HZ Her/Her X-1: Study of the light curve dips
Abstract
The HZ Her/Her X-1 X-ray binary exhibits rapid and variable X-ray absorption features. These were noticed soon after the discovery of its periodic flux variations, such as X-ray pulsations and eclipses, and were named light curve dips by Giacconi et al. 1973. Their properties were analyzed, debated and documented ever since. The largest existing set of detailed observations of Her X-1 are contained in the data archive of NASA's Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA). From this entire light curve, several hundred new light curve dips were documented, based on X-ray Softness Ratio (SR), making this thesis the most extensive study of HZ Her/Her X-1's dips to date. The dips were classified into 12 different categories in order to study their statistical distribution, intensity, duration, symmetry and SR evolution. Some dips properties depend on Her X-1's 35-day X-ray cycle, which is caused by the precessing disk around the neutron star. The 35-day phase of dips was determined using Turn-On (TO) times calculated from the February 1996 - December 2009 RXTE/All Sky Monitor (ASM) light curve. 147 TOs were found by cross-correlation with X-ray cycle templates, and the 22 Burst and Transient Source Experiment TOs were confirmed. Thus this study also has the longest time period yet for the analysis of the 35-day X-ray cycle. The set of 147 TOs does not correlate with 0.2 or 0.7 orbital phases, disproving the reports over the past 30 years. The ASM-based 35-day cycle lengths range from 33.2 to 36.7 days, with an average of 34.7 +/- 0.2 days. The observed timing of dips is illustrated in the 35-day phase vs. orbital phase plot, and compared to models. The current large set of dips gives much better detail than that of Crosa & Boynton 1980. A model for dips is developed here, which takes dips to be caused by blockage of the line of sight to the neutron star by the site of the accretion stream - disk collision. An extensive investigation of the model's dependence on parameters showed that only the outer accretion disk radius had a major influence on dip timing. In this model a variable disk size is equivalent to disk penetration. The orbital phase of TO does not have a significant effect. The model can explain several dip properties including the marching trend, small number of anomalous dips, the dips' spread across orbital phase, observed dip egress duration, the larger number of pre-eclipse dips and their higher intensities, and reproduce the phase and timing of any type of dip. Additional mechanisms must be modeled in order to reproduce all the observed properties of dips.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2011
- Bibcode:
- 2011PhDT.........8I
- Keywords:
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- Physics, Astronomy and Astrophysics