3.1 X-ray Spectral Hardness

We begin with hardness ratio analysis to determine whether we can reasonably co-add all spectra for analysis. We used the aglc  program³originally developed for the Chandra  Transmission Grating Data Catalog (TGCat; Huenemoerder et al., 2011) to create light curves in 5000-s bins for three bands: the soft band (S: 0.4-1.1 keV), the medium band (M: 1.1-2.6 keV), and the hard band (H: 2.6-8 keV). The MEG data were used to compute the light curves in the S band, while the light curves in the M and H bands were extracted from a combination of the MEG and HEG data over the spectral regions of 0.4-4 and 4-8 keV, respectively. Positive and negative first-order spectra for HEG and MEG were also combined to generate the light curves of the broad band on the 0.4-8 keV spectral region (the light curve in Figure1, upper panel).

Figure 2: The hardness ratios $\mathrm {HR_{1}}=(M-S)/(S+M+H)$ (upper panel), and $\mathrm {HR_{2}}=(H-M)/(S+M+H)$ (lower panel) plotted against the light curve of all the energy bands ($S+M+H$).

We define the count rate hardness ratios ( $\mathrm{HR_{1}}$ and $\mathrm{HR_{2}}$) using the soft (S), medium (M), and hard (H) light-curve bands as follows:

$$\mathrm{HR_{1}} = \frac{M-S}{S+M+H},$$ (1)

$$\mathrm{HR_{2}} = \frac{H-M}{S+M+H},$$ (2)

The uncertainties on the hardness ratios are estimated by propagating the band errors assuming Gaussian statistics. By these definitions, as the X-ray spectra become harder, the hardness ratio $\mathrm{HR_{2}}$ increases. The disk blackbody is expected to dominate the soft band, while the powerlaw and warm absorber are the dominant features in the medium band. We note that the powerlaw dominates the hard band.

The hardness ratios $\mathrm{HR_{1}}$ and $\mathrm{HR_{2}}$ are plotted against the sum of the light curve of all the energy bands ($S+M+H$) in Figure 2. The hardness ratio $\mathrm{HR_{1}}$ increases with the count rate of the M band, and decreases with the count rate of the S band, so a decrease in $\mathrm{HR_{1}}$ may be related to a stronger soft excess of an accretion disk. In contrast, the hardness ratio $\mathrm{HR_{2}}$ decreases with the count rate of the M band and increases with the count rate of the H band, so an increase in $\mathrm{HR_{2}}$ may indicate a hardening of the powerlaw spectral index. The source tends to be slightly softer during the 4th observation (on April 13) when it has the highest luminosity. The variability of the source will be studied in a separate paper. For the time-averaged analysis, we excluded the 4th observation from the combined observations, and restrict ourselves to the five lowest flux spectra. The time-averaged spectrum of PG1211+143 is obtained from combining $\sim 390$ks of exposure time.