X-ray Line Emission in Hercules X-1

We find line emission from the hydrogen-and/or helium-like ions of Ne, O, N and C in the low and short-on states of Her X-1, using the XMM-Newton Reflection Grating Spectrometer. The emission line velocity broadening is 200 < σ < 500 km s −1. Plasma diagnostics with the Ne IX, O VII and N VI Heα lines and the radiative re-combination continua of O VII and N VII, indicate the gas is heated by photoionization, with an electron temperature of T = 6 ± 2 eV. We use spectral models to measure element abundance ratios of [N/O] = 9.5 ± 1.3, [C/O] = 0.68 ± 0.16, and [Ne/O] = 2.4 ± 1.2 times solar, which quantify CNO processing in HZ Her. Photoexcita-tion and high-density effects are not differentiated by the measured Heα lines, as shown by calculations that use previous UV photometry. We set limits on the location of the line emission region, and find an electron density n e > 10 11 cm −3. The narrow emission lines can be attributed to reprocessing in either an accretion disk atmosphere and corona or on the X-ray illuminated face of HZ Her. In the main-on state, the bright continuum only allows the detection of interstellar absorption, plus O VII Heα emission lines with σ = 3200 ± 700 km s −1 and complex profiles. Other broad lines may be present. The broad lines may originate in a region near the pulsar magneto-sphere. In such a case, periodic occultations of the Her X-1 magnetosphere must occur with 35 d phase, consistent with the precession of the accretion disk. Fe L lines are not detected.