Abstract
We present the results of ROSAT PSPC observations of three active galactic nuclei (AGN) with extremely strong Fe IT emission (PHL 1092, lRAS 07598 + 6508 and I Zw 1) and two AGN with very weak Fe IT emission (Mrk 10 and 110). The weak Fe II emitters have X-ray spectra typical of Type 1 AGN (OC = 1.35 and 1.41, where oc is the spectral energy index). Of the strong Fe II emitters, two have steep spectra (PHL 1092 has oc = 3.5, and I Zw 1 has = 2.0) and the third, lRAS 07598 + 6508, is barely detected and so is extremely X-ray-quiet (ocox =2.45). During our observations, PHL 1092 varied by a factor of 4, unusually fast for such a high luminosity object, and requiring an efficiency of matter-to-energy conversion of 2 per cent or more. Compiling recently published data on other strong Fe II emitters, we find that they are always X-ray-quiet, and usually X-ray-steep. Adding these data to the complete uvx: -selected quasar sample of Laor et aI., we find a statistical connection of Fe IIIHP with ex.: but not a simple relationship: weak Fe II emitters always have flat spectra, but strong Fe II emitters can be either flat or steep. A much cleaner relationship exists between Fe II strength and X-ray loudness, as quantified by OCix' the spectral index between 11lII1 and 2 keY. We also confirm that Fe IIIHP anticorrelates with Balmer line velocity width, which in tum correlates well with both ex.: and OCix in the sense that AGN with narrow lines are X-ray-quiet. There is also marginal evidence that Fe IIIHP correlates with both optical continuum slope and the curvature of the optical-UV-X-ray continuum: strong Fe IT objects tend to have steeper continua and weaker 'blue bumps'. The amount of extinction required to explain the optical steepening compared to normal quasars [E (B - V) in the range 0.2 to 0.6] suggests absorbing columns in the range (1- 3) x 1021 cm -2, just about the right amount to reduce the ROSAT-band X-ray flux by enough to explain
Highlights
The Fe II emission lines which dominate the optical-UV spectra of many active galactic nuclei (AGN) have been seen as a worrying puzzle in two ways.(i) Photoionization models have so far failed to replicate convincingly the strength of Fe II relative to the Balmer lines
We analysed the image using the point source searching routine (PSS). This does a sliding fit using an empirical point-spread function (PSF), which depends in turn on the PHA channels included
For the Galactic H I column in this direction, NH=4.3 x 1020 cm- 2, we find that this corresponds to a monochromatic flux density at 1 keV of 3.8 ± 1.3
Summary
The Fe II emission lines which dominate the optical-UV spectra of many active galactic nuclei (AGN) have been seen as a worrying puzzle in two ways.(i) Photoionization models have so far failed to replicate convincingly the strength of Fe II relative to the Balmer lines (see Joly 1993, and references therein). (ii) The relative strength of Fe II emission varies widely amongst different AGN, and shows intriguing correlations with other AGN properties: X-ray slope (Wilkes, Elvis & McHardy 1987), Balmer line velocity width (Wills 1982; Gaskell 1985; Zheng & O'Brien 1990; Zheng & Keel 1991; Boroson & Green 1992), narrow-line strength (Gaskell 1987; Boroson & Green 1992), radio loudness (Osterbrock 1977; Grandi & Osterbrock 1978; Bergeron & Kunth 1984) and, within radio-loud objects, core dominance (Joly 1991). As they seem to be important clues to the AGN phenomenon, it is important to clarify their reality and nature
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