BeppoSAX spectroscopy of the NGC 7078 ( M 15 ) globular cluster

Results of a 1999 November 16–17 BeppoSAX observation of the low-mass X-ray binary X 2127+119 located in the globular cluster M15 are presented. The system is believed to be one where the central neutron star is normally obscured by the accretion disk, and only X-rays scattered into our line of sight by an extended accretion disk corona (ADC) are observed. The 0.1–10 keV lightcurve is energy dependent and shows two partial eclipses separated by the 17.1 hr orbital period. The 0.1– 100 keV spectrum is unusually complex, but can be successfully modeled using a partially covered power-law and disk-blackbody model. Together with a column consistent with the interstellar value to M15, ∼60% of the source is covered by an additional column of ∼10 22 atom cm −2. The absorbed component may be X-rays that pass through the outer layers of the accretion disk. The energy dependent intensity variations by a factor of ∼2 may be modeled as due to a changing normalization of the disk-blackbody. None of the other spectral parameters appear to clearly depend on luminosity. The same spectral model is also able to fit an archival ASCA spectrum. We demonstrate that during the luminous (∼ Eddington) X-ray burst observed from X 2127+119 by Ginga, material located in the outer regions of the accretion disk could have been temporarily ionized, so allowing the central neutron star to be viewed directly.