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

TL;DR

This study conducts detailed pulse phase resolved spectroscopy of Her X-1 using RXTE data, revealing how spectral parameters vary with pulse and 35-day phases, and providing insights into the system's geometry and neutron star precession.

Contribution

It presents the first detailed pulse phase and 35-day phase-resolved spectral analysis of Her X-1, linking spectral variations to system geometry and neutron star precession.

Findings

Spectral parameters modulate strongly with pulse phase.

Iron line flux disappears near highest continuum flux, suggesting hollow cone geometry.

Cyclotron line energy shows moderate increase around pulse phase 0.7, with no significant 35-day phase dependence.

Abstract

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 the centroid 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. We analyzed RXTE observations of the Main-On of November 2002. 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 phase 0.03, 0.10, 0.15, and 0.20, respectively. 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, 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. The variation of spectral parameters as a function of the pulse phase provides important information about the system: 1. 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.