Study of the Broadband X-ray Lightcurves of Hercules X-1/HZ Hercules

Abstract

Hercules X-1 / HZ Hercules (Her X-1/HZ Her) is an X-ray binary system monitored by multiple X-ray missions since last century. The neutron star Her X-1 is a pulsar with 1.24-second period, and HZ Her is the optical companion. The orbital period of the binary system is 1.7 days. The accretion disk around Her X-1 has a twist-tilted geometry that causes the well-known 35-day super-orbital cycle in the X-ray lightcurve of Her X-1/HZ Her. With the abundance of long-term observations, four broadband X-ray missions are analyzed in this thesis to study the lightcurve of Her X-1/HZ Her on the timescales of both the 35-day cycle and the 1.7-day binary orbit. The observational data selected are: RXTE/ASM (2-12 keV), Swift/BAT (15-50 keV), MAXI (2-20 keV), and Fermi/GBM (12-50 keV). An updated list of 35-day cycles is presented utilizing the method of Cross-Correlation (CC) on Swift/BAT observations. Instead of Turn-On (TO) when the count rate starts to rise in 35-day cycles, Main-High (MH) peak times are reported, and the average difference between TO and MH peak is calculated. The disk geometry is found to be rather stable during the last two decades, and no preferred orbital phase of TO is identified. Averaged 35-day lightcurves are obtained in multiple energy bands of X-ray. For the 1.7-day orbit, a complete set of multi-band orbital lightcurves is created during the six states of the 35-day cycle, which illustrates in detail the changing lightcurve caused by the rotating twisted-tilted accretion disk surrounding the neutron star. The orbital lightcurves during MH are analyzed in 0.05-long 35-day phase intervals, and show the regular occurrence of pre-eclipse dips which march to earlier orbital phase as 35-day phase increases. A clustering of anomalous dips is found near orbital phase 0.6 during early MH. For each 35-day state, the time-average orbital phase dependence of column density for photoelectric absorption and energy-independent transmission is derived. The X-ray lightcurves during Low States (LSs) are similar in shape to the optical LS lightcurve, but X-ray leads optical by '0.04 to 0.08 in orbital phase. The regular pre-eclipse dips in the orbital lightcurves during MH are compared to the existing accretion stream model. The marching of pre-eclipse dips towards earlier orbital phase at later 35-day phases is well predicted, but the observed dips are in general at earlier orbital phases. The geometry of the accretion disk in the orbital plane has minor influence on the prediction. A more curved stream is required to match the stream model with the observed pre-eclipse dips.