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

Abstract

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 Civ and Lyalpha are highly correlated with the UV continuum, and are delayed with respect to the continuum variations by 50-110d with the red wing of both Civ and Lyalpha leading the blue wing. (2) The Civ/Lyalpha 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 Lyalpha and Civ 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/Lyalpha variations might be related to this hard ionizing continuum in 3C 390.3. Our results suggest: (i) the broad Civ and Lyalpha emitting gas is infalling towards the central object in 3C 390.3; (ii) the overall behaviour of the Civ/Lyalpha 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 light-days from the central source; (iv) under these assumptions and with the derived circular velocity of v_rot~=2850 km s^-1, the central mass inside this radius is confined to 1.3x10^8Msolar< M_CM<4.0x10^8 Msolar (v) comparing our results with those obtained from very long-baseline interferometry (VLBI) and observations of the FeKalpha line suggests the association of the BLR with a disc, inclined at 98 deg+/-12 deg with respect to the direction of superluminal motion of the radio blobs.