2.1 Chandra -HETGS Observation

We observed PG1211+143 over six visits (Proposal 16700515, PI: Lee) from 2015 April 9 (MJD 57121.362) to April 17 (MJD 57130.351) with the High Energy Transmission Grating Spectrometer (HETGS ; Canizares et al., 2005; Weisskopf et al., 2002) using the Chandra  Advanced CCD Imaging Spectrometer (ACIS; Garmire et al., 2003). The observation log is listed in Table 1. The total useful exposure time of the observations was about $433$ks, with individual exposure times ranging from 42-105ks. The HETGS has two grating assemblies, the medium energy grating (MEG ) and the high energy grating (HEG ). The MEG has a full-width at half-maximum (FWHM) resolution of 0.023 Å and covers 0.4-7keV, whereas the HEG has a FWHM resolution of 0.012 Å and covers 0.8-10keV. The MEG response is therefore more efficient in the soft band, while the HEG can more effectively measure the hard band.

We used the CIAO package (v.4.8; Fruscione et al., 2006), along with calibration files from the CALDB (v 4.7.0), to process the HETGS data in the standard way, which produces the plus and minus first-order ($m=\pm 1$) MEG and HEG data and the response files. Spectra files were extracted using the CIAO tool tgextact from the $-1$ and $+1$ arms of the MEG and HEG . Redistribution and response files were generated using the CIAO tools mkgrmf and fullgarf , respectively.

We regridded the HEG spectra to match the MEG bins, and then combined the HEG and MEG data using the combine_datasets function in the Interactive Spectral Interpretation System (ISIS ) package² v.1.6.2-35 (Houck & Denicola, 2000) for spectral fitting. We further rebinned the combined data (see discussion in the Appendix), starting at 0.4 keV, to a minimum signal-to-noise of 4 and a minimum of 4 spectral channels per bin (i.e., approximately the spectral resolution of the MEG detector). The signal-to-noise criterion determined the binning below $\sim 1$keV, while the minimum channel criterion determined the binning above $\sim 1$keV. While not ideal, this was necessary for the signal-to-noise required of our analysis, although a blind-line search with uniform MEG binning is consistent with the results presented in this paper (see Appendix). We fit our spectral models only in the 0.5-6.75 keV range (all energy ranges above refer to energies in the observer frame), which corresponds to the 0.54-7.3 keV range in the rest frame.