The relxillDCp model combines the ionized disc reflection code xillverDCp (García et al. 2013, 2016) with the convolution model relconv (Dauser et al. 2013). The relconv model determines the relativistic effects in the reflection spectrum and assumes a broken power-law emissivity profile for the illumination of the disc by an X-ray corona. The emissivity profile has the following form: $\epsilon (r)\propto r^{-q_{{\rm in}}}$ for rin ≤ r ≤ rbr, and $\epsilon (r)\propto r^{-q_{{\rm out}}}$ for $r_{\rm br}\le r\le r_{{\rm {\rm out}}}$, where qin and qout represent inner and outer emissivity indices, respectively, rbr is the break radius, rin and rout are the inner and outer disc radii, respectively. The other parameters that characterize the disc reflection model are: black hole spin (a), the inclination angle (θ○) of the disc to the observer, reflection fraction (refl_frac), iron abundance (AFe), ionization state (log ξ), and number density (ne) of electrons in the disc atmosphere. The disc irradiation profile is considered Newtonian over the outer regions of the disc, and hence we fixed the outer emissivity index at 3. We fixed the break radius at 6rg, which is a typical AGN coronal radius (e.g. Wilkins & Fabian 2011; Mallick et al. 2021). We assume the solar abundance of iron and fixed the inner and outer radii of the accretion disc at the innermost stable circular orbit (risco) and 1000rg (rg = GMBH/c2), respectively. We set refl_frac=−1 in the relxillDCp model to solely describe the disc reflection. Since relxillDCp considers the thermal Comptonization model nthComp (Zdziarski, Johnson & Magdziarz 1996; Źycki, Done & Smith 1999) as the irradiating (primary) continuum, we replaced the zpowerlw continuum with nthComp. The slope of the nthComp and relxillDCp components are tied. The seed photon temperature in the nthComp model was set at the maximum possible disc temperature (see column 8 of Table 2) for each source. We fixed the electron temperature of the hot coronal plasma at a typical value of 100 keV. The relative strength of reflection was measured as a ratio of the disc reflected flux to the irradiating primary source flux in the 0.3–10 keV band. We find that the broad-band best-fitting spectral model is Tbabs×(relxillDCp+nthComp) for all the sources in the sample, except for J1559 and POX 52. The source, J1559, showed an absorption feature at ∼0.7 keV, which was modeled by a Gaussian absorption line gabs. The gabs component improves the fit statistic by Δχ2 = 92.9 for 3 free parameters. The expression for the broad-band best-fit model of J1559 is Tbabs×gabs×(zgauss+relxillDCp+nthComp). To model the absorption curvature present in the 1–2 keV band of POX 52, we included an ionized partial covering absorption component (zxipcf; Reeves et al. 2008). The zxipcf model has three free parameters: column density (NH,wa), ionization state (log ξwa) of the absorbing medium and covering fraction (Cf). The inclusion of the ionized absorption component provided an improvement of Δχ2 = 237.5 for three additional free parameters. The maximum-likelihood ratio (MLR) test suggests that the zxipcf model component is ≥99.99 per cent significant. The best-fitting model for POX 52 has the following expression: Tbabs×zxipcf×(relxillDCp+nthComp). We find that the relativistic reflection from an ionized, higher density accretion disc can self-consistently explain the soft X-ray excess emission for the low-mass AGN sample. The best-fitting unfolded spectral models, model components, and data-to-model ratio plots are shown in Fig. 1. We show the broad-band photon count spectra, best-fitting models, and residuals plots in Fig. A2.
Citation: Zdziarski, Johnson & Magdziarz 1996
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