天蝎座X-1的海王星外天體X射線掩星事件光度曲線中繞射效應計算的應用

Applications of Diffraction Pattern Computation for Trans-Neptunian Objects Occultation of Scorpius X-1 in X-rays

陳冠廷(Kuan-Ting Chen)

指導教授 ： 張祥光

國立清華大學/理學院/物理學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00367

摘要

Serendipitous stellar occultation search is so far the only way to detect the existence of very small, very dim, remote objects in the Solar system. Up to date, however, there are only very few reported detections for trans-Neptunian objects (TNOs) in optical bands. In the X-ray band, with the RXTE/PCA data of Sco X-1 taken from June 2007 to October 2009, we found three X-ray occultation events of Sco X-1 including two events with possible instrumental indications and one without. Through the diffraction pattern fitting, we suggested that one might be by a main-belt asteroid (MBA) of 15 m in size, and another one events might be caused by a decametre Centaur or MBA or by a very nearby object of meter size with relative speed at a few kilometres per second. For the event without instrumental indication, it might be due to a TNO of 150-m size, but with a rare retrograde orbit, or it might be due to an MBA of 40-m size, but the associated detection rate is incredibly high, or it might be due to a very nearby object of meter size moving at a relative speed of a few kilometers per second. Given all these uncertainties, we conservatively made a conclusion of no detection of TNOs occultation events and can proceed to estimate the upper limits to the size distribution of TNOs and MBAs at the level of assuming one detection.

To investigate the feasibility of determining X-ray emitting region size in Sco X-1, we study the condition under which the occultation light curves can be distinguished that caused by background point source for a Sco X-1 X-ray emitting region of 100,000 km which is the largest one proposed in the literatures. RXTE/PCA can achieve to distinguish the size from a point source for a central-crossing occultation event if its count rate is about 20 times more and the diameter of the TNO is in kilometer size. For other impact parameters, a count rate of 600 or more is needed. For distant occultation events, e.g. 1000 AU, the central crossing event can provide a good opportunity. The current RXTE/PCA count rate is enough to distinguish the two light curves when the occulting body diameter is about kilometer size but a couple of higher is needed for other impact parameters. For the case of a very large occulting body, it is extremely unlikely to probe the X-ray emitting region size of Sco X-1 if the body is at 40 AU. However, if the very large occulting body is at large distance, such as 1000 AU, an instrument with about 10 times RXTE/PCA count rate will be able to distinguish the occultation light curves produced by a X-ray emitting region size of 100,000 km at Sco X-1 and by a point source.

關鍵字

掩星 ； 古柏帶 ； 中子星 ； X光雙星

並列摘要

偶發性掩星事件是目前監測搜尋非常小、黯淡且遙遠的太陽系天體存在的唯一方法。然而，在可見光波段，至今只有非常少的研究報告了偵測到海王星外天體(TNOs)的掩星事件。在X光波段，於2007六月到2009年十月RXTE/PCA觀測天蝎座X-1(Sco X-1)的數據中，我們發現了三個天蝎座X-1的掩星事件，包含兩個有可能是儀器造成的與一個無儀器因素的事件。藉由繞射曲線的擬合，在兩個可能是儀器造成的事件中，我們認為其中一個事件可能是由15公尺大小的小行星帶天體(MBA)造成，另一個可能是由大小約十米的半人馬小行星(Centaur)或小行星帶天體造成，也可能由相對速率每秒數公里，距離非常近的公尺級大小物體造成，兩個事件均無法排除由儀器造成的可能性。而對於沒有儀器因素的事件，則有可能是由具罕見逆行軌道的150公尺大小的海王星外天體造成，也可能是由40公尺大小的小行星帶天體造成，但是對應的相關聯偵測率會極高，又或者是由相對速率每秒數公里，且距離非常近的公尺級大小物體造成。基於以上所有不確定性，我們保守做出無偵測到海王星外天體掩星事件的結論並估計海王星外天體與小行星帶天體大小分佈的上限。
為了探討天蝎座X-1的X光輻射區域大小測定的可能性，考慮天蝎座X-1的X光輻射區域為文獻上所提出最大100,000公里的大小時，我們研究掩星光度曲線在此情況下可被從背景點光源中區別出來的條件。對於有通過中央亮帶的掩星光度曲線來說，當物體大小是公里級的海王星外天體時，約20倍於RXTE/PCA的光子接收率(photon count rate)可以達到區別條件，而對於其他的撞擊參數來說，至少要600倍以上才能達到；當考慮較遠的掩星事件時，例如在距離1000天文單位(AU)的地方，目前RXTE/PCA的光子接收率對於公里級大小的掩體且有通過中央亮帶的掩星光度曲線事件中是足以達到區別條件的，而在其他的撞擊參數下，則需數倍於RXTE/PCA的光子接收率。當考慮非常大的天體為掩體時，在40 AU地方的事件是非常不可能用來探測天蝎座X-1的X光輻射區域大小的；而在較遠的地方(例如在距離1000 AU的地方)時，那麼約十倍於RXTE/PCA光子接收率的儀器就可以分辨掩星光度曲線是由天蝎座X-1的X光輻射區域大小100,000 km產生還是由點光源產生的。

並列關鍵字

occultation ； Kuiper Belt ； neutron star ； X-ray binaries

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