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Relativistic Emission Lines from Accreting Black Holes A&A 413, 861–878 (2004) Astronomy DOI: 10.1051/0004-6361:20031522 & c ESO 2004 Astrophysics Relativistic emission lines from accreting black holes The effect of disk truncation on line profiles A. M¨uller and M. Camenzind Landessternwarte Koenigstuhl, 69117 Heidelberg, Germany Received 30 July 2003 / Accepted 22 September 2003 Abstract. Relativistic emission lines generated by thin accretion disks around rotating black holes are an important diagnostic tool for testing gravity near the horizon. The iron K–line is of special importance for the interpretation of the X–ray emission of Seyfert galaxies, quasars and galactic X–ray binary systems. A generalized kinematic model is presented which includes radial drifts and non–Keplerian rotations for the line emitters. The resulting line profiles are obtained with an object–oriented ray tracer operating in the curved Kerr background metric. The general form of the Doppler factor is presented which includes all kinds of poloidal and toroidal motions near the horizon. The parameters of the model include the spin parameter, the inclination, the truncation and outer radius of the disk, velocity profiles for rotation and radial drift, the emissivity profile and a multi–species line–system. The red wing flux is generally reduced when radial drift is included as compared to the pure Keplerian velocity field. All resulting emission line profiles can be classified as triangular, double–horned, double–peaked, bumpy and shoulder–like. Of particular interest are emission line profiles generated by truncated standard accretion disks (TSD). It is also shown that the emissivity law has a great influence on the profiles. The characteristic shoulder–like line profile observed for the Seyfert galaxy MCG–6–30–15 can be reproduced for suitable parameters. Key words. galaxies: active – galaxies: Seyfert – accretion – relativity – black hole physics – line: profiles 1. Introduction Broad X–ray emission lines originate from the hot inner parts of accreting black hole systems, typically from radii com- Emission lines originating from regions where the influence of parable to the radius of marginal stability. Naturally, these lines curved space–time can not be neglected are found in accret- hint for a proximity of hot and cold material: hot material ing black hole systems like Active Galactic Nuclei (AGN), can be found in an optically thin corona, a place where cold microquasars and Galactic Black Hole Candidates (GBHC). seed photons are Comptonized to hard radiation. It is expected These astrophysical systems exhibit hot plasma falling into that the corona becomes optically thick at super-Eddington ac- the deep potential well of black holes. The ion species in cretion rates and is dominated by Comptonization. This lat- the plasma still have some electrons on lower shells so that ter scenario seems to hold for narrow-line Seyfert-1 galax- there are several transitions between electron states that gen- ies (NLS1s), whereas classical broad-line Seyfert-1 galaxies erate emission lines in the X–ray range. The most prominent (BLS1s) accrete at sub-Eddington to Eddington rates. line is the Fe Kα line at 6.4 keV rest frame energy (near- neutral iron). Additionally, the Fe Kβ line at 7.06 keV and Relatively cold material hides in the radiatively–efficient, rarely Ni Kα at 7.48 keV and Cr Kα at 5.41 keV can be ap- geometrically thin and optically thick accretion disk, the so plied to fit X–ray emission line systems. The fluorescence pro- called standard disk, hereafter SSD (Shakura–Sunyaev Disk) cess and its transition energies depend on the ionization state (Shakura & Sunyaev 1973). The resulting structure equations of the species. One can distinguish hot and cold regimes of line for this standard disk have been generalized to the relativistic emission which can be described by an ionization parameter case later (Novikov & Thorne 1974). The hard coronal input (Ballantyne et al. 2001). Generally, X-ray astronomers observe radiation usually described by a power law illuminates the cold a multi–component emission line complex of these species. disk. As a consequence, the surface of the disk is ionized in a The Fe Kα line dominates the line system due to its high rela- thin layer. In a fluorescence mechanism the hard radiative input tive strength and is often indicated in unreduced global X-ray is absorbed (threshold at 7.1 keV) and re–emitted in softer fluo- spectra. rescence photons. But in the dominant competitive process, the auger effect, the excited ions emit auger electrons and enrich Send offprint requests to:A.M¨uller, the environment with hot electrons. One of the first applications e-mail: [email protected] of this X–ray fluorescence was performed at the microquasar Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20031522 862 A. M¨uller and M. Camenzind: Relativistic emission lines from accreting black holes Cyg X–1 (Fabian et al. 1989). These observations were supple- mented by radio–quiet AGN, e.g. Seyfert galaxies, showing the same X–ray reflection signatures (Pounds et al. 1990; Tanaka et al. 1995; Reynolds & Nowak 2003). But there are also sources exhibiting narrow X–ray emis- sion lines. The narrow lines do not show distortion by relativis- tic effects. It has been proposed that narrow X–ray emission lines can be explained by reflection at the large–scale dusty torus (Turner et al. 1997). Here, the narrow line is overimposed on broad line profiles. This interpretation has been applied to some sources, like Seyfert galaxies, e.g. MCG–6–30–15 (Wilms et al. 2001), MCG–5–23–16 (Dewangan et al. 2003) and Quasars, e.g. Mrk 205 (Reeves et al. 2001). The absence of the line feature in several Sefert–1 galaxies (Pfefferkorn et al. 2001) can be explained by global accretion models. The first possible interpretation is that the inner edge of the disk seems to be far away from the black hole. If the Fig. 1. Prototype X–ray spectrum of a type–1 AGN (illustration idea fluorescence process is still possible in that distance, the skew- taken from Fabian 1998). The spectrum is dominated by the global ness of the X–ray emission line vanishes and becomes rather Comptonized continuum. At low energies, there is the soft excess,in Newtonian-like. In even larger distances, it is supposed that the principle a composition of some black bodies belonging to the disk di- vided into rings. At energies around 1 keV, one can recognize a com- emission line feature disappears completely because irradiation plex of absorption dips, originating from the warm absorbers.There- and fluorescence is reduced. Accretion theory states that this flection component consists of a broad bump peaking around 20 keV scenario holds for low accretion rate: then the transition radius and the prominent fluorescence emission line complex around 6.4 keV. between standard accretion disk and optically thin disk is large (Esin et al. 1997). The mass determinations for Seyfert black holes confirm this interpretation while being in the intermedi- independently from the accretion model! This is accessable by ate range of 106 to 108 M . These lighter supermassive black analytical investigations. holes seem to accrete very weakly. Another basic ingredient to calculate emission lines and X–ray spectra of AGN and X–ray binaries are dominated spectra in general is the emissivity law that determines the by broad Comptonized continua. The properties of these spec- emission behaviour of the line emitting region. In Sect. 5, alter- tra are discussed in the next section. The first step for as- native emissivity shapes besides th classical single power law tronomers is to reduce the observed data files and to extract the are applied. line profile by subtracting this global continuum emission. This Ray racing provides in a first step disk images. It may be procedure is already complicated because the intrinsic curva- possible to resolve these images observationally in near future. ture of the Comptonized radiation is still topic of an ongoing In any case, disk images as presented in Sect. 6 are very instruc- debate. tive unrevealing GR effects and influence of plasma dynamics. The resulting emission line profile serves for diagnostics In Sect. 7, the calculation and basic properties of the emis- and to constrain essential parameters of the accreting black sion lines are discussed. hole system. So, astronomers found an X–ray imprint of black In the final Sect. 8, the parameter space of relativistic emis- holes and can derive properties concerning the relative position sion lines is investigated and suitable criteria to fix line features from the inner accretion disk (standard disk) to the observer, are presented. One can apply these criteria to observed line pro- the inclination i or the rotational state of the black hole, the files and derive a classification in the line jungle. Kerr rotational parameter a, the extension of the geometrically thin and optically thick standard disk, like inner radius rin and 2. X–ray spectra and topology of type–1 AGN outer radius rout, the velocity field of the plasma, separated in Φ Θ r toroidal v , poloidal v and radial v components and the rela- The data quality increased conspicuously with the launch of the tive position of hot corona to cold accretion disk. space–based X–ray observatories Chandra and XMM–Newton. Let us follow up the theoretical steps from black hole the- The challenge in interpreting X–ray spectra of AGN and XRBs ory needed here until one gets simulated emission line pro- is to correctly reduce the data and subtract the continuum. files: Sect. 3 introduces the metric of rotating black holes, the Afterwards one can discuss the formation of other spectral Kerr geometry, and provides the standard co–ordinate system components, such as emission lines.
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