Pulse phase and precession phase resolved spectroscopy of Hercules X-1: studying a representative Main-On with RXTE

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: vasco@astro.uni-tuebingen.de
² Sternberg Astronomical Institute, Lomonossov University Moscow, Russia

Received: 7 August 2012
Accepted: 22 December 2012

Abstract

Aims. We performed a detailed pulse phase resolved spectroscopy of the accreting binary X-ray pulsar Her X-1 in the energy range 3.5–75 keV and have established pulse phase profiles for all spectral parameters. For three parameters, the centroid energy of the cyclotron line, the photon index and the flux of the 6.4 keV iron line, we have studied the variation as a function of 35 d phase.

Methods. We analyzed RXTE observations of the Main-On of November 2002 using data from the PCA and the HEXTE instruments. Four different time intervals of about  ~1 d duration were selected to provide a good coverage of a complete Main-On. The intervals are centered at 35 d phases 0.03, 0.10, 0.15, and 0.20, respectively.

Results. All spectral parameters show a strong modulation with pulse phase. While the centroid energy of the cyclotron line follows roughly the shape of the pulse profile, showing higher values close to the peak of the X-ray pulse, both the photon index and the iron line intensity exhibit distinct minima around the peak of the X-ray pulse. With respect to variations of the observed profiles with 35 d phase, we find that there is a clear evolution of the shape of the pulse profiles (flux versus pulse phase), a moderate increase of the maximum cyclotron line energy (found around pulse phase 0.7), but no significant evolution of the shape of the pulse phase profiles of the cyclotron line energy, the spectral power law index or the iron line intensity.

Conclusions. The variation of spectral parameters as a function of the pulse phase provides important information about the system: i. the disappearance of the Fe line flux near the highest continuum flux may be an indication of a hollow cone geometry of the accretion structure; ii. the apparent non-dependence of the cyclotron line energy profiles on 35 d phase provides a new possibility to test the model of free precession of the neutron star, proposed to be responsible for the systematic variations in the pulse profiles.