2000MNRAS.317L..11R

It has recently been suggested that Compton down-scattering may give rise to the broad iron Kα line seen in the X-ray spectrum of the Seyfert 1 galaxy MCG-6-30-15. This model suggests that the Comptonizing optically thick plasma surrounding the central X-ray source has a temperature of 0.5keV and a large radius of 10¹⁴cm. This offers an alternative to the standard model whereby the broadening of the iron line is solely the result of strong general relativistic effects. We revise the Comptonization model and show that statistically the disc-line model gives a much better fit to the time average of the data analysed by Iwasawa et al. in 1996 and 1999. We also demonstrate that the Comptonization model has problems with simultaneous fitting of the redshifted tail and the core of the line. We show that, in the case of the 1996 data, the best-fitting Thomson depth τ∼1.6 is consistent with the lack of continuum break, which is constrained to be at photon energies E≳100keV. However, the total amount of power in the UV component required to cool the Comptonizing cloud exceeds the Eddington limit. For large black hole masses relativistic effects are important and for small masses the Eddington limit is exceeded by a larger factor. In the case of the 1999 data, the best-fitting Thomson depth is τ∼5.7; this would imply the existence of a break in the continuum at E∼16keV, which is not observed. (However, we point out that the down-scattering break may be diluted if a fraction of the continuum is observed directly.) This rules out Comptonization as the principal mechanism to explain the shape of the Fe Kα line in MCG-6-30-15.