76 Acta Polytechnica CTU Proceedings 2(1): 76–80, 2015 76 doi: 10.14311/APP.2015.02.0076 Probing the Accretion Processes in Soft X-Ray Selected Polars I. Traulsen1, K. Reinsch2, A. D. Schwope1 1Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany 2Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany Corresponding author: itraulsen@aip.de Abstract High-energy data of accreting white dwarfs give access to the regime of the primary accretion-induced energy release and the different proposed accretion scenarios. We perform XMM-Newton observations of polars selected due to their ROSAT hardness ratios close to -1.0 and model the emission processes in accretion column and accretion region. Our models consider the multi-temperature structure of the emission regions and are mainly determined by mass-flow density, magnetic field strength, and white-dwarf mass. To describe the full spectral energy distribution from infrared to X-rays in a physically consistent way, we include the stellar contributions and establish composite models, which will also be of relevance for future X-ray missions. We confirm the X-ray soft nature of three polars. Keywords: cataclysmic variables - polars - spectroscopy - photometry - X-rays - individual: AI Tri, QS Tel, RS Cae. 1 Introduction Accretion onto magnetic white dwarfs involves plasma under extreme physical conditions, in particular high temperatures up to millions of Kelvin. X-ray observa- tions of the discless AM Her-type systems (“polars”) provide direct insight into the accretion processes and the opportunity to study related system properties. Hard X-ray emission (E > 0.5 keV) arises from the cooling accretion column above the white dwarf and soft (E < 0.5 keV) from the heated accretion region on the white-dwarf surface. Model calculations and re- cent spectral analyses reveal complex structures of the emission regions and a wide range of temperatures and densities. Several systems are found at excesses of soft over hard X-ray flux by factors up to 1 000, which can be interpreted as a sign of inhomogeneous accretion. Full understanding of the accretion processes and the bi- nary system requires multi-wavelength data, since the different system components dominate the spectral en- ergy distribution (SED) at different wavelengths from infrared up to X-rays. In a campaign of dedicated XMM-Newton and op- tical observations of selected AM Her-type systems, we spectrophotometrically study the parameters and flux contributions of their components. We concentrate on the conditions in the emission regions in the post-shock accretion column and on the heated white dwarf, flux and luminosity ratios and their strong dependence on the choice of the underlying spectral models. Here, we summarize our work on three soft X-ray selected polars and our efforts to establish consistent multi-wavelength models. 2 Observed SEDs Starting in 2005, we obtained XMM-Newton X-ray and ultraviolet data of AI Tri, QS Tel, and RS Cae (obs. IDs 0306840901, 0306841001, 0404710401, 0554740801; Traulsen et al., 2010, 2011, 2014), covering one to five orbital cycles per object. On the basis of optical mon- itoring, the TOO observations were triggered during high and intermediate high states of accretion. Opti- cal photometry and – for QS Tel – spectroscopy were performed (quasi)simultaneously. To construct the full long-term SEDs of the objects from infrared to X-ray wavelengths, we use publicly available archival data, in particular of the WISE, 2MASS, HST, FUSE, and ROSAT archives. Figure 1 shows all data, the main system components being marked according to their ap- proximate flux maxima in the middle panel as follows: a. the secondary star in the IR, b. the cyclotron emis- sion in the IR to optical, c. the accretion stream and d. the white-dwarf primary in the optical to UV, e. the accretion-heated region on the white-dwarf surface in the extreme UV to the soft X-ray regime, and f. the post-shock accretion column in hard X-rays. All panels include both high-state and low-state data, identifiable by their different flux levels. The distinct soft X-ray / EUV components at high states are clearly visible, compared for example to the low-state SED of EF Eri 76 http://dx.doi.org/10.14311/APP.2015.02.0076 Probing the Accretion Processes in Soft X-Ray Selected Polars (Schwope et al. 2007) or to the serendipitously discov- ered X-ray hard polar 2XMMp J131223.4+173659 (Vo- gel et al. 2008). At times of high soft X-ray flux, the optical and soft X-ray data show pronounced short-term variability (“flickering”). 10 5 10 4 10 3 10 2 10 1 Wavelength [Å] 10−6 10−5 10−4 10−3 10−2 10−1 AI Tri XMM ROSAT EUVE GALEX FUSE IUE HST ground 2MASS WISE 10−6 10−5 10−4 10−3 10−2 10−1 E ne rg y fl ux [ k eV c m − 2 s− 1 ] QS Tel a b c d e f 10−6 10−5 10−4 10−3 10−2 10−1 Energy [keV] 0.001 0.01 0.1 1.0 10.0 RS Cae Figure 1: Observed SEDs of three X-ray soft polars from IR to X-rays: our XMM-Newton and optical plus archival data at different epochs and accretion states (cf. Traulsen et al. 2010, 2011, 2014). The X-ray spec- tra are unfolded using the best-fit models. 3 Multi-Wavelength Modeling To consistently describe the spectral energy distribu- tion and multi-band light curves, we synthesize spectral models for one uniform set of system parameters and calculate the corresponding light curves. For the differ- ent system components, we adopt (referring to the la- bels used in Sect. 2 and Fig. 1): a. a PHOENIX stellar atmosphere model of an M star (Hauschildt & Baron 1999), b. a cyclotron component of a stratified post- shock accretion column (Fischer & Beuermann 2001), c. a simplified accretion-stream component of a 3D bi- nary model (Staude et al. 2001), d. a non-LTE white- dwarf atmosphere model (Werner & Dreizler 1999), e. a single- or multi-temperature black body or hot white- dwarf atmosphere, f. a single- or multi-temperature plasma model of the accretion column. The input parameters of the models are determined from observational data, where possible, and estimated as typical values for primary white dwarf and secondary M star otherwise (e.g. Townsley & Gaensicke 2009, Knigge 2006). In particular, orbital period, inclina- tion, and magnetic field strength are available from op- tical spectroscopy and polarimetry; parameters of the accretion-induced emission are fitted to the X-ray spec- tra (cf. Sect. 4 and Table 1). Table 1: Parameters of the best fits to the XMM- Newton spectra. Bolometric ROSAT flux ratios at fixed temperatures are calculated from (a)Schwarz et al. 1998, (b)Schwope et al. 1995, (c)Burwitz et al. 1996, applying corrections of κbb = 2.5, κbr = 4.8. AI Tri QS Tel RS Cae black body multi-T single-T single-T plasma model multi-T two-T single-T NH,ISM [cm −2] ∼ 1020 ∼ 1020 2±1×1019 kTbbody [eV] 44 ± 5 20 ± 4 36 ± 1 kTplasma [keV] 1 .. 20 0.2 .. 4.0 7 ± 3 NH,intr [cm −2] 3±2×1023 1022..1023 ∼ 4×1023 log Ṁ [M�/yr] −11.. − 9 ∼ −10 ∼ −10 Fbb/Fplasma ∼ 3..200 ∼ 10..100 ∼ 11 Fbb/Fbr, ROSAT ∼ 170(a) ∼ 80(b) ∼ 90(c) Synthetic cyclotron light curves are derived from the phase-dependent model spectra of the accretion- column (b) by folding them with the Johnson and XMM-Newton OM filter bandpasses, and white-dwarf and accretion-stream light curves from model compo- nents c and d. By comparing them with observational data, we determine their respective phase shifts and in- tensities. As an example, Fig. 2 shows the cyclotron spectra calculated for RS Cae, Fig. 3 the corresponding synthetic UBV RI light curves. Using a consistent set of parameters for all com- ponents, we thus establish a physically realistic and consistent multi-wavelength model of the whole binary system, missing only the unknown spectral contribu- tion of the accretion stream. We describe its success- ful application to the multi-wavelength data of RS Cae in Traulsen et al. (2014). The most relevant limita- tion relates to the different observational epochs of the high-state data. While we need low-state spectra in the IR and UV to identify the secondary and (unheated) 77 I. Traulsen, K. Reinsch, A. D. Schwope primary star, all high-state data should be, ideally, ob- served simultaneously, due to the high variability of the accretion processes. 4 Probing the Accretion Processes in X-Rays As described above, X-ray data are not the sole, but the main source of information on the accretion mechanisms in magnetic CVs. They give us access to characteris- tic determinants of the systems and their evolution like mass accretion rate, component masses, and bolometric fluxes / luminosities, which let us distinguish between accretion scenarios like standing or buried shocks or inhomogeneous accretion. These objectives, however, are limited by the complexity of the puzzle and by the energy resolution and signal-to-noise ratio of currently available X-ray data. Several model approaches have been developed, each of them focusing on different as- pects, as Cropper et al. (1999, mass determination and effective spectral fitting), Fischer & Beuermann (2001, column structure, SED coverage). Mass and flux deter- mination particularly depend on the underlying spectral models (see also Cropper et al. 1999). 103 104 105 Wavelength [Å] 10−21 10−20 10−19 10−18 10−17 10−16 10−15 S im ul at ed f lu x [e rg c m − 2 s− 1 Å − 1 ] U B V R I Figure 2: Spectral models of cyclotron emission for B = 36 MG, ṁ = 0.01, 1.0 g cm−2 s−1, and the es- timated accretion geometry of RS Cae: orbital mean (red) and phase-resolved from ϕmag = 0.0 (light) to 0.5 (dark). Dotted: Johnson filter bandpasses, used to de- rive the light curves in Fig. 3. The X-ray soft component is usually modeled by an absorbed black body, which is on the one hand an appropriate approach for CCD data, not resolving the line features. On the other hand, non-LTE processes and the metal-richness of the hot photosphere have a non-negligible effect on the spectral continuum in the UV to X-rays (cf. Rauch 2003 and references therein). The relevance of consistent non-LTE modeling, con- sidering line-blanketing by heavier elements has been demonstrated for non-accreting hot white dwarfs (e.g. Traulsen et al. 2005). Bolometric fluxes derived from black-body fits to XMM-Newton data are lower by fac- tors up to five than from non-LTE models, which typi- cally yield lower effective temperatures and hydrogen absorptions. Multi-temperature models, designed to reproduce the temperature gradient in the heated ac- cretion region, result in increased bolometric fluxes by at least 50 % with respect to single-temperature mod- els. To illustrate the differences between the models, we simulate pointed observations with the upcoming eROSITA mission (Merloni et al. 2012), which will have a higher effective area at energies between 0.2 and 2 keV than 0.0 0.5 1.0 1.5 Magnetic phase 21 20 19 18 17 M ag ni tu de ( V eg a sy st em ) 0.0 0.5 1.0 1.5 X−ray dip phase Figure 3: Simulated and observed light curves of RS Cae. Left: UBV RI models (bottom to top), de- rived from the spectra in Fig. 2. Right: SMARTS/B, XMM-Newton/U and UVW1. Dashed: cyclotron emis- sion, dotted: white dwarf and accretion stream. XMM-Newton and ROSAT. Figure 4 shows 50 ks eROSITA spectra based on fits to the XMM-Newton data of AI Tri (reduced χ2 between 0.96 and 1.01). For the X-ray hard post-shock spectra, we adopt the radiation-hydrodynamic models by Fischer & Beuer- mann (2001). They are valid for shallow accretion columns, both for the shock scenario dominated by bremsstrahlung cooling and the bombardment scenario dominated by cyclotron cooling. To make them avail- able for automated spectral fitting of X-ray CCD and grating data, we incorporate their temperature and den- sity distributions in xspec, using the local mass flow densities ṁ and magnetic field strengths B listed in their paper and white-dwarf masses MWD = 0.6, 0.8, and 1.0 M�. We parameterize the distributions for 30 layers of a stratified column, and add up 30 apec plasma components to the final combined column spec- trum. Our models include velocity shifts and broad- ening of the emission lines by stream motion, gravity, orbital motion, and the changing viewing angle. Adding a pexmon reflection component (Nandra 2007) and multiplying it with the same orbital velocity term, we get a comprehensive description of the phase-dependent emission induced by the accretion column, in particu- lar of the iron lines between 6.4 and 6.9 keV. Figure 5 shows composite accretion-column models compared to 78 Probing the Accretion Processes in Soft X-Ray Selected Polars a 50 ks synthetic spectrum of an (illustrative) polar with similar fluxes to AM Her. The models include the same reflection term and different column parameters. The unabsorbed bolometric fluxes of the column models are typically by about 50 % higher than of the corresponding single-temperature fits. The intrinsic ab- sorption and reflection components have a considerable impact on the fluxes (cf. Cropper et al. 1999), which increase by factors up to 15 compared to pure plasma models. The soft-to-hard flux ratios, thus, may signif- icantly vary for the same object and observation, de- pending on the model choice. 0.0 0.5 1.0 1.5 2.0 2.5 N or m al . c ou nt s s− 1 ke V − 1 −5 0 Energy [keV] ∆ χ 0.2 0.3 0.4 0.5 + Single−T black body + Multi−T black body + NLTE WD atmosphere Figure 4: Synthetic spectra of a 50 ks eROSITA pointed observation, based on different models for the heated white dwarf in the X-ray soft polar AI Tri. 3 4 5 6 7 8 9 10 Energy [keV] 10 −2 10 −1 1 N or m al iz ed c ou nt s s− 1 ke V − 1 – MWD = 0.6 – 1.0 MO • – B = 100 – 10 MG – m = 0.01 – 100 g cm–2 s–1 ˙ Figure 5: Simulated 50 ks eROSITA observation of a polar at ṁ = 1.0 g cm−2 s−1, B = 30 MG, MWD = 0.8 M�, scaled to the XMM-Newton spectrum of AM Her (black: data, yellow: model). Each colored line represents one parameter that is varied while the other parameters and the reflection component are fixed (dashed: lowest, solid: highest value). 5 Conclusions Our XMM-Newton observations confirm the soft X-ray excess of the three selected polars and indicate inho- mogeneous accretion processes. We develop composite models including the contributions of the stellar atmo- spheres and the X-ray emitting accretion regions for a physically realistic description of the binary system and the accretion processes. Simultaneous multi-λ observa- tions are relevant for a fully consistent SED fitting. The upcoming eROSITA survey will significantly increase the total number of known systems and of systems for that reliable soft-to-hard ratios can be derived. Sur- vey and pointed observations will enable us to better distinguish between different models of the accretion processes, refine them, and push our knowledge about the physical properties of the X-ray emission regions of polars. Acknowledgement Our research was supported by DLR under grant num- bers 50 OR 0501, 50 OR 0807, and 50 OR 1011. References [1] Burwitz, V. et al.: 1996, A&A 305, 507 [2] Cropper, M. et al.: 1999, MNRAS 306, 684 doi:10.1046/j.1365-8711.1999.02570.x [3] Fischer, A., Beuermann, K.: 2001, A&A 373, 211 [4] Hauschildt, P. H., Baron, E.: 1999, J. Comp. Appl. Math. 109, 41 doi:10.1016/S0377-0427(99)00153-3 [5] Knigge, C.: 2006, MNRAS 373, 484 doi:10.1111/j.1365-2966.2006.11096.x [6] Merloni, A. et al.: 2012, arXiv:1209.3114 [7] Nandra, K. et al.: 2007, MNRAS 382, 194 doi:10.1111/j.1365-2966.2007.12331.x [8] Rauch, T.: 2003, A&A 403, 709 [9] Schwarz, R. et al.: 1998, A&A 338, 465 [10] Schwope, A. D. et al.: 1995, A&A 293, 764 [11] Schwope, A. D. et al.: 2007, A&A 469, 1027 [12] Staude, A., Schwope, A. D., Schwarz, R.: 2001, A&A 374, 588 [13] Townsley, D. M., Gänsicke, B. T.: 2009, A&A 693, 1007 [14] Traulsen, I. et al.: 2005 in 14th European Workshop on White Dwarfs, D. Koester & S. Moehler (eds.), ASP Conf. Ser. 334, 325 [15] Traulsen, I. et al.: 2010, A&A 516, A76 [16] Traulsen, I. et al.: 2011, A&A 529, A116 79 http://dx.doi.org/10.1046/j.1365-8711.1999.02570.x http://dx.doi.org/10.1016/S0377-0427(99)00153-3 http://dx.doi.org/10.1111/j.1365-2966.2006.11096.x http://dx.doi.org/10.1111/j.1365-2966.2007.12331.x I. Traulsen, K. Reinsch, A. D. Schwope [17] Traulsen, I. et al.: 2014, A&A 562, A42 [18] Vogel, J. et al.: 2008, A&A 485, 787 [19] Werner, K., Dreizler, S.: 1999, J. Comp. Appl. Math. 109, 65 DISCUSSION CHRISTIAN KNIGGE: Your data on AI Tri seems to show that the hard X-ray flux actually decreases as the accretion rate (and soft X-rays) increase. What is the interpretation of this? KLAUS REINSCH: It is related to the response of the accretion shock height to the changing specific ac- cretion rate. In addition, a higher fraction of inho- mogeneous accretion events can suppress X-ray hard emission. The first observation of AI Tri during an ex- tremely soft state does not cover a full binary orbit. 80 Introduction Observed SEDs Multi-Wavelength Modeling Probing the Accretion Processes in X-Rays Conclusions