4.1 Continuum Modeling

We used XSPEC package v.12.9.0 (Arnaud, 1996) in ISIS  (v.1.6.2-35; Houck & Denicola, 2000) to model the intrinsic continuum of the combined MEG and HEG spectra in the (rest frame) energy range of 0.54-7.3 keV. We matched the continuum with a phenomenological model of $ \textsf{tbnew}~\times \textsf{highecut}~\times
(\textsf{diskbb}~+ \textsf{zpowerlw}~)$. The soft excess continuum below 1.1keV is dominated by the XSPEC accretion disk model diskbb  (Mitsuda et al., 1984; Makishima et al., 1986), consisting of multiple blackbodies, that has one physical parameter: the peak temperature ( $ T_{\rm in}$) at the apparent inner disk radius ( $ R_{\rm in}$) (see Kubota et al., 1998; Makishima et al., 1986). The hard excess continuum above 1.1keV is dominated by the XSPEC power-law model zpowerlw , which describes the Comptonization corona of the accretion disk with the photon index ($ \Gamma$). To account for foreground absorption affecting the shape of the soft X-ray continuum, we used the tbnew component (Wilms et al., 2000) for better flexibility in fitting the continuum shape in our phenomenological model. However, it does not necessarily represent any quantitative physical property of our Galaxy or the host galaxy of PG1211+143 . A high-energy exponential cutoff (highecut with cutoff energy $ E_{c}$ and folding energy $ E_{f}$ listed in Table 2) was also used to account phenomenologically for the curvature in the hard X-ray spectrum. Table 2 details the best fit parameters based on this model.

Figure: The X-ray spectrum taken with the Chandra -HETGS in 2015 April averaged over 390ks (i.e., excluding the 4th observation). The upper panel shows the first-order spectrum (black line) and the best-fitting continuum model plus iron emission lines (red line; tbnew *highecut *(diskbb +zpowerlw +$ \sum$ zgauss )), while the lower panel plots the $ \chi ^2$ residuals between the observation and the best-fitting model.
\includegraphics[width=3.2in, trim = 0 0 0 0, clip, angle=0]{figures/fig3_continuum_fit.eps}

Ashkbiz Danehkar