Broadening of the iron emission line in MCG-6-30-15 by comptonization

Abstract

We show that the iron K emission line from MCG-6-30-15 could be broadened through Comptonization by a surrounding highly ionized cloud with radius ~10¹⁴ cm. We calculate the temperature of the cloud to be ~0.21 keV, provided that a reasonable estimate of the UV flux is made. The X-ray/γ-ray emission observed from the source is compatible with this model. Such a cloud should be highly ionized, and strong absorption edges are not expected from the source. For a 10⁶ M⊙ black hole the size of the cloud corresponds to about 300 Schwarzschild radii. The intrinsic line could then be emitted far from the black hole, and gravitational redshift and Doppler effects would be negligible. If the black hole mass is much larger than 10⁶ M⊙, gravitational/Doppler redshifts would also contribute significantly to the broadening. We argue that the broad red wing observed in the source does not by itself imply emission from regions close (R < 5r_S) to the black hole. However, Comptonization cannot produce a double peak. The presence of such a feature is a clear sign of inner disk emission influenced by gravitational and Doppler effects, perhaps broadened by the Comptonization. We note that simultaneous broadband (2-100 keV) study of this source can also reveal (or rule out) the presence of such a Comptonizing cloud.