Anomalous Orbital Period Change of The Low- Mass X-ray Binary EXO 0748-676

• Main Authors: Ching-Ju Chien, 簡淨茹
• Other Authors: Chou, Yi
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/85826485220462517971

碩士 === 國立中央大學 === 天文研究所 === 98 === EXO 0748-676 is an eclipsing low-mass X-ray binary（LMXB）system with a neutron star accreting matter from the Roche lobe-filling main-sequence star. The source was discovered by European X-ray Observatory Satellite（EXOSAT）in Feb 1985. This source exhibits a ~ 3.8hr orbital period with a ~492s eclipse durations, irregular X-ray dips and type I X-ray bursts. The purpose of this thesis is to investigate the orbital period change of this source. We analyzed the properties of the eclipses for EXO 0748-676 using the data collected by RXTE between 2001 and 2006. A two-hypobaric-tangent model was applied to model the eclipse profile to get the parameters of eclipse, including mid-eclipse times, the fiducial marker of binary orbital motion. The mid-eclipse times were then analyzed with O-C method to reveal its orbital period variation. However, combined with historical data, we found no simple polynomial ephemeris may explain the variation of the residuals from O-C method. The O-C values during MJD 50000-54500 were better fitted to two different models, including an sinusoidal function and piecewise constant period model. The first model indicated that orbital period changes are probably caused by existence of a third star. The O-C values were fitted by sinusoidal function, and gave the long-term period of about 8.35 year and an amplitude of 12.6 lt-s. If this system is presence, the mass function of the third object to is f(M3)=2.31x10-7 M☉. From the fitting result of the latter model, the orbital period change is △P/P∼ ±4 x10-7 , and the changes seems to have the regular changes between the RXTE & USA 1996-2007 data . Thus, the orbital period change of EXO 0748-676 system was probably resulted from a variable quadrupole moment produced by magnetic activity in companion star.