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 gratingspectrometer of IRAS 13349+2438, which has one of the richest absorptionspectra of a quasar outflow. Absorption from almost all charge states of Fe isdetected. This allows for a detailed reconstruction of the absorption measuredistribution (AMD), which we define as the continuous distribution of columndensity as a function of ionization parameter. We find a double peaked AMD forIRAS 13349+2438 with a total (ionized) column density of N_H = (1.2 +- 0.3)*10^{22} cm^-2} assuming solar iron abundance. For comparison, we perform asimilar analysis on the well studied HETGS spectrum of NGC 3783. Both sourcesfeature a deep minimum in column density consistent with no absorption from gasat temperatures of 4.5 < log T < 5 (K). We interpret the minima asobservational evidence for thermal instability in this temperature regime.Comment: 25 pages, 7 figure