uvx) = 0.89, indicating a very weak or no big blue bump. The unusual behaviour of CIV/Ly alpha variations might be related to this hard ionizing continuum in 3C 390.3. Our results suggest: (i) the broad C IV and Ly alpha emitting gas is infalling towards the central object in 3C 390.3; (ii) the overall behaviour of the C IV/Ly alpha ratio and the absence of a big blue bump strongly indicate the coexistence of optically thick as well as optically thin BLR clouds; (iii) assuming circular symmetry and predominantly circular motion, the BLR gas is situated at 83 +/- 25 lightdays from the central source; (iv) under these assumptions and with the derived circular velocity of upsilon(rot) similar or equal to 2850 km s(-1), the central mass inside this radius is confined to 1.3 x 10(8) M. < M-CM < 4.0 x 10(8) M.; (v) comparing our results with those obtained from very long-baseline interferometry (VLBI) and observations of the Fe K alpha line suggests the association of the BLR with a disc, inclined at 98 degrees +/- 12 degrees with respect to the direction of superluminal motion of the radio blobs.