Absorption Measure Distribution of the Outflow in IRAS 13349+2438: Direct Observation of Thermal Instability?

We analyze the Chandra X-ray spectrum, obtained with the HETGS grating spectrometer, of IRAS 13349+2438, which has one of the richest absorption spectra of a quasar outflow. Absorption from almost all charge states of Fe is detected. This allows for a detailed reconstruction of the absorption measure distribution (AMD), which we define as the continuous distribution of column density as a function of ionization parameter. We find a double-peaked AMD for IRAS 13349+2438, with a total (ionized) column density of NH = (1.2 ± 0.3) × 1022 cm-2, assuming solar iron abundance. For comparison, we perform a similar analysis on the well-studied HETGS spectrum of NGC 3783. Both sources feature a deep minimum in column density that is consistent with no absorption from gas at temperatures of 4.5 < log T < 5 (K). We interpret the minima as observational evidence for thermal instability in this temperature regime.

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