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| - The disc—jet relation in strong-lined blazars
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| - The relation between emission from the accretion disc (thermal) and the jet (non-thermal) in blazars is still a mystery as, typically, the beamed jet emission swamps the disc emission even in the ultraviolet band where disc emission peaks. In this paper we estimate the accretion disc component for 136 flat-spectrum radio quasars selected from the Deep X-ray Radio Blazar Survey. We do this by deriving the accretion disc spectrum from the mass and accretion rate on to the central black hole for each object, estimated using the emission linewidths and the power emitted from the broad-line region. We find that non-thermal emission dominates the optical/UV band of our sources. The thermal component is in fact on average ∼15 per cent of the total and ≳ 90 per cent of the objects in the sample have a thermal component <0.5 of the total luminosity. We then estimate the integrated disc and kinetic jet powers and find that on average the disc luminosity is ∼1 to 20 times the jet power (depending on the uncertainties in the estimation of the latter quantity). A comparison with previous, independent results favours a scenario in which jet and disc powers are of the same order of magnitude. Extraction of energy from a rotating black hole via the ‘Blandford—Znajek’ mechanism fails to explain the estimated jet power in the majority of our sources. Finally, we find that the typical masses for our sources are ∼5 × 108 M⊙ and that, contrary to previous claims, about one quarter of our radio quasars have relatively small (<3 × 108 M⊙) black hole mass.
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