HETG Observations of AC211 in the Globular Cluster M15
Tim Kallman (NASA/GSFC, Laboratory for X-ray Astrophysics) , L. Angelini, N. E. White (NASA/GSFC), J. Sepinsky (NWU)
We report on a 60 ksec observation of the low mass X-ray binary AC211/4U2127+119 in the Globular CLuster M15 using the Chandra HETG. The presence of the confusing source 3 arc-seconds away necessitates careful observation planning and analysis to disentangle the two spectra. The observation spans approximately one binary orbit, and the HETG spectrum shows emission lines from highly ionized Si, Mg, Ne, and O,and also features which correspond to the K lines of iron. The ratios of the helium-like lines from Mg and Si provide a lower limit on the density or the UV radiation field in the line emitting region. The relative strengths of the H- and He-like lines are consistent with photoionization equilibrium, with a narrow range of allowed ionization parameters. This provides an upper limit on the distance of the line-emitting gas from the source of X-ray continuum. The line emission is crudely consistent with that expected from an accretion disk corona, although there are important differences between this source and other likely ADC sources. One example is the strength of the iron line. We discuss these differences, and interpret them in the context of other information about this source: its orbital parameters and the nature of the companion star.