XMM-Newton EPIC and Optical Monitor observations of Her X-1 over the 35-d beat period

Abstract

We present the results of a series of XMM-Newton European Photon Imaging Camera (EPIC) and Optical Monitor observations of Her X-1, spread over a wide range of the 35-d precession period. We confirm that the spin modulation of the neutron star is weak or absent in the low state - in marked contrast to the main or short-on states. During the states of higher intensity, we observe a substructure in the broad soft X-ray modulation below ∼ 1 ke V, revealing the presence of separate peaks which reflect the structure seen at higher energies. The strong fluorescence emission line at ∼6.4 keV is detected in all observations (apart from one taken in the middle of eclipse), with higher line energy, width and normalization during the main-on state. In addition, we report the detection of a second line near 7 keV in 10 of the 15 observations taken during the low-intensity states of the system. This feature is rather weak and not significantly detected during the main-on state, when the strong continuum emission dominates the X-ray spectrum. Spin-resolved spectroscopy just after the rise to the main-on state shows that the variation of the Fe Kα at 6.4 keV is correlated with the soft X-ray emission. This confirms our past finding based on the XMM-Newton observations made further into the main-on state, and indicates the common origin for the thermal component and the Fe Kα line detected at these phases. We also find that the normalization of the 6.4-keV line during the low state is correlated with the binary orbital phase, having a broad maximum centred near φoribit ∼ 0.5. We discuss these observations in the context of previous observations, investigate the origin of the soft and hard X-rays and consider the emission site of the 6.4-keV and 7-keV emission lines.