EFFECTS OF STRONG GRAVITY ON THE X-RAY SPECTRA OF AGN AND BHCs A. MARTOCCHIA 1, V. KARAS 2 and G. MATT 3 (1) SISSA-ISAS Via Beirut 2/4, I-34014 Trieste (Italy) (2) Astronomical Institute, Charles University Prague, V Holesovickach 2 CZ - 180 00 Praha (Czech Republic) (3) Dipartimento di Fisica, Universita' degli Studi "ROMA TRE" Via della Vasca Navale 84 - I-00146 Roma (Italy) Email: martok@sissa.it, karas@mbox.troja.m.cuni.cz, matt@haendel.s.uniroma3.it Abstract. We computed iron line proles and Compton reection continua from X{ray illuminated, relativistic accretion discs around the black hole in AGN and BHCs. A fully relativistic treatment in Kerr metric is adopted for both the photons impinging on the disc, assumed to originate from a central primary X-ray source, and for the radiation reaching the distant observer. The local emission is modelled through Monte Carlo simulations of the re-processing in the optically thick accreting matter. The relativistic eects manifest themselves mainly in the smearing and broadening of any narrow feature, such as emission lines and absorption edges. The dependencies of the observables on the parameters of the system are illustrated. 1. Introduction In recent years much eort has been devoted to put forward practical methods which could discriminate between static and spinning Black Holes [BHs]. Substantial energy shift and lensing of photons from accretion discs in BH metric oer such a promising tool. Both stellar mass objects like Black Hole Candidates [BHCs] and supermassive objects like Active Galactic Nuclei [AGN] are well suited for this task. In the early 70s pioneering works (refs. [1], [2]) where performed in the standard framework of accretion disc theory, developed in those years (Shakura & Sunyaev 1973 [3], Thorne 1974 [4]), which can still be considered as a useful tool for estimating the eects of radiation shift and focusing in BH spacetimes. Eects of General Relativity on the emission line proles have been modelled by many authors in Kerr metric, i.e. in the general case of spinning BHs (Laor 1991 [5], Kojima 1991 [6], Hameury et al. 1994 [7], Karas et al. 1995 [8], Bromley et al. 1997 [9], Fanton et al. 1997 [10], Dabrowski et al. 1997 [11]). Observational evidence for relativistic discs has also already been sought, and found in AGN where X-ray spectra with iron K features (around E = 6:4 kev) are often detected.
2 This is e.g. the case of the Seyfert 1 galaxy MGC-6-30-15, observed by ASCA (Tanaka et al. 1995 [12]), with very broad and asymmetric line prole. This result has been recently conrmed by BeppoSAX observations (Guainazzi et al. 1999 [13]). Not much attention, however, has been paid on eects related to the reection continuum which is produced along with the iron line following illumination of the disc by a primary X-ray source, to be most probably identied with a hot corona. Selfconsistent calculations of the shape of iron lines and continuum together are useful to impose further restrictions on physical properties of the system, which will thus be less ambiguous. Here we show our recent results obtained with a fully relativistic code in Kerr metric (see also: Martocchia, Karas & Matt 1999 [24]). 2. Line plus Compton-reected Continuum: Results Averaged Local Emissivity (Arbitrary Units) 0.0005 0.00045 0.0004 0.00035 0.0003 0.00025 0.0002 0.00015 0.0001 5e-05 Emission angle decreasing Cosine of emission angle: 0.05 0.25 0.45 0.65 0.85 1.00 0 2 3 4 5 6 7 8 9 10 Energy (kev) Figure 1. Examples of local spectra about the iron line rest energy. Only dierencies related to the observer's inclination, i.e. the angle with respect to the symmetry axis, are shown here. Compton reection is the mechanism which generates the emission bump at high energies observed in Seyfert galaxies, whereas the iron line is produced by uorescence. To compute local spectra we performed Montecarlo simulations including reection and uorescence with such standard assumptions on the disc structure and ionization (Matt et al. 1991 [14]; emitted spectra are shown in Fig. 1). Disc emissivity, i.e. the radial dependence law, and local angle dependence law follow from fully relativistic computations of the illuminating ux (Martocchia 1996 [15]; Martocchia & Matt, 1996 [16]). Figures 2-3 show \narrow" spectra around the line rest energy calculated for an observer at innity and two dierent values of the primary source height h (100 and 4 gravitational radii). Usually, the line proles in real data are tted assuming an underlying reection continuum which is not in itself treated in the proper, relativistic way. It is then important to look for combined eects on both line and continuum.
3 Flux (arbitrary units) 2.5 10.5 2.5 10.5 Figure 2. In this as well as in the following picture "narrow-band" (2:5? 10:5 kev) spectra are shown for both minimally (left) and maximally (right) rotating BHs. The disc emitting area is comprised between the innermost stable orbit ( 6m and 1:24m, respectively) and r out = 100m (thick line), while contributions from three subsections (up to 34, 34-67 and 67-100) are also plotted. The observer's inclinations = cos(i) considered here are, from top to bottom: 0.1, 0.3, 0.5 (60 degrees), 0.7 and 0.9. Here h = 4m. The Compton-reected continuum and its dependence on i and a can be better seen in the broad band from 2 up to 220 kev, like in Figure 4 and 5. Flux (arbitrary units) 2.5 10.5 2.5 10.5 Figure 3. The same as before, but for h = 100m As an eect of the Doppler shift of the photons, even the continuum emission as a whole is spread through, more and more "enlarged" with increasing inclination. Apart
4 a/m = 0.001m a/m = 0.9981m (a) (d) θ o = 5 Count Rate (arbitrary units) (b) (c) (e) (f) θ o = 85 θ o = 45 Figure 4. The overall spectra (2?220 kev) for a disc extending from the innermost stable orbit up to 100 gravitational radii, and for three observer inclinations. Following conventions have been used for the source heights: h = 100m red, h = 20m green, h = 10m cyan, h = 6m magenta, h = 4m blue. a/m = 0.001m a/m = 0.9981m (a) (d) Count Rate (arbitrary units) θ o = 5 (b) (c) (e) (f) θ o = 85 θ o = 45 Figure 5. As before but for discs extending up to 34m only. from aecting the line prole in the expected way, this shifts towards lower or higher energies also cause smearing of the photoelectric edge into broad troughs. Therefore both the line and the iron edge are progressively smoothed down with increasing inclination. This behaviour is in qualitative agreement with previous claims (i.e. Sincell 1998 [19], who considered the distortion of the Lyman edge in Kerr metric, and Ross et al. 1996 [20]). All the mentioned eects on the spectrum have been computed for a larger range of
5 0.16 0.14 0.12 10-1000.dat 6-1000.dat 1-1000.dat 10-100.dat 6-100.dat 1-100.dat 0.1 0.08 0.06 0.04 0.02 0 0 0.2 0.4 0.6 0.8 1 Figure 6. Graph of equivalent widths for the case h = 20m. The upper three curves correspond to disc emission when r out = 1000m, those below to emission when r out = 100m. In both cases the curves refer to a = 0:9981m and, from top to bottom: r > r in = 1:23m (the innermost stable orbit in Kerr metric); r > r in = 6m (the innermost stable orbit in Schwarzschild metric); r > r in = 10m. In the last two cases the results come out to be identical when using the static BH metric: the diagrams would overlap each other. the parameters, and are shown in Martocchia, Karas & Matt (1999) [24] together with results concerning the iron line integral quantities. 0.22 0.2 th30_a0 th30_a0.5 th30_a1 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 2 4 6 8 10 12 14 16 18 20 Figure 7. EW vs. h for an observer inclination xed at 30 degrees. From top to bottom: a = 0:9981m, 0:5m and 0:001m. The outer disk edge is 100m. Due to re-emission from the innermost orbits, in the extreme Kerr situation one can have much enhanced EWs at low source heights (for comparison: Dabrowski 1998).
6 0.2 0.18 r34_a0 r34_a0.5 r34_a1 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 2 4 6 8 10 12 14 16 18 20 Figure 8. As before but for r out = 34m. To compare models with data of moderate quality, it may be better to consider line integral quantities like centroid energy, width and above all equivalent width [EW], i.e. the line width with respect to the underlying continuum. In Figure 6 we show the EW dependence from observer's inclination for some dierent emitting regions. The curves obtained for the static BH case are in agreement with those of Matt et al. 1992 [18]. Figs. 7 and 8 present the dependence of EW on h for dierent cases. 3. Conclusions Good to high energy resolution, high sensitivity X-ray spectra will be the goal of near future missions like AXAF, XMM and ASTRO-E, after the preliminary but quite encouraging data provided by the japanese satellite ASCA. The work done so far in modelling iron emission lines has to be matched with a more careful treatment of the underlying continuum component, so that self-consistent calculations, including reected continuum and line proles, will be used to t the data. This issue is important also because the observed line proles are usually obtained by subtraction of a continuum component for which important relativistic eects, such as the iron edge smearing, have not been taken into account. References [1] Cunningham C.T. & Bardeen J.M. (1973), ApJ 183, 273 [2] Cunningham C.T. (1975), ApJ 202, 788 [3] Shakura N.I & Sunyaev R.A. (1973), A&A 24, 337 [4] Thorne K.S. (1974), ApJ 191, 507 [5] Laor A. (1991), ApJ 376, 90 [6] Kojima Y. (1991), MNRAS 250, 629 [7] Hameury J.M. et al. (1994), A&A 287, 795 [8] Karas V. et al. (1995), ApJ 440, 108
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