在伽瑪射線(Gamma-ray)波段的天空中,脈衝星一直被認為是一個穩定的源。然 而,唯一且第一個變動性伽瑪射線脈衝星 PSR J2021+4026 的發現,卻有可能挑戰 我們對於脈衝星的認知。在本研究中,我們發表利用七年的 Fermi 伽瑪射線望遠鏡 的資料,進而分析有可能是下一個變動性伽瑪射線脈衝星 PSR J0248+6021。

脈衝星 PSR J0248+6021 是一個由自身轉動能量發出射線的脈衝星,它處在 密度較高分子雲的環境,且在我們的觀測中,PSR J0248+6021 有發生兩次的瞬間頻 率變動 (glitch)。 在分段光譜以及脈衝圖的分析中,我們發現兩者在 glitch 前後 發生變化。為了進一步分析光譜的變化之原因,進行了脈衝解析的光譜分析,此分 析的結果顯示原先光譜變動的結果是因為在第一段時間 (P1) 內的非來自派衝星射 線太弱的關係,並非由 glitch 引起。我們總結 PSR J0248+6021 沒有顯著的變動和 gltich 有關。

關於非來自脈衝星的射線,之前的研究根據光學波段發現 PSR J0248+6021 附近有延伸結構,顯示該射線可能來自於脈衝星風驅動星雲 (PWN),因此我們利用 Fermi 的資料進行空間分析以及利用 Chandra 望遠鏡觀測 X-ray 波段。PSR J0248+6021 在 X-ray 波段下,被認定是極暗的 X-ray 脈衝星。且非來自脈衝星射 線有著低能量型態的伽瑪射線波段光譜。這兩件事實弱化脈衝星風驅動星雲 (PWN) 存在之可能性。

在無線電波 (radio) 以及 X-ray 波段中,許多脈衝星變動的事件和光譜、 光度變化有著密切關係,然而在伽瑪射線波段中,唯有 PSR J2021+4026 被發現類 似的變動特性。 我們對於 PSR J2021+4026 的變動性尚未了解,持續搜尋類似的伽 瑪射線脈衝星,有助於了解對伽瑪射線脈衝星所發出射線的機制。

Pulsars are usually known as steady sources in gamma-ray sky. However, the discovery of the first variable gamma-ray pulsar PSR J2021+4026 challenges our understanding to pulsar astronomy. We here present the results of a detailed analysis of ∼ 7 years of Fermi-LAT data of the variable gamma-ray pulsar candidate, PSR J0248+6021, a rotation-powered pulsar located in a dense gas cloud region with two glitches after the Fermi satellite being launched. We found spectral variation before and after the first glitch and a marginal change in its rotational light curve (pulse profile). To inspect the origin of the spectral change, phase-resolved spectral analysis have been performed while the results show the origin might be due to a weak detection of off- pulse emission during the period before the first glitch (P1). We conclude that no significant variation of pulsar properties of PSR J0248+6021 has been observed to be associated with glitches. For the unpulsed emission, in previous works, a potential pulsar wind nebula (PWN) has been proposed on the basis of extended feature in op- tical wavelengths, and hence we performed spatial analysis with Fermi-LAT data and revisited the X-ray observation from Chandra. PSR J0248+6021 is an underluminous X-ray pulsar with a soft powerlaw spectrum obtained in gamma-ray regime seemingly weaken the possibility PWN. Spectral changes and flux variability associated with pulsar irregularities have been found in radio and X-ray energy bands, while to date, only PSR J2021+4026 is claimed as a variable gamma-ray pulsar showing different modes before and after a significant flux jump. The intrinsic origin of the flux jump occurred in PSR J2021+4026 is still unclear, searching for more pulsar behaving in this way would help us understand the gamma-ray pulsar emission mechanism.

Chapter 1 Introduction .......................... 1
1.1 Irregularities in Pulsars.......................... 1
1.1.1 Glitch.................................................. 2
1.2 PSR J2021+4026.................................. 4

Chapter 2 Instrumentation......................... 9
2.1 Fermi-LAT............................................. 9
2.1.1 Performance...................................... 10

Chapter 3 Data Analysis and Results........ 14
3.1 Gamma-ray Observations.................... 15
3.2 Spectral Analysis................................. 17
3.3 Timing Analysis................................... 19
3.4 Analysis for Different Periods Divided by Glitches.......................... 22
3.4.1 Long-term Light Curve...................... 22
3.4.2 Pulse Profiles in Different Periods.... 29
3.4.3 Spectra in Different Periods............. 35
3.4.4 TS Evolution..................................... 41
3.5 Analysis the Off-pulse Emission.......... 43
3.5.1 Spatial Analysis................................. 43
3.5.2 Flux Variability and Spectral Analysis. 49

Chapter 4 Discussion and Conclusion....... 52
4.1 Glitch.................................................... 52
4.2 The Origin of the Off-pulse Emission... 53

Chapter 5 Future Work .............................. 56

Appendix ................................................... 61
A Overvies of Likelihood Analysis .............. 61

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