Jets, corona and accretion disk in the black hole source SS433: Monte-Carlo simulations

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1 Jets, corona and accretion disk in the black hole source SS433: Monte-Carlo simulations Yu. M. Krivosheyev, G. S. Bisnovatyi-Kogan,, A. M. Cherepashchuk and K. A. Postnov Space Research Institute of the RAS (IKI), Profsoyuznaya str. 84/32, Moscow , Russia Moscow Engineering Physics Institute, Kashirskoye sh. 31, Moscow , Russia Sternberg Astronomical Institute, Moscow State University, Universitetsky pr., 13, Moscow , Russia Abstract. We propose a physical model of the X-ray emitting region of the Galactic microquasar SS433 that includes thermal emission from the accretion disk, jets and hot corona where the photons of different origin are comptonized. We have employed a Monte-Carlo technique to calculate X-ray continuum spectra of comptonized thermal plasma and applied it to describe the broadband X-ray continuum of SS433 observed by INTEGRAL. From comparison with INTEGRAL observations, we estimate physical parameters of the X-ray emitting region in SS433 and present model spectra for different viewing angles of the object. Keywords: X-ray binaries, SS433, Monte-Carlo, scattering, bremsstrahlung PACS: Dd, Sd, Jx, Pq, Nv, Gz, Lf, Jp, Fs 1. INTRODUCTION. Presently, SS433 is recognized as a galactic microquasar with precessing supercritical accretion disk around a relativistic compact object with relativistic jets [1]. The optical companion V1343 Aql was first identified in the survey of stars exhibiting H α emission of [2]. The system is located at a distance of about 5.2 kpc [3] nearly in the galactic plane. SS433 is a close binary system with an orbital period of 13.1 days [4]. The uniqueness of this source comes from the presence of narrow oppositely directed subrelativistic jets emitting red and blue-shifted periodically variable lines. The optical star fills its critical Roche lobe supplying powerful and almost continuous flow of gas onto the relativistic star. A supercritical accretion disk forms together with narrow jets of gas propagating perpendicular to the disk plane from the central regions of the disk and having a relativistic speed of 0.26c. Here we will concentrate on the broad-band X-ray spectrum of SS433. In the last years the source was intensively observed by RXTE [5, 6], XMM [7], Chandra [8] and INTEGRAL [9, 10]. INTEGRAL observations of SS433 discovered a hard X-ray spectrum from the source up to 100 kev visible over the whole precessional period 162 days. This suggests the presence of an extended hot region in the central parts of the supercritical accretion disk [10].

2 jet 0 13 r jet = 10 cm r disk 12 = 2 10 cm disk 0 FIGURE 1. Schematic picture of the model adopted for SS MODEL OF SS433 The schematic picture of the source is presented in Fig.1. The collimated, oppositely directed jets were proposed by [11] and [12] to explain the observed moving emission lines at unusual wavelengths. The X-ray imaging observations of [13] also support the jet model. The jets must be supplied with matter from some source, so a hypothesis of a compact star in a close binary system was proposed. The discovery of the 13- day periodicity in the radial velocity of the "stationary" SS433 emission-line system [14] proved this picture. The donor star fills its critical Roche lobe providing powerful flow onto the compact object. Thus, an accretion disk forms with a pair of narrow jets propagating perpendicular to the disk plane. The presence of a hot corona above the inner regions of the accretion disk was suggested to explain the hard X-ray spectrum of SS433 observed by INTEGRAL over the entire precession period [10] Supercritical accretion disk In SS433, the optical star fills the Roche lobe and supplies matter to the accretion disk at a rate of 10 4 M /yr. The most emission from the disk is generated in its central

3 regions, so the exact value of the accretion disk radius is of small importance. Far away from the central parts, the standard accretion disk theory [15] can be used to estimate its geometrical thickness. Taking M = 7M, Ṁ 2Ṁ cr and α = 0.1 we obtain that in the outer gas-dominated region with free-free opacity, which lies beyond the radius r bc = cm for the adopted parameters, ( ) 3/20 ( ) Ṁ M 9/10 ( ) R 9/8 h = α 1/10 M cr M Sun 3R g [ ( ) ] R 1/2 3/20 1 (1) 3R g For the disk height h growing linearly with radius the disk opening angle θ disk can be directly found from the relation: h = r tan θ disk 2 At the accretion disk radius R disk = cm the half-thickness of the disk is cm, so the angle between the disk surface and the equatorial plane in (2) is θ disk /2 2. We will consider an optically thick disk, so in our calculations this conical surface will be used as an opaque screen for incident photons. (2) 2.2. Thermal X-ray jets The evidence for X-ray emitting jets in SS433 was found in X-ray observations of the source. The EXOSAT satellite detected Doppler shifted iron lines moving periodically in accordance with the precessional Doppler curve of the source [16], suggesting the X-ray emission origin from the base of jets. We assume X-ray jets to have a conical shape, as inferred from the coincidence of the jet opening angle θ j in the optical and X-rays [17], with the jet opening angle θ j = 1.2. We also assume that the jet temperature changes with height according to the law corresponding to the jet cooling due to adiabatic expansion of ideal monoatomic gas (3). The temperature at the base of the jet is assumed to be the same as that of the corona and is set to be T 0 = 19keV, the best-fit value found in our calculations. ( r0 ) 4/3 T = T 0,T ρ 2/3,ρ r 2 (3) r The density dependence along the jet follows from the continuity equation. The jet mass outflow rate M j = nm p V S, where n is the number density of protons at r, m p is the proton mass, V = 0.26c and S = πr 2 tanθ jet /2 is the jet cross-section. So we can write ( r0 ) 2, n = n 0 (4) r

4 where M jet n 0 = πm p Vr0 2 tan 2 θ jet 2 (5) 2.3. Corona The corona is hot and has a low density compared to the accretion disk. In the present paper we assume a spherical isothermal corona with temperature 19keV = K. For simplicity, the radial density dependence in the corona is assumed to obey the same law as in the jet but with the different initial value at r = cm. Then the optical depth for Thomson scattering in the corona is r cor τ cor = σ T n 0 r 0 r 2 0 dr (6) r2 The radius of the corona can be expressed in terms of its optical depth as ( r cor = r 0 / 1 τ ) cor σ T n cor r 0 n cor = ( ) cm 3 σ T = cm (7) The analysis of the observed wide primary orbital hard X-ray eclipse and the precessional variability suggests large visible size of the hot corona comparable to that of the accretion disk, but it is not absolutely certain. We have fitted the observed X-ray spectra with two models of corona: 1) the corona is not eclipsed by the precessing accretion disk and has r cor = cm; 2) the corona is fully eclipsed with r cor = cm. Both models can reproduce the observed X-ray spectrum, but the first model gives a slightly better fit The structure of the computational domain We consider the accretion disk with a spherical corona above its inner regions, and two jets propagating perpendicular to the disk plane. This picture is symmetric with respect to the disk plane, so we will consider only its upper half (Fig.1). The computational domain is bounded by the surface of the accretion disk (from bottom) and a sphere with radius equal to the jet height. The inner accretion disk is assumed to have constant h/r ratio. The conical jet, disk and corona begin at the coordinate origin. Both jet and corona are assumed to consist of fully ionized hydrogen plasma. Guided by observations of the accretion wind photosphere [17], we set the visible jet base at r 0 = cm and assume the inner spherical boundary of the corona to lie at the same radius.

5 10-2 I, phot/cm 2 /sec/kev h, kev FIGURE 2. The comparison of the Monte-Carlo simulated X-ray continuum of SS433 with observations. θ obs = 60 0, only the jet and corona emission are considered. The radius of spherically-symmetric corona is r cor = cm, the mass loss rate in the jet is M jet = g/s. The viewing angle of the system is θ obs = RESULTS OF SIMULATIONS 3.1. Observational data For spectral analysis, we have made use INTEGRAL observations of SS433 in May 2003 obtained simultaneously by IBIS/ISGRI ( kev) and JEM-X (3-20 kev) telescopes [9, 10]. The source was observed near the precessional phase 0 (the T3 moment) when the accretion disk is maximally open and the flux from the source is also maximal. The angle between the line of sight and the blue jet (i.e. the jet pointing to the observer) at this phase is The Doppler redshift at this precessional phase is z 0.1. Spectral lines near 7 kev were ignored. The X-ray data were processed using the standard OSA 5.1 INTEGRAL software developed by the INTEGRAL science data center. A model fit of the observed spectrum is accepted by achieving the minimal reduced χ 2 (χ 2 red = χ2 /N), N = n n par is the number of the degrees of freedom Bremsstrahlung comptonization model In this section we present the results of X-ray spectral modeling neglecting the radiation from the disk. The distance to SS433 is fixed at d = 5 kpc = cm. The

6 TABLE 1. Physical parameters of two corona models Parameter Eclipsing corona Non-eclipsing corona r cor, cm τ cor n cor, cm Ṁ j, g/s 3 4 L kin, erg/s χ CL 91.6% 95.4% emerging X-ray spectrum in the 3-90 kev range is assumed to be formed due to freefree emission of corona and jet with account for Compton scattering. The temperature of the corona and the jet base is T cor = 19 kev. Fig.2 shows the comparison of the computational results with the observed broadband X-ray spectrum of SS433 for a corona which is not fully eclipsed by the disk. For the model with non-eclipsing corona χ 2 = 21.4 shown in Fig. 2 the reduced χred 2 = 0.630, and the confidence level of the fit at N do f = 34 is 95.4%. For the second model with fully eclipsing corona χ 2 = for 34 degrees of freedom, so the reduced χred 2 = and the corresponding confidence level of the fit is 91.6%. Thus the model with non-eclipsing corona with smaller χ 2 seems to be more realistic. All model parameters are listed in Table Influence of the accretion disk emission The two-component model described in the previous section gives satisfactory results, but the accretion disk emission could also affect the emergent X-ray continuum. To test this in our calculations, we included the emission from the inner parts of the accretion disk with the following spectrum. L ν L ν ( ) = 4L disk /7ν ννc 1/3,ν c < νc ) = 4L disk /7ν c exp (1 ν,ν > ν νc c ν c = ht e f k (Here L disk is the total luminosity of the accretion disk.) For the assumed disk parameters we obtain that T e f K 0.05 kev. The results of the three-component model simulations are presented in Fig. 3. It is clear that generally the disk radiation can strongly change the emerging spectrum, but in the energy range kev the spectral difference is insignificant because of a comparatively low effective temperature T e f 0.05 kev of the disk emission. In principle, the spectrum at lower energies can be used to estimate the accretion disk physical parameters.

7 I, phot/cm 2 /sec/kev ,01 0, h, kev FIGURE 3. Comparison of two- and three-component emission models. The solid curve corresponds to the model with disk radiation, the dashed one to the model without disk radiation. r cor = cm, M jet = g/s. 4. CONCLUSION We have developed a Monte-Carlo code for modeling X-ray (3-90 kev) continuum of SS433 based on the realistic physical model of the source, including the accretion disk, thermal jet and hot scattering corona. We found the model parameters that provide good qualitative and quantitative agreement with INTEGRAL observations. More details about the model, Monte Carlo techniques and fitting procedure can be found in [18]. We conclude that the observed broadband X-ray continuum of SS433 is dominated by thermal emission of the jet and rarefied hot corona around the inner parts of the supercritical accretion disk, with addition from the inverse Compton scattering on hot electrons of the corona. The jet mostly contributes in the soft X-ray band, while hard X-ray emission is dominated by thermal emission from the corona and inverse Compton scattering of soft thermal X-ray photons. The emission from inner regions of the accretion disk also contributes to the spectrum, but mostly at energies below 1 kev. ACKNOWLEDGMENTS The work is partially supported by the Russian Foundation for Basic Research under grants , , by the Russian Federation President s grant for supporting of leading scientific schools NSh

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