X-ray observations of X2127+119/AC 211 in the metal-poor globular cluster M15: an X-ray measure of metallicity?

We present results from observations of the 17.1 hr low-mass X-ray binary X2127+119 (AC 211) in M15 obtained with the broadband X-ray telescope (BBXRT) in 1990 December and observations obtained with the solid state spectrometer (SSS) together with the monitor proportional counter (MPC) on the Einstein Observatory in 1978 and 1979. The continuum can be well described with the two-component model of a blackbody (kT ~ 1 keV) and a power law that was found to fit the EXOSAT medium-energy data. Short-term variability observed by the SSS and MPC was related to changes in the intensities of both components in this description. The SSS and MPC observed factor of 10 differences in column density in observations separated by a few binary orbits. These observations confirm correlations between the steepness of the spectrum and the column density, and column density with phase. The percentage contribution of the blackbody is anticorrelated with the factor of 4 variations in the apparent luminosity, but other correlations are weak. A 1000 s observation by BBXRT at photometric phase 0.9 provides an upper limit of ~150 eV to narrow Fe K line emission. BBXRT spectra are generally inconsistent with the overall cluster metallicity of 1% of solar, although complex spectral models can be more consistent with the cluster metallicity. A model separating the absorption of H and He from the metals gives 1.25+0.03-0.2x1018 cm-2 for the column density of metals in the absorbing gas associated with the binary system. The noted correlations between spectral parameters, luminosity, and their relation to orbital phase are consistent with a disk plus moderate-sized corona interpretation.