| dc.description.abstract | 4U1820-30 is a low mass X-ray binary (LMXB) system, located near the center of the globular cluster NGC 6624, composed of a neutron star and a ~20% degeneracy white dwarf. Besides its 685s orbital period, it also has a ~ 170d long-term variation in X-ray band. Chou & Grindlay (2001) proposed the hierarchical triple model for 4U1820-30 system based on the stability of ~171d periodicity.
The purpose of this thesis is to investigate the long-term modulation of this source. The X-ray light curve adopted for this study was from the observations of All-Sky Monitor (ASM) on board Rossi X-ray Timing Explorer (RXTE) from 1996 to 2009. We also used the data from Proportional Counter Array (PCA) on RXTE and Swift Burst Alert Telescope (BAT) to compare the results between different energy bands. The Lomb-Scargle periodogram was applied to verify the periodicity of long-term modulation. We also performed dynamic power spectrum analysis to trace the modulation power variation, as well as the period change. On the other hand, phase evolution evaluated by Observed – Calculated (O-C) method was also included to study long-term modulation of 4U1820-30.
However, this period is likely shift to ~ 168d from it power spectrum after 2004. The phase evolution also shows negative drift after 2004. But neither of the abrupt period change( ~ 2%) nor the period derivative caused by binary or third companion’s orbital period, can be explained by triple model. These evidences above all imply that the period of long-term modulation of 4U1820-30 is not as stable as the one proposed by Chou & Grindlay (2001), which is a challenge of the triple model for this system. Furthermore, ~ 170d power between TJD 11000 and TJD 12300 became weaker in the dynamic power spectrum, due to enhancements of secondary minima. Nevertheless, ~ 167d modulation from phase evolution shows unreasonable discontinuous phase during weak power interval, which is also unlikely to be explained with triple model.
The period detected from RXTE/ASM and PCA is consistent, but the data from Swift/BAT show period shift to ~160d after 2004. Although Farrell et al. (2008) proposed that ~ 170d period of 4U1820-30 could be detected only up to 24 keV, it still cannot explain why the period shifts. In our studies, we also checked the time of occurrence of X-ray bursts. All the X-ray bursts detected within ±22d of the intensity minima rather than predicted minima proposed by Chou & Grindlay (2001), which indicates that the long-term modulation is a change in the accretion rate.
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