Hot dust on the outskirts of the broad-line region in Fairall 9

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Abstract

Since 1978, the Seyfert 1 galaxy F9 has been observed 54 times in the far-ultraviolet and optical (FES) range with the IUE, and at 27 different epochs at J, H, K, and L. The UV continuum underwent dramatic variations, its intensity decreasing by a factor 33, from a maximum in 1978 to a deep minimum in mid-1984. The near-IR and optical fluxes changed by a factor of ~3 and in the same sense as the ultraviolet. Variations in the optical and J band are diluted by starlight but are in phase with those in the UV. This contrasts with the variations at K and L which are delayed by 400 +/- 100 days. We interpret the K and L emission as thermal radiation from dust lying at ~ 1 lt-yr from the UV source. The equilibrium temperature of the grains is 1730 +/- 230 K, which implies that they are made of graphite. The mass of hot dust is small and does not contribute significantly to the extinction. The Lyα λ1216, C IV λ1550, and Mg II λ2800 emission-line intensities also vary in the same sense as the UV continuum but with a lag of 155 +/- 45 days. This strongly suggests that the broad-line region (BLR) gas is photoionized and lies at ~ 150 lt-day from the UV source, i.e. inside the dust shell. Strong correlations exist between the Lyα/C 1V and Lyα/Mg II line ratios, on the one hand, and the intensity of the ultraviolet continuum, on the other. This is likely to be due to the factor of ~10 increase of the X-ray to UV luminosity ratio which accompanied the 1978-1984 decline of F9. It is probably the strongest evidence to date that the Mg II λ2800 line emissivity depends primarily on the density of soft X-ray photons, as postulated in optically thick photoionization models of the BLR. Despite large intensity variations, the spectral index of the UV continuum remains constant, α = -0.46 +/- 0.11, equivalent to a blackbody temperature T_bb_ = 26,500 +/- 900 K. The constancy of α or T_bb_ is a new important constraint on models which aim at explaining the origin of the "big blue bump" in AGNs.

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