Free access article

A&A 441, 855-861 (2005)
DOI: 10.1051/0004-6361:20053451

The relativistic shift of narrow spectral features from black-hole accretion discs

T. Pechácek¹, M. Dovciak², V. Karas^{1, 2} and G. Matt<sup>3

1</sup>  Charles University, Faculty of Mathematics and Physics, V Holesovickách 2, 180 00 Prague, Czech Republic
²  Astronomical Institute, Academy of Sciences, Bocní II, 141 31 Prague, Czech Republic
    e-mail: vladimir.karas@cuni.cz
³  Dipartimento di Fisica, Università degli Studi "Roma Tre", via della Vasca Navale 84, 00146 Roma, Italy

(Received 17 May 2005 / Accepted 14 June 2005 )

Abstract
Transient spectral features have been discovered in the X-ray spectra of Active Galactic Nuclei, mostly in the 5-7 keV energy range. Several interpretations were proposed for the origin of these features. We examined a model of Doppler boosted blue horns of the iron line originating from a spot in a black hole accretion disc, taking into account different approximations of general relativistic light rays and the resulting shift of energy of photons. We provide a practical formula for the blue horn energy of an intrinsically narrow line and assess its accuracy by comparing the approximation against an exact value, predicted under the assumption of a planar accretion disc. The most accurate approximation provides excellent agreement with the spot orbital radius down to the marginally stable orbit of a non-rotating black hole.

Key words: line: profiles -- accretion, accretion discs -- black hole physics -- X-rays: galaxies

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© ESO 2005

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