EXO 0748-676是顆低質量X光雙星系統，由充滿洛希瓣的主序星和從伴星吸積物質的中子星所組成。 這顆星體在1985年時被EXOSAT觀測發現，並展現出3.8小時週期性現象且過程492秒的食現象，以及不規則的X光光陷和第一類型X光爆發現象。 本論文的目的在於研究EXO 0748-676的軌道週期的演變，可作為探討LMXB可能演化的參考。 我們使用2001-2007年間RXTE的觀測資料來分析EXO 0748-676的掩食特性，並採用雙曲正切的模型來做擬合以求得掩食參數，如掩食時間中點，並作為雙星軌道運動的基準點。 然後，利用O-C分析的方法來分析掩食中點時間，以顯出軌道週週期的變化。 然而，結合歷史的觀測資料後，我們發現簡單的星曆表無法解釋從O-C分析方法得出的O-C值變化。 我們對O-C圖中MJD 50000-54500間的資料點分別採用正弦波模型和分段擬合模型去擬合。 前者表示軌道週期改變可能是由於第三顆星的存在所影響。 在 O-C值被正弦波模型擬合後，可得到8.35年的長週期和投影軌道半徑12.6 光秒， 假設這種系統是存在的，則我們可得到質量函數為f(M3)=2.31x10-7 M?。而從後者模型的分析中，我們得到軌道週期變化值約為△P/P? ±4 x10-7，其中似乎展現了週期性的變化。 最後，推測可能導致EXO 0748-676有軌道週期變化的原因是，由伴星的磁場活動的變化造成重力四極矩改變所致。 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.
