梁云峰

(中國科學(xué)院紫金山天文臺(tái)南京210008)

在現(xiàn)代的天體物理觀測中,有許多天文現(xiàn)象包括星系旋轉(zhuǎn)曲線、星系團(tuán)的質(zhì)量測量、宇宙微波背景輻射的角功率譜等,難以用現(xiàn)有模型進(jìn)行解釋.它們跨越多個(gè)尺度—小至星系尺度,大至整個(gè)宇宙尺度,需要在模型中額外引入一定的質(zhì)量才能令觀測現(xiàn)象得到很好的解釋,而提供這些質(zhì)量的“物質(zhì)”卻難以用通常的電磁波或中微子手段探測到.這部分物質(zhì)被稱為“暗物質(zhì)”.根據(jù)現(xiàn)有的觀測結(jié)果,它們應(yīng)具有正常的引力相互作用,沒有強(qiáng)相互作用和電磁相互作用,壽命很長(顯著長于宇宙的壽命).對(duì)暗物質(zhì)粒子的成功探測可望引發(fā)一場新的物理學(xué)革命.目前暗物質(zhì)探測的最主要方法之一是所謂的間接探測,其目的是探測暗物質(zhì)湮滅或衰變產(chǎn)生標(biāo)準(zhǔn)模型中的粒子,包括伽馬光子、正反中微子、正反電子、正反質(zhì)子等.在間接探測中,基于伽馬光子的探測受到廣泛的關(guān)注—因?yàn)橘ゑR射線可以直接示蹤輻射區(qū)的具體方位,有助于研究這些輻射與空間暗物質(zhì)分布的關(guān)聯(lián)性,而且對(duì)它們的探測效率顯著地高于中微子.但探測過程仍然面臨著很多潛在的挑戰(zhàn),其中一個(gè)主要的挑戰(zhàn)是天體物理過程導(dǎo)致的高能伽馬輻射對(duì)暗物質(zhì)信號(hào)的污染,因此搜尋具有獨(dú)特能譜特征的暗物質(zhì)信號(hào)就顯得尤為重要.伽馬射線線譜就是這樣一種信號(hào),因?yàn)樯袩o已知的天體物理機(jī)制能產(chǎn)生此類信號(hào),所以它一旦被可靠探測到,就意味著新物理的發(fā)現(xiàn).為此在攻讀博士學(xué)位期間,我致力于在Fermi-LAT(Large Area Telescope)數(shù)據(jù)中搜尋(暗物質(zhì))線譜信號(hào).

論文的第1章是對(duì)一些相關(guān)的背景進(jìn)行介紹,包括暗物質(zhì)觀測證據(jù)、暗物質(zhì)探測方法、暗物質(zhì)間接探測、Fermi-LAT儀器及數(shù)據(jù)、暗物質(zhì)信號(hào)搜尋研究進(jìn)展等.

論文第2章到第4章介紹我們在伽馬射線線譜方面的4個(gè)工作:(1)利用Fermi-LAT的85個(gè)月的觀測數(shù)據(jù)分析了16個(gè)近鄰星系團(tuán)方向的伽馬射線輻射,我們在～43 GeV能量處發(fā)現(xiàn)了疑似的線譜信號(hào),該信號(hào)一旦在將來得到證實(shí),將是暗物質(zhì)間接探測領(lǐng)域的重要進(jìn)展.由于該信號(hào)仍較弱,我們給出了對(duì)暗物質(zhì)湮滅到線譜速度的平均截面(即?σv?χχ→γγ)的限制并指出除非星系團(tuán)普遍具有很高的增強(qiáng)因子(BF,對(duì)于一個(gè)樣本群體的平均值記為,否則星系團(tuán)給出的限制要顯著弱于銀心觀測給出的限制;(2)針對(duì)我們發(fā)現(xiàn)的43 GeV疑似線譜信號(hào),在假定暗物質(zhì)起源的前提下應(yīng)該有?σv?～5×10?28/103)?1cm3·s?1,我們討論了幾種可能的模型,發(fā)現(xiàn)要解釋觀測到的截面通常需要一個(gè)較大的耦合參數(shù);(3)我們通過分析Fermi-LAT對(duì)銀河系衛(wèi)星星系(包括所有已知的矮星系和大、小麥哲倫星系)長達(dá)91個(gè)月的伽馬射線輻射觀測來搜尋暗物質(zhì)湮滅產(chǎn)生的線譜信號(hào),并給出了相應(yīng)的線譜湮滅截面的上限,相較于之前基于4 yr Pass 7數(shù)據(jù)得到的結(jié)果,我們的限制顯著增強(qiáng);(4)基于分析Via Lactea IIN體數(shù)值模擬結(jié)果得到的暗物質(zhì)子結(jié)構(gòu)的分布和相關(guān)關(guān)系,我們模擬了一批銀河系附近區(qū)域的暗物質(zhì)子結(jié)構(gòu)群體,并計(jì)算了它們的J因子,再結(jié)合Fermi-LAT的91個(gè)月的觀測數(shù)據(jù),我們給出了對(duì)線譜湮滅截面的限制,這是首次結(jié)合N體模擬的結(jié)果與高能伽馬射線衛(wèi)星的觀測來對(duì)線譜湮滅截面做出限制.

在第5章中扼要介紹了我們在暗物質(zhì)湮滅產(chǎn)生的伽馬射線連續(xù)譜信號(hào)搜尋方面的幾點(diǎn)研究進(jìn)展,包括銀心GeV超出的物理真實(shí)性研究、矮星系Tuc III方向的疑似GeV輻射信號(hào)、2個(gè)非認(rèn)證點(diǎn)源的暗物質(zhì)暈物理起源的可能性研究.

此外,在攻讀博士期間我還參加了暗物質(zhì)粒子探測衛(wèi)星(悟空號(hào))項(xiàng)目并負(fù)責(zé)伽馬射線分析軟件的開發(fā).在第6章我們介紹了悟空號(hào)伽馬射線分析軟件的數(shù)學(xué)原理、軟件結(jié)構(gòu)、研發(fā)進(jìn)展,并展示了一些初步的結(jié)果.我們的結(jié)果能很好地吻合Fermi-LAT的觀測,這表明我們的伽馬射線分析軟件以及悟空號(hào)探測器本身、數(shù)據(jù)重建、事例篩選、探測器模擬等都是可靠的.

最后我們對(duì)基于悟空號(hào)以及Fermi-LAT數(shù)據(jù)的伽馬射線線譜搜尋的未來進(jìn)行了展望.

Search for Gamma-ray Line Features with Fermi-LAT Data and Development of DAMPE Gamma-ray Science Analysis Software

LIANG Yun-feng
(Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210008)

Many astrophysical and cosmological phenomena,such as the flat rotation curves of galaxies,the discrepancy between luminosity masses and kinematic masses of galaxy clusters,and the cosmic microwave background power spectrum,indicate the existence of a large amount of so-called dark matter(DM)in the Universe.It is known that none of the currently known standard model(SM)particles can be viable for the DM particle,thus the discovery of the DM may open a new window for our fundamental physics.However,though it is well established that the DM consists～26%of the total energy density of the current Universe,its nature is still far from clear since all the evidence or properties are inferred from gravitational effects.It is highly necessary to find non-gravitational evidences of the DM.One way to search for such non-gravitational evidences is the DM indirect detection,by which we search for the annihilation or decay products of DM,including photons,electrons and positrons,protons and antiprotons,and neutrinos and antineutrinos.Among all kinds of possible DM annihilation signals,monochromatic gamma-ray lines,generated by DM annihilating to double photons directly,play an important role in the DM indirect detections.Since no known astrophysical process could generate a line-like gamma-ray signal,it can be clearly discriminated from astrophysical backgrounds.

In the first chapter,we introduce some general background of the dark matter indirect detection.

In the second and third chapters,we introduce our works on searching for gamma-ray line emission originated from the dark matter annihilation in some nearby massive Galaxy Clusters and the Milky Way satellites,respectively.No compelling evidence for the line signal has been identi fied,and upper limits are set on the annihilation cross section of dark matter particles into mono-energetic gamma-rays.The only potential candidate is a～43 GeV line-like emission in the directions of 16 nearby massive Galaxy Clusters,and the possible dark matter interpretations of such a signal have been examined.In the fourth chapter,we have set the limits on the cross sections of dark matter annihilation to the gamma-ray lines with subhalo distributions in N-body simulations and Fermi-LAT(LargeArea Telescope)data.

In the fifth chapter,we introduce some of our works in the continual signal search of gamma-ray DM,including the dependence of the Galactic GeV excess on the diffuseemission background,the tentative GeV emission in the direction of dwarf spherical galaxy Tuc III,and the possibilities of 3FGL J2212.5+0703 and 3FGL J1924.8-1034 being dark matter subhalos.

In these years I have also been concentrated on developing the gamma-ray science analysis software of DAMPE(Dark Matter Particle Explorer)satellite,the first space based high energy cosmic ray andγ-ray detector of China.In the sixth chapter,I introduce the mathematical foundation,the framework,and the current status of this software.Moreover,some preliminary results obtained in the DAMPE gamma-ray data analysis by this software are shown.The DAMPE results are well consistent with Fermi-LAT,indicating that not only our software is valid for science analysis but also all the aspects of DAMPE satellite are in good conditions.We suggest that our understanding of the dark matter can be greatly improved with the help of DAMPE,Fermi-LAT,HERD(High Energy cosmic Radiation Detection),GAMMA-400,and other high energy detectors.