The Fe Kalpha Line in Seyfert 1 Galaxies

Following the launch of the ASCA satellite 1993, it became increasingly clear that the Fe emission line was much broader than might be expected.

Instead of a relatively narrow peak at 6.4 keV (only broadened by approx 100 eV due to the spectral resolution of detector), a broad excess was seen superimposed on the underlying continuum. The excess often appeared assymetric and preferentially shifted to lower energies (hence it is sometimes refered to as a broad red wing), and in some cases stretched down as far as approx 4 keV.

On the left is shown the summed line profiles for a sample of Seyfert 1 galaxies observed by ASCA. The upper panel shows the entire sample, the lower panel has the sources NGC 4151 and MCG-6-30-15 removed. Both show a characterstic shape: a narrow component at 6.4 keV (broadened by the instrumental resolution), and a broad "hump" of emission between about 5 and 7 keV.

The favoured interpretation for the broad component is that they are indeed the result of Fe Kalpha emission, but these photons are emitted in an accretioin disk close to a central black hole. The photons therefore suffer the (severe) effects of general relativity en route out of the nucleus.

The effects of both the gravtitation redshift (ie General Relativity) and the large orbital velocities pocessed by matter in the inner regions of the accretion disk (Special Relativity) lead to characteristic shifts in the (observed) energies of the Fe Kalpha photon originating from different parts of the accretion disk.

The latest results prehaps indicate the Blandford-Znajek effect in action whereby rotational energy can escape from the black hole as it is braked by the magnetic fields threading the hole and accretion disk.

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