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

¹ Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Prague, Czech Republic
² Astronomical Institute, Academy of Sciences, Boční 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.