Properties of the Accretion Disk Corona in Her X-1
LI JI (MIT Kavli Institute for Astrophysics & Space Research) , N.S. Schulz (MKI), M. Nowak(MKI) & H.L. Marshall(MKI)
We present an analysis of six high-resolution Chandra grating observations of the X-ray binary pulsar Her X-1. With a total exposure of 170 ks, the observations are separated by years and cover three combinations of orbital and super-orbital phases. Our goal is to determine the properties of the photoionized emission and its dependence on phase-dependent variations of the continuum. We find that the continua can be described by a partial covering model which is consistent with the high energy spectra observed with RXTE. Moreover, an additional thermal blackbody component is required to fit the soft band below 12 Å( 1 keV). Most of the variability is caused by high absorption columns varying from (1 - 3) x 1023 cm-2. Most spectra show strong line emission stemming from a photoionized accretion disk corona. We model the line emission with generic thermal plasma models as well as with the photoionization code XSTAR and determine changes of the ionization balance with orbital and superorbital phases. Most spectra also show strong and variable fluorescence line emission from near neutral Fe,likely associated with the cooler, outer portions of the disk.