極速快閃觀測之X光望遠鏡之詳盡模擬研究

Abstract

極速快閃觀測（UFFO）是一個太空望遠鏡觀測，主要在於偵測伽瑪射線暴（GRBs）一分鐘以內的紫外與光學信號。該伽瑪射線暴是最極端高能爆炸，其總能量可達10的54次方ergs。 UFFO突發預警望遠鏡（UBAT）是一個以X-ray 波段來進行伽瑪射線暴的方向追蹤。 為了研究UBAT的性能，首先是建立UBAT的電腦模擬模型。 在軟體方面我們使用Geant4，它是基於C + +語言來撰寫的軟體，主要用於模擬物質與高能粒子的交互作用。它的應用領域包括高能源，核能和加速器物理，以及研究在醫療和太空科學。 而探測器幾何形狀和材料都必須加入電腦模擬中。像是鎢制編碼遮罩，鋁製外罩和支撐結構也同時加到程式碼裡面。在 Geant4的設計之下，可以模擬UBAT一些關鍵的性能。 在模擬注射高能量的粒子如X-ray/Gamma-ray光子，宇宙射線，或是電子/正電子時，電腦程式會模擬粒子與UBAT的交互作用並且記錄反應位置和能量損耗。有了這個模擬程式，我們可以研究UBAT的材料反應，方向追蹤的精準度，甚至於GRBs的偵測率。 而為了估計UBAT的GRBs偵測率，檢測突發和瞬態源實驗（BATSE）的GRB光曲線可當作一個重要的參考指標。其實驗包括8個相同的探測器模組並且搭載於稱為康普頓伽瑪射線觀測（CGRO）的衛星上，這8個偵測器位於該衛星的每一個角落（左，右，正面和背面，頂部和底部）。每個偵測器則含有碘化鈉晶體（TL）大面積探測器（LAD）和碘化鈉光譜探測器。 從BATSE 304 GRB光曲線樣本下，UBAT的GRB偵測率14.09/year，如果加上是否有成功的偵測到GRB的方向,其偵測率為5.7/year。而當GRB的信號夠強的時候，其角精度可達2.2角分。

The Ultra Fast Flash Observatory (UFFO) is a space telescope to observe the prompt UV/optical signal from Gamma Ray Bursts (GRBs) exploring the subminute regime. The GRBs are the most brightest objects which are associated with extremely energetic explosions with total energy 1054ergs and last from ten milliseconds to several minutes. It is the most powerful comic phenomena that we have known in the universe up to now. The UFFO Burst Alert Telescope (UBAT) is a X-ray coded mask camera for trigger and localization of GRB events for follow-up observation by a UV/optical observations. To do the simulation, the UBAT model has been constructed with Geant4 tool kit which is based on the C++ language for the simulation of the passage of particles through matter. Its areas of application include high energy, nuclear and accelerator physics, as well as studies in medical and space science. The detailed parameters can be taken into consideration such as geometry and the material of the detector. The tungsten coded mask, the aluminum hopper and the even the supporting structure are one by one built to the simulation package. With the Geant4, one can simulate the performance of each critical components with high energy particles such as X-ray/Gamma-ray photons, cosmic rays, or electrons/positrons. As the simulation starts, the computer start to generate particles and simulate its interaction with UBAT and records the information such as position and energy. With this simulation code, the shielding e ciency of the hopper can be tested; the photon counting rate with a given ux can be obtained; the algorithm for tracking the direction can be tested; and even the event rate can be estimated with additional information. To estimate the event rate of GRB, light curves of GRB detected by Burst And Transient Source Experiment (BATSE) are converted to UBAT according to its geometrical acceptance. It was on Compton Gamma Ray Observatory (CGRO) with eight identical detector modules, one at each of the satellite's corners (left, right; front and back; top and bottom). Each module consisted of both a NaI(Tl) Large Area Detector (LAD) and a NaI Spectroscopy Detector so that BATSE has a full sky monitoring capability to observe bursts which were typically detected at rates of roughly one per day over the 9-year mission. Within 304 GRB samples from BATSE, the UBAT has a GRB event rate of 2 14:9 per year if a 4 SNR threshold is set, and it should be able to locate the direction as well with an event rate of 5:7 per year under a 8 second exposure time. And its the angular accuracy of GRB could be 2:2 arcmin with a strong burst signal. 3