球狀星團之X射線光源的多波段研究

本研究包含兩大部分：（一）研究球狀星團中X射線光源的分佈；（二）在球狀星團中尋找中質量黑洞。以下為其摘要。（一）球狀星團中的雙星系統分佈對於研究星團的動力演化有重要貢獻。由於球狀星團中的X射線光源大多屬於緻密雙星系統，因此藉由研究星團內部的X射線光源，可以有效得知其雙星系統的組成。前人研究得知，星團中的X射線光源主要包含有：低質量X射線雙星、激變雙星、毫秒脈衝星、以及活躍色球層雙星。本研究利用錢卓X射線太空望遠鏡以及哈伯光學太空望遠鏡觀測數個球狀星團中的X射線光源，用以判斷及分類X射線光源的本質。為了探討星團中X射線光源的可能形成管道。我們利用18個球狀星團的物理參數（核半徑、核密度和金屬含量）以及其中所搜尋到的X射線光源數量，來尋找及建構星團特性與X射線光源產生率之間的可能關聯。若是X射線光源的產生和星團特性有正相關，我們便能利用此相關性來預測星團中X射線光源的形成管道 -- 判斷它們為原生雙星系統或是經由動力演化而形成的雙星系統。（二）我們利用從觀測歸納出的黑洞活動的基本特性，來預測球狀星團中可能存在的中質量黑洞所發出的電波輻射。在假設星團含有足夠的氣體含量的前提之下，理論預測位於星團中心的中質量黑洞能夠吸積附近的游離氣體，而發出電波射線。黑洞活動的基本特性指出，在特定的吸積狀態之下，黑洞所發出的X射線與電波射線會成一定正相關，而此正相關又與黑洞質量有關，因此定下黑洞質量和X射線強度，我們便能預測中質量黑洞發射出的電波射線強度，經過計算，此電波強度能夠被最新一代電波望遠鏡所偵測。因此本研究中利用澳洲的緻密陣列電波望遠鏡，來探測銀河系中兩大球狀星團（47 Tucane 和 Omega Centauri）中的電波訊號，用以尋找可能存在於星團中的中質量黑洞。儘管我們未能偵測到任何星團中央的電波訊號，我們仍然得以估計中質量黑洞的質量上限，此為現今對於 47 Tucane 和 Omega Centauri 中的黑洞質量約束力最強之觀測。結果顯示中質量黑洞仍然可能存在於 47 Tucane 和 Omega Centauri 之中，而未來若能利用靈敏度更高的電波觀測達到更強的質量限制，將得以提供球狀星團內中質量黑洞存在與否的最直接證據。

關鍵字

球狀星團；低質量X射線雙星；激變雙星；活躍色球層雙星；毫秒脈衝星；中質量黑洞

並列摘要

This research is composed of two parts: (I) Studying the X-ray sources populations in globular clusters (GCs), and (II) Searching for the intermediate-mass black holes (IMBHs) in GCs. They are summarized as below. Part I: The binary content in GCs is crucial for the dynamical evolution of GCs. Studying the X-ray sources population would be an efficient method to explore the binary content since most of the X-ray sources in GCs are close binaries. The X-ray sources population contains various exotic binaries: low mass X-ray binaries, cataclysmic variables, millisecond pulsars, or chromospherically active binaries. By using Chandra and HST data, we would be able to observe and identify the X-ray sources in GCs. In order to study the possible formation origins of the X-ray sources in GCs, we construct the correlation for the X-ray source number and the physical parameters (core radius, core density, and metallicity) from 18 GCs. With the correlation, we would be able to predict the X-ray source number in GCs and to constrain their formation mechanisms – if they are formed through dynamical interactions or from primordial binaries. Part II: We make use of the fundamental plane of black hole activities to predict the possible radio emissions from IMBHs in GCs. If GCs contain some amount of ionzied gas, the IMBH at the center will accrete the gas and emit radio emission. With the assumed ratio of X-ray to radio flux and the assumed accretion state of black holes, we could predict the radio emission from the central IMBH, which is observable with current radio facilities. In our study, we use Australis Telescope Compact Array (ATCA) to search for the possible radio emissions from IMBHs in the two most massive GCs – 47 Tucane and Omega Centauri. Although we did not detect any central radio source in both GCs, we set an upper limit for the black hole masses and the existence of IMBHs in GCs is still possible. Either a tighter upper limit for the black hole masses or a detection of central radio source in GCs will be the direct and solid evidence against or for the existence of IMBHs in GCs.

並列關鍵字

globular clusters ； low mass X-ray binaries ； cataclysmic variales ； chromospherically active binaries ； millisecond pulsars ； intermediate-mass black holes

參考文獻

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Hui, C. Y., Cheng, K. S., Wang, Y., Tam, P. H. T., Kong, A. K. H., Chernyshov, D. O., & Dogiel, V. A. 2011, ApJ, 726, 100

國際替代計量

球狀星團之X射線光源的多波段研究

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