| dc.description.abstract | In this thesis, I present the result of the phase shifts of the X-ray pulses in different energy ranges of the first known accretion-powered millisecond pulsar SAX J1808.4-3658 in its 2002 outburst observed by Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA), and compare to the similar phenomenon seen in 1998 outburst.
To compare the pulse arrival times of different energy ranges, the precise orbital and spin parameters which reveal the correct pulse profiles are required. These parameters were refined through minimizing the variation of the pulse peak phases, yielded by folding the non-burst, 2 – 10 keV events with a model consisting of orbital Doppler Effect plus possible pulse phase drift described as a polynomial. I found the discontinuity in parameters after day 14, hence it was hard to get the orbital and spin parameters for whole 2002 outburst. Moreover, Burderi et al. (2006) reported that there was a “phase jump” during the day 14 – 17 of the 2002 outburst. Thus, I decided to remove the data in the phase jump and divided 2002 outburst into two sections, before and after the phase jump, then obtained their parameters separately. I subsequently applied these parameters to fold the 11 energy bands of data allocated by event energy to obtain their fine pulse profiles. The phase delays relative to the softest band were derived through cross-correlating the corresponding pulse profiles. The results show a soft pulse lag in the first ~14 days’ data up to 0.08 spin phase (~200 μs) and it saturates at > 9 keV, consistent with the one in 1998 outburst. On the other hand, the similar phenomenon is also seen after the phase jump but the soft lag is up to 0.12 spin phase (~300 μs), much more than the one we found in first 14 days, and the phase lag doesn’t saturate.
It is believed that the energy dependent phase pulse arrival time would be caused by the spectrum on the pulsar’s hotspot. The energy dependent pulse phase behavior before the phase jump in 2002 outburst is consistent with it in 1998 outburst, but the case after the phase jump shows a huge difference – in which it shows longer soft pulse lags and no sign of saturation. It seems that the spectrum around the hotspot after the phase jump in 2002 outburst might be softer. Based on this result, we conclude that even in the same source, it might have different behaviors due to the construction of the spectra of the hotspot environments.
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