# Krennrich (2009) - Gamma ray astronomy with atmospheric Cherenkov telescopes: the future
###### tags: `γ-astro` `IACT` `journal` `translating` `annotating`

> 契忍科夫望遠鏡與伽馬射線天文學——未來展望

## :star2: Highlights *重點整理*
https://hackmd.io/@siniu-thesis/krennrich2009-summary

## :information_source: Information *資訊*
* URLs
* [[IOP Science] *New J. Phys.* **11** 115008 (2009)](https://iopscience.iop.org/article/10.1088/1367-2630/11/11/115008)
* https://doi.org/10.1088/1367-2630/11/11/115008
* BibLaTeX

    @article{krennrich-2009,
        author    = {Frank Krennrich},
        title     = {Gamma ray astronomy with atmospheric Cherenkov telescopes: the future},
        journal   = {New Journal of Physics},
        volume    = {11},
        year      = {2009},
        month     = {11},
        pages     = {115008},
        publisher = {{IOP} Science},
        doi       = {10.1088/1367-2630/11/11/115008},  
    }

## :page_with_curl: Table of Contents *目錄*
[TOC]

---

## 1. Introduction 簡介
Imaging atmospheric Cherenkov telescopes (IACTs) provide the most powerful tool for probing the high-energy universe in the teraelectronvolt (TeV) regime and are founded on pioneering advances that were made in the last two decades. The IACT technique was first successfully implemented in the Whipple 10 m γ -ray telescope and image analysis, where a single instrument led to a tenfold improvement in flux sensitivity over any previous installations [^1], and opened a window to galactic and extragalactic TeV astronomy [^2]. Another tenfold improvement in sensitivity was made in the last 5 years through stereoscopic imaging with arrays of large- size (12 m diameter) telescopes, namely the HESS [^3] and the VERITAS instruments [^4]. In addition, the combination of a very large mirror and fast timing is used in the 17 m MAGIC telescope [^5], which has also demonstrated a complementary approach to improving sensitivity. A worldwide community has established a catalog of more than 90 TeV sources [^6][^7] of a wide range of classes, while IACTs provide unprecedented astronomical capabilities at TeV energies: resolved images of some galactic sources, the measurement of flux variations as short as 200 s, and measurement of energy spectra up to 70 TeV. The IACT technique already provides the best angular resolution ($0.1^\circ$) of any astronomical technique above 0.1 MeV and has the potential for dramatic improvements. The recent revolution in TeV astronomy was enabled by a tenfold increase in flux sensitivity, while the capital investment for instrumentation increased a factor of 10 compared with previous telescopes. Further improvements are possible and would allow flux sensitivity at the $1~\text{mCrab}$ level ($10^{-13}~\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$). These advances are likely to use new technologies that have recently become available.

成像式大氣切倫科夫望遠鏡 (IACT) 為探測兆電子伏特 (TeV) 範圍內的高能宇宙提供了最強大的工具，並且建立在過去二十年取得的開創性進展之上。 IACT 技術首先在「Whipple 10米伽馬射線望遠鏡」和圖像分析中成功實施，其中一台儀器使通量靈敏度比以前的任何裝置提高了十倍 [^1]，並為銀河和河外 TeV 天文學打開了一扇窗。[^2]在過去 5 年中，透過使用大尺寸（直徑 12 公尺）望遠鏡陣列進行==立體成像==，即 HESS [^3] 和 VERITAS 儀器 [^4]，靈敏度又提高了 10 倍。 此外，在 17 m MAGIC 望遠鏡 [^5] 中使用了大型鏡面和快速計時的組合，這也展示了提高靈敏度的補充方法。一個全球社區已經建立了一個包含 90 多個 TeV 來源 [^6][^7] 的廣泛類別的目錄，而 IACT 在 TeV 能量方面提供了前所未有的天文能力：一些銀河內光源的解析圖像，通量變化的測量短至 200 秒，能譜測量高達 70 TeV。IACT 技術已經提供了任何高於 0.1 MeV 的天文技術的最佳角解析率 ($0.1^\circ$)，並且具有顯著改進的潛力。 最近 TeV 天文學的革命是由通量靈敏度提高十倍促成的，而與以前的望遠鏡相比，儀器的資本投資增加了 10 倍。 進一步的改進是可能的，並且將允許在 $1~\text{mCrab}$ 水準（$10^{-13}~\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$）的通量靈敏度。這些進步可能會使用最近出現的新技術。

In the following ([section 2](#2-Science-requirements-and-key-specifications-科學要求和關鍵規格)), I proceed to give a brief discussion of the science requirements that are drivers for the key specifications of future instruments. In section 3, the large array concept and its principle design considerations are presented, including the fundamental limitations associated with the detection of Cherenkov light from air showers. In section 4, I will discuss the most promising technologies for a future IACT system, including new optical designs and modular camera concepts and related research and development efforts that may be critical for the next generation of IACTs. Finally, in section 5, I briefly refer to specific array concepts that are being considered and are currently at the conceptual design stage.

* 在下文（[第 2 節](#2-Science-requirements-and-key-specifications-科學要求和關鍵規格)）中，我將簡要討論驅動未來儀器關鍵規格的科學要求。
* 在第 3 節中，介紹了大型陣列的概念及其主要設計考慮因素，包括與從空氣簇射中檢測切倫科夫光相關的基本限制。
* 在第 4 節中，我將討論未來 IACT 系統最有前途的技術，包括新的光學設計和模組化相機概念以及可能對下一代 IACT 至關重要的相關研發工作。
* 最後，在第 5 節中，我簡要介紹了正在考慮中且目前處於概念設計階段的特定陣列概念

## 2. Science requirements and key specifications *科學要求和關鍵規格*
Future IACTs will become an important follow-up to the all-sky survey at $E > 0.1~\text{GeV}$ provided by the Fermi space telescope [^8]. The excellent point source sensitivity and angular resolution of the IACT technique already enable in some cases symbiotic Fermi/IACT studies with current instruments [^9]. However, a 5–10 year Fermi exposure of our galaxy with well over $10^3$ (see also review by [R. Johnson and R. Mukherjee in this volume]()) objects will require a sensitive follow-up with a future IACT to extend statistical studies of source classes well into the TeV regime, thereby providing a combined coverage of six orders of magnitude in energy. This is particularly promising since IACTs allow detailed morphological and spectral studies well into the tens of TeV regime. <font color=red>This is critical for probing particle acceleration and the identification of particle populations in perhaps the most interesting regime, where electrons and protons distinguish themselves through their vastly different synchrotron cooling properties and may help break the degeneracy of leptonic and hadronic emission models.</font>

繼費米太空望遠鏡提供 $E > 0.1~\text{GeV}$ 全天巡天之後，未來的 IACT 將成為其重要的後續行動。[^8] IACT 技術擁有出色的點光源靈敏度和角解析率，在某些情況下已經研究者已經使用目前儀器進行了==費米/IACT 互利性（symbiotic）研究== [^9]。儘管費米衛星已對本星系 $10^3$ 個天體曝光了 5 到 10 年，在未來我們還需要利用 IACT 對它們進行靈敏的後續追蹤（另見 R Johnson 和 R Mukherjee 在本卷中的評論），以便將伽馬射線源分類的統計研究擴展到 TeV 體系，亦提供六個數量級的能量綜合覆蓋。這是大有可為的，因為 IACT 讓詳細的形態學和光譜研究得以進入數十個 TeV 的範圍。<font color=red>這對於在高能量區域中探測粒子加速和識別粒子居量來說是關鍵；在這些令人感興趣的高能量區域中，電子和質子的同步輻射冷卻特性大相徑庭，並可能有助於打破輕子和強子發射模型的簡併性。</font>

In the following, I provide a brief science motivation for the most important instrument parameters of future IACTs. For more details on the scientific results achieved with current generation instruments see the review by J Hinton [^10]. Owing to the fact that TeV astronomy provides information on a broad range of astrophysical objects, it is important to identify common instrument parameters that are critical for the individual science topics. A brief discussion of the science is followed by a summary and table (table 1) describing improvements, necessary for future major scientific breakthroughs in non-accelerator high-energy physics and TeV astronomy.

在下文中，我為未來 IACT 最重要的儀器參數提供了一個簡短的科學動機。 有關目前世代儀器取得的科學成果，更多詳細訊息請參閱 J. Hinton 的評論 [^10]。由於 TeV 天文學提供了廣泛的天文物理訊息，因此確定對個別科學主題至關重要的通用儀器參數非常重要。對科學的簡要討論之後是摘要和表格（表 1），描述了改進，這些改進是未來在非加速器高能物理學和 TeV 天文學方面取得重大科學突破所必需的。

::: spoiler **Table 1**
:::
\
**表 1.** 儀器能力改進和科學主題的對應。括號中的數字指的是 TeV 能量下的目前伽馬射線源的數量 [^6]。符號「+」表示重要，「++」表示關鍵，未分配符號表示它不是最優先項目。

|科學|天體|$S_{\leq.2~\text{TeV}}$|$S_{\geq.2~\text{TeV}}$|角解析度|F.O.V.|面積|
|---|---|:---:|:---:|:---:|:---:|:---:|
| 宇宙射線起源 | 超新星殘骸 (7) | + | ++ | ++ | ++ | ++ |
| 宇宙射線起源 | 黑暗加速器 (28) | | ++ | ++ | + | + |
| 宇宙射線起源 | 星團 (1) | | ++ | ++ | + | + |
| 宇宙射線起源 | 銀河內彌散伽馬射線源 | | ++ | | ++ | + |
| 宇宙射線起源 | 銀心 (1) | | ++ | ++ | ++ | ++ |
| 宇宙射線起源 | 星爆星系 (1) | | ++ | | | + |
| 宇宙射線起源 | 星系團 (0) | | ++ | | ++ | + |
| 緻密天體 | 脈衝星 (1) | ++ | | | | ++ |
| 緻密天體 | 脈衝星風星雲 (10) | + | ++ | ++ | + | ++ |
| 緻密天體 | 雙星系統 (3) | ++ | ++ | | | ++ |
| 相對論性噴流 | 耀變體 (24) | ++ | ++ | | + | ++ |
| 相對論性噴流 | 電波星系 (2) | ++ | ++ | ++ | | + |
| 相對論性噴流 | 伽馬射線爆 (0) | ++ | + | | ++ | + |
| 相對論性噴流 | 微類星體 (?) | ++ | ++ | | | + |
| 暗物質 | 矮星系 (0) | ++ | ++ | | | + |
| 暗物質 | 微星系暈 (0) | ++ | ++ | ++ | ++ | + |
| 暗物質 | 中質量黑洞 (0) | ++ | ++ | ++ | ++ | + |
| 銀河外背景 (EBL) | | + | ++ | | | ++ |
| 磁場 | | | ++ | | | + |
| 量子重力 | | ++ | ++ | | | ++ |

### 2.1. Galactic astrophysics *星系天文物理*

#### 2.1.1 Cosmic accelerator (I) *宇宙加速器（一）*
Our galaxy harbors particle accelerators that exceed the energies reached by any man-made machine, including the large hadron collider [^11], by several orders of magnitude. A key science goal of TeV astronomy with γ rays and neutrinos [^12] is to ==understand the origin of cosmic rays [^13][^14] and the nature of their sources [^15][^16]==. Cosmic rays should not escape their sources without leaving a trace of γ rays through the interaction with molecular gas in the vicinity of cosmic accelerators and subsequent neutral pion ($\pi^0\to\gamma\gamma$) and charged pion ($\pi^\pm\to\cdots\to\nu$) production. Identification of γ-ray sources and mapping their angular extent are most promising, e.g. for testing the paradigm that the remnants of supernova explosions are responsible for producing protons and other hadrons up to a few times $10^{15}~\text{eV}$ (PeV) [^17][^18]. ==Angular-resolved studies of individual sources [^19][^20][^21] trace the spatial distribution of particles and are key to mapping the physical conditions in the γ-ray emission region==. While a hadronic origin generally requires γ rays and molecular gas to be co-spatial, TeV γ rays could also be produced by relativistic electrons via inverse Compton scattering of ambient photons, e.g. the cosmic microwave background (CMB). In this case, a spatial correlation between x-ray emission and TeV emission is expected due to the dual role of electrons producing synchrotron (x-ray) and inverse Compton (TeV) emissions. Angular-resolved spectroscopy of TeV photons needs to approximately match <font color=red>the fine structure of shells</font> that have been unveiled at the sub-arcminute scale with x-ray studies [^22] and would be the most powerful tool to identify electron populations [^23] and distinguish hadronic from leptonic origin.

我們的銀河系擁有一些粒子加速器，它們的能量比任何人造機器（包括大型強子對撞機 [^11]）所能達到的能量還高，而且高出好幾個數量級。使用 γ 射線和微中子 [^12] 的 TeV 天文學的一個關鍵科學目標是==了解宇宙射線的起源 [^13][^14] 及其來源的性質 [^15][^16]==。宇宙射線在逃離它們的產地時一定會留下「痕跡」，留下痕跡的方式就是透過跟宇宙加速器附近的分子氣體產生交互作用，隨後又透過中性介子（$\pi^0\to\gamma\gamma$）和帶電介子（$\pi^\pm\to\cdots\to\nu$）產生交互作用。「識別 γ 射線源並繪製其的角度範圍」是檢驗一些理論典範的利器，舉例來說，可以測試超新星爆炸的殘骸是否如理論預測，產生高達數個 $10^{15}~\text{eV}$（PeV）的質子和其他強子。[^17][^18]==單個 TeV 源的角度解析研究 [^19][^20][^21] 可追蹤粒子的空間分佈，是描繪 γ 射線發射區物理條件==的關鍵。雖然強子源通常需要 γ 射線和分子氣體佔有相同空間，但 TeV γ 射線也可以由相對論性電子、經環境光子（例如宇宙微波背景）的逆康普頓散射產生。在這種情況下，由於電子產生同步輻射（X 射線）和逆康普頓（TeV）發射的雙重作用，我們預期 X 射線發射和 TeV 發射之間存在空間相關性。TeV 光子的角解析光譜需要大致匹配 X 射線研究 [^22] 在次角分尺度（sub-arcminute scale）上所揭示的<font color=red>殼層的精細結構</font>，這將是識別電子數量的最強大工具 [^23]，亦可區分強子和輕子的起源。

#### 2.1.2 Cosmic accelerator (II) *宇宙加速器（二）*
A larger collection area is necessary to make use of a better angular resolution to provide sufficient photon statistics. ==An order of magnitude increase in collection area paired with a factor of 2--3 better angular resolution== would be a major step forward towards resolving the non-thermal emission regions in our galaxy and identifying cosmic accelerators. A ten times better sensitivity is required for ==broad studies with several hundred galactic TeV sources== [^24] and is key for ==attributing the cosmic accelerator phenomenon to a specific class/classes of astrophysical objects==. Furthermore, in order to map supernova remnants of a large angular size (angular size $\sim$ few degrees), e.g. [Vela Junior](https://en.wikipedia.org/wiki/RX_J0852.0%E2%88%924622) [^19] and to detect regions where multiple supernovae have occurred, ==a larger field of view than currently employed by IACTs== is desirable. Additional evidence for extended sources of several degrees across was also provided by the Milagro experiment [^25].

需要更大的集光區域，才能用更精細的角解析度來提供足夠的光子統計數據。如果讓集光面積增加一個數量級，使得角解析度提升 2 到 3 倍，將使我們邁出的重要一步，得以分辨銀河系中非熱輻射區域，並找出宇宙加速器。如果將靈敏度提高十倍，就能==廣泛研究數百個銀河系內 TeV 源 [^24] 進行，並把宇宙加速器現象歸因於特定類別的天體==。此外，為了==繪製大角距（好幾度）的超新星遺跡==，例如 [Vela Junior](https://zh.wikipedia.org/wiki/RX_J0852.0-4622) [^19]，並檢測發生多顆超新星的區域，所需的視野必須大於 IACT 目前所使用的視野。Milagro 實驗 [^25] 也提供了其他證據，證明有橫跨數個角度的展源。
#### 2.1.3 Compact objects *緻密天體*
Other particle accelerators in our galaxy include compact objects such as pulsars [^26][^27][^28], pulsar wind nebulae [^29][^30], pulsar wind binaries [^31] and possibly microquasars [^4][^32][^33]. The γ-ray detection of rapidly spinning neutron stars (*viz.* pulsars) is currently the domain of space-based telescopes (Fermi, AGILE). Nevertheless, the IACT technique offers unique capabilities, as was demonstrated by a recent detection of the [Crab pulsar](https://en.wikipedia.org/wiki/Crab_Pulsar) above 25 GeV [^26]. The inherently large collection area of IACTs could be used to provide large photon statistics for mapping the cut-offs of pulsed emission, thereby probing ==particle acceleration in pulsar magnetospheres==. This provides a key motivation to improve the sensitivity of IACTs in the 20--100 GeV regime. TeV γ rays also probe ==the interaction of the relativistic wind from a pulsar with its surrounding interstellar medium or a massive binary star==, as found in high-mass x-ray binaries. Particle acceleration in shocks do potentially generate PeV electrons that produce TeV emission via inverse Compton scattering of ambient radiation fields. Given the angular extent of some of these pulsar wind nebula [^30][^36], an angular resolution improvement of a factor of a few would allow mapping of the particle distribution and diffusion processes in many objects.

我們銀河系中的其他粒子加速器也包括緻密天體，例如脈衝星 [^26][^27][^28]、脈衝星風星雲 [^29][^30]、脈衝星風雙星 [^31] 和可能存在的微類星體 [^4][^32][^33]。快速旋轉中子星（即脈衝星）的 γ 射線目前屬於太空望遠鏡（費米、AGILE）的探測領域。然而，IACT 技術提供了獨特的探測能力；最近對[蟹狀星雲脈衝星](https://zh.wikipedia.org/wiki/%E8%9F%B9%E7%8A%B6%E6%98%9F%E4%BA%91%E8%84%89%E5%86%B2%E6%98%9F) 25 GeV 以上的探測證明了這一點 [^26]。IACT 本身具有大範圍的集光區域，可用於提供大量的光子統計數據，用來繪製脈衝發射的截止點（cut-off），使我們得以研究==脈衝星磁層中的粒子加速機制==。這點引起我們產生關鍵性的動機來提高 IACT 在 20 到 100 GeV 範圍內的靈敏度。TeV γ 射線還可以用來探測==來自脈衝星的相對論風與其周圍星際介質或大質量雙星的交互作用==；這出現在大質量 X 射線雙星中。衝擊中的粒子加速確實可能產生 PeV 電子，這些電子透過環境輻射場的逆康普頓散射產生 TeV 發射。在給定其中一些脈衝星風星雲的角範圍 [^30][^36] 的條件下，若角解析度提高幾倍，將允許繪製許多物體中的粒子分佈和擴散過程。
#### 2.1.4 TeV electrons *TeV 電子*
The understanding of the origin of cosmic TeV electrons is also critical for the interpretation of possible dark matter signatures that might contribute to the locally measured cosmic-ray electron spectrum. A recent report of an excess around 600 GeV in the ATIC balloon experiment [^37] could be interpreted as a signature for dark matter; however, the result is not confirmed by data from Fermi [^38]. IACTs also measure the electron spectrum up to multi-TeV energies; however, they provide high statistics [^39] using a complementary technique. ==A putative excess could be interpreted as a dark matter self-annihilation signature, or could also arise from a relatively nearby pulsar [^40][^41] or have its origin in a nearby supernova remnant [^42][^43] or microquasar [^44]== and highlights the importance of understanding astrophysical cosmic-ray backgrounds.

了解宇宙 TeV 電子的起源也有助於解釋可能暗物質特徵，這種特徵可能出現在局部測量的宇宙射線電子能譜中。最近一份關於 ATIC 氣球實驗 [^37] 測量超過 600 GeV 的報告可以解釋成暗物質的特徵；不過費米望遠鏡 [^38] 的數據並未證實這一結果。IACT 還可以測量高達數個 TeV 能量的電子能譜，但它們是利用一種互補的技術來提供大量統計數據 [^39]。==假若有過量的宇宙 TeV 電子，那麼它可以被解讀為暗物質自我湮滅（self-annihilation）的特徵，它也可能來自相對較近的脈衝星 [^40][^41]，或起源於附近的超新星遺跡 [^42][^43] 或微類星體 [^44]==，以上種種解讀都體現了天文物理學理解宇宙射線背景的重要性。

#### 2.1.5

#### 2.1.6

### 2.2. Extragalactic astrophysics *河外天文物理*

## 3. The large array concept *大型陣列的概念*
#### 3.0.1
Current IACT arrays built for TeV γ-ray astronomy are the HESS and VERITAS observatories, and most recently the MAGIC-II stereo system [^67]. HESS and VERITAS each consist of four telescopes with a spacing of $\sim$ 100 m. ==The key instrument design goals are: a factor of ten better sensitivity and collection area, a better angular resolution, and a lower energy threshold compared with current instruments.== A natural progression of the IACT technique is to consider large arrays of imaging telescopes. Simplistic scaling of the number of telescopes suggests that ==sensitivity improves with $\sqrt{N}$== ($N =$ number of telescopes). This would require approximately 400 telescopes to reach an order of magnitude better sensitivity. In the following, we discuss the principal design considerations that show that ==large arrays provide significantly better performance improvements than expected== from reconstruction capabilities of air showers whose shower cores fall within the array boundaries, thereby providing ‘contained’ events. As a consequence, much fewer telescopes (∼100) are required to reach the stated design goals, opening the path for a cost effective construction of IACT arrays that meet the design goals stated in the previous section.

目前為 TeV 伽馬射線天文學建造的 IACT 陣列有 HESS、VERITAS 天文台，以及最近的 MAGIC-II 立體系統 [^67]。HESS 和 VERITAS 各由四台望遠鏡組成，間距約為 100 公尺。==儀器設計的主要目標是：與現有儀器相比，靈敏度和集光區域提高十倍，角解析度提高，能量閾值降低==。IACT 技術的自然發展是考慮**大型成像望遠鏡陣列**（large arrays of imaging telescopes）。望遠鏡數量的簡單化縮放（scaling）表明==靈敏度隨 $\sqrt{N}$ 而提高==（$N=$ 望遠鏡數量）。根據這點，我們需要大約 400 台望遠鏡才能將靈敏度提升到下一個數量級。在下文中，我們將討論主要的設計考慮因素，這些因素顯示==大型陣列提供的性能改良明顯優於 $\sqrt{N}$ 縮放所預期的性能改良==。這主要是因為核心落在陣列邊界內的空氣簇射具有更好的重建能力，從而提供「包含」（contained）事件。因此，用更少的望遠鏡（$\sim100$ 台）就可以達到指定的設計目標，這有如開闢了一條捷徑，使我們建造具有成本效益的 IACT 陣列，以符合上一節所述設計目標。
#### 3.0.2 Differential Flux Sensitivity *微分通量靈敏度*
To identify the most effective means of improving upon sensitivity, it is instructive to consider the limitations of existing IACT arrays. Figure 1 shows the differential flux sensitivity2 of current generation IACTs (HESS and VERITAS), the Fermi Gamma-Ray Space Telescope and an approximate estimate of future IACTs such as AGIS[^68]/Cherenkov telescope array (CTA)[^69]. The physical limitation for flux sensitivity essentially occurs for three distinct reasons corresponding to different energy regimes:

為了找出最有效方法來提高靈敏度，最好考慮現有 IACT 陣列的局限性以得到啟發。圖 1 顯示了目前世代的 IACT（HESS 和 VERITAS）、以及費米伽馬射線太空望遠鏡（Fermi Gamma-Ray Space Telescope）的**微分通量靈敏度**（differential flux sensitivity），以及對未來 IACT（例如 AGIS[^68]、契忍可夫望遠鏡陣列〔CTA〕[^69]）的近似估計。通量靈敏度的物理限制基本上是起因於對應不同能量狀態的三個不同原因：
* The sensitivity above $\sim 5~\text{TeV}$ is ==photon-count limited== (<font color=blue>blue line</font> in figure 1) and can only be improved by increasing the collection area. A ten times larger area would increase the sensitivity by an order of magnitude, since background contamination becomes negligible due to a falling cosmic-ray spectrum and reconstruction improves for bright—thereby well defined—Cherenkov light images.

高於 $\sim 5~\text{TeV}$ 的靈敏度==受限於光子計數的稀少性==（圖 1 中的<font color=blue>藍線</font>），只能透過增加集光面積來提高靈敏度。十倍大的面積會使靈敏度提高一個數量級；這是因為宇宙射線通量下降，而且對於明亮（而定義良好的）契忍科夫光而言，圖像重建有所改善，使得背景的污染變得可以忽略。

* The medium energy regime between $100~\text{GeV}$ and $5~\text{TeV}$ (<font color=green>green line</font> in figure 1) is generally ==dominated by background that arises from cosmic-ray showers==. A better discrimination between the characteristics of hadronic cosmic-ray showers and γ-ray showers is required. In addition, a better angular resolution helps to reject cosmic rays (electrons and hadrons) with random arrival directions.

$100~\text{GeV}$ 和 $5~\text{TeV}$ 之間的中等能量區域（圖 1 中的<font color=green>綠線</font>）通常由==宇宙射線簇射產生的背景==支配。所以我們有必要更仔細地區分「強子宇宙射線簇射」和「γ 射線簇射」的特徵。此外，較佳的角解析度也有助於拒斥來自隨機方向的（電子和強子）宇宙射線。

* The energy regime below $100~\text{GeV}$ (<font color=red>red line</font> in figure 1) is limited by an additional background component. While cosmic rays are still a significant contributor, ==the most severe background arises from Poisson fluctuations of the **night sky background** (NSB) light==. For a given telescope mirror area, the latter poses a ‘wall’ of accidental triggers and sets a limit for triggering a Cherenkov telescope and its ability to detect low-energy γ-ray showers characterized by low-light levels. While the night sky becomes a dominant limitation at $100~\text{GeV}$ for a 12 m telescope, the energy threshold $E_\text{thres.}$ can be lowered. $E_\text{thres.}$ can be reduced by ==increasing the amount of Cherenkov light collected either by using a larger light collector, higher quantum efficiency (QE) photodetectors and all measures that minimize light losses in the instrument==. The energy threshold of IACTs scales approximately with $A^{−1/2}_\text{mirror}$ and $\text{QE}^{−1/2}$, the mirror area and the QE, respectively. However, when approaching energies as low as $30~\text{GeV}$, intrinsic shower fluctuations (a few secondary particles) in the Cherenkov light signal from a γ ray leads to poor hadron rejection. Furthermore, the Cherenkov light images contain significant contamination from accidental NSB photoelectrons compromising the image analysis.

低於 $100~\text{GeV}$ 的能量區域（圖 1 中的<font color=red>紅線</font>）受到額外背景成分的限制。雖然宇宙射線仍然是一個重要的貢獻者，但==最嚴重的背景是**夜空背景** (night sky background，NSB) 光的泊松波動（Poisson fluctuations）造成的==。對於給定的望遠鏡鏡面區域，後者變成了一種會偶然觸發的「牆」，並抑制了契忍科夫望遠鏡的觸發，也限制了它檢測低能量 γ 射線簇射的能力，其中低能量 γ 射線的特徵是低光水平（low-light levels）。雖然對於 12 米望遠鏡來說，夜空成為在 $100~\text{GeV}$ 的主要限制，但是它的能量閾值 $E_\text{thres.}$ 還可以再降低。如果要降低 $E_\text{thres.}$，就要==使用更大的集光器和量子效率 (QE) 更高的光電探測器，並想盡辦法減少儀器中損失的光子，才能增加收集到的契忍科夫光==。IACT 的能量閾值分別大致正比於 $A^{−1/2}_\text{mirror}$ 和 $\text{QE}^{−1/2}$，其中 $A_\text{mirror}$ 為鏡面面積。然而，當能量低至 $30~\text{GeV}$ 時， γ 射線簇射的契忍科夫光訊號本身具有一些（次級粒子造成的）波動，這會導致拒斥強子簇射的效果較差。再者，此時的契忍科夫光圖像包含來自偶發 NSB 光電子的嚴重污染，連累了圖像分析。

<font size=1> **Figure 1.** The differential flux sensitivity $S$ ($5\sigma$ point source detection per quarter decade in energy is required) is shown in units of erg cm^−2^ s^−1^ (power) for various existing/future γ-ray telescopes: Fermi (1 year), the HESS or VERITAS (50 h) observatory and a future large array of IACTs (AGIS/CTA). Note that improved sensitivity means a lower flux can be detected. The flux sensitivity for a future IACT is only an idealized approximation [^70] in the mid-(green) and high-(blue) energy regimes and does not take into account additional background from NSB at low (red) energies. More accurate sensitivity estimates are currently under study based on extensive Monte Carlo simulations and specific cases can be found in Bernlöhr [^71], which are close to the curve presented here at mid and high energies.

**圖 1.** 各種現有以及未來的 γ 射線望遠鏡的微分通量靈敏度 $S$（對於所需能量，每四分之一個 decade 的 $5\sigma$ 點光源檢測），單位為 erg cm^−2^ s^−1^（功率）。這些 γ 射線望遠鏡包含了：費米太空望遠鏡（1 年)、HESS 或 VERITAS (50 小時) 天文台和未來的大型 IACT 陣列 (AGIS 和 CTA)。請注意，==提高靈敏度意味著可以檢測到較低的通量==。未來 IACT 的通量靈敏度只是中能量（綠色）和高能量（藍色）區域的理想化近似值 [^70]，並且不考慮 NSB 在低能量（紅色）下的額外背景。目前研究人員正在使用大量的蒙地卡羅模擬來獲得更準確的靈敏度估計，具體案例可以在 Bernlöhr [^71] 中找到，那些靈敏度估計接近這張圖呈現的中、高能量曲線。</font>

Design considerations for a large array require first and foremost a good understanding of the lateral distribution of Cherenkov light produced by the electromagnetic cascade from a γ -ray shower. In figure 2(a), we show the simulated lateral distribution of Cherenkov light for γ -ray events of various energies. The density of photoelectrons versus core distance remains relatively flat for γ -ray primaries below 1 TeV and exhibits a characteristic ‘rim’ [^73] at approximately 120 m for an observing altitude of 2 400 m above sea level. The rim arises from an increasing Cherenkov angle with air density, as particles penetrate deeper into the atmosphere. For example, the Cherenkov angle increases from 0.3 ◦ at 15 km atmospheric height to 0.8 ◦ at 7 km, while the distance between the emission height and detector elevation decreases. Therefore, Cherenkov light from particle tracks close to the shower axis is accumulated at a characteristic core distance, namely the ‘rim’. The region within the rim is often referred to as the ‘Cherenkov light pool’ and covers an area of ∼50 000 m2 .

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*[ATIC]: Advanced Thin Ionization Calorimeter，先進稀薄游離量熱計
*[PAMELA]: Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics，反物質探索和輕核天文物理學酬載

*[AGILE]: Astro‐Rivelatore Gamma a Immagini Leggero，伽瑪射線輕型探測器
*[AGN]: Active Galaxy Nucleus，活躍星系核
*[ARGO-YBL]: Astrophysical Radiation with Ground-based Observatory at YangBaJing，羊八井天文物理輻射地面天文台
*[CAT]: Cherenkov Array at Thémis，泰美斯契忍科夫陣列
*[CTA]: Cherenkov Telescope Array，契忍科夫望遠鏡陣列
*[EAS]: Extensive Air Shower，廣域空氣簇射
*[GRB]: Gamma Ray Burst，伽馬射線爆
*[FOV]: Field of View，視野
*[F.O.V.]: Field of View，視野
*[HAGAR]: High Altitude GAmma Ray experiment，高海拔伽馬射線實驗
*[HAWC]: High-Altitude Water Cherenkov Observatory，高海拔水契忍科夫天文台
*[HEGRA]: High-Energy-Gamma-Ray Astronomy，高能伽馬射線天文學實驗
*[HESS]: High Energy Stereoscopic System，高能立體視野望遠鏡
*[H.E.S.S.]: High Energy Stereoscopic System，高能立體視野望遠鏡
*[HiSCORE]: Hundred Square-km Cosmic ORigin Explorer，一百平方公里宇宙起源探測者
*[IACT]: imaging atmospheric/air Cherenkov telescope，成像式大氣契忍科夫望遠鏡
*[LAT]: Large Area Telescope，大面積望遠鏡
*[LHAASO]: Large High Altitude Air Shower Observatory，大型高海拔空氣簇射天文台
*[LST]: Large-Sized Telescope，大口徑望遠鏡
*[MACE]: Major Atmospheric Cherenkov Experiment，大氣契忍科夫實驗
*[MAGIC]: Major Atmospheric Gamma Imaging Cherenkov Telescope，神奇伽馬射線望遠鏡
*[MST]: Medium-Sized Telescope，中口徑望遠鏡
*[NSB]: Night Sky Background，夜光背景
*[PMT]: Photomultiplier tube ，光電倍增器管
*[QE]: quantum efficieny，量子效率
*[SiPM]: Silicon PhotoMultiplier，矽光電倍增器
*[SNR]: Supernova Remnant，超新星殘骸
*[SST]: Small-Sized Telescope，小口徑望遠鏡
*[SSTCAM]: Small-sized Telescope Camera，小口徑望遠鏡相機
*[UHE]: Ultra-High-Energy，超高能
*[UHECR]: Ultra-High-Energy Cosmic Rays，超高能宇宙射線
*[VERITAS]: Very Energetic Radiation Imaging Telescope Array System，
*[VHE]: Very-High-Energy，甚高能
*[WCT]: Water Cherenkov Telescope，水契忍科夫望遠鏡