夏子晴

(中國科學院紫金山天文臺南京210023)

目前已經(jīng)有很多觀測證據(jù)表明宇宙中存在著大量暗物質(zhì), 其能量密度占據(jù)了目前宇宙總能量密度的1/4. 根據(jù)高精度的數(shù)值模擬和引力透鏡觀測, 我們已經(jīng)對從矮星系到星系團中的暗物質(zhì)空間分布有了較好的理解, 但是對于暗物質(zhì)究竟是什么我們還一無所知.

由此, 物理學家提出了很多假想的粒子模型. 其中比較著名的粒子模型有: 弱相互作用大質(zhì)量粒子(WIMP)、軸子和類軸子(ALP). 弱相互作用大質(zhì)量粒子只存在弱相互作用和引力相互作用, 可以相互湮滅(或者衰變)成穩(wěn)定的高能粒子, 包括伽馬光子、帶電粒子和中微子. 從而使我們可以通過探測其湮滅(或者衰變)產(chǎn)生的高能粒子來間接探測弱相互作用大質(zhì)量粒子. ALP可以在電磁場中與光子相互轉化, 這一特性使得我們可以通過尋找伽馬射線能譜中的光子-類軸子振蕩結構來間接探測類軸子.本文中的研究主要是利用公開的費米大面積望遠鏡(Fermi Large Area Telescope, Fermi-LAT)的數(shù)據(jù)和已發(fā)表的大氣切倫科夫望遠鏡High Energy Stereoscopic System (H.E.S.S.)能譜數(shù)據(jù), 對暗物質(zhì)粒子(軸子和類軸子、弱相互作用大質(zhì)量粒子)進行間接探測.

銀河系中廣泛存在著磁場, 因此在河內(nèi)源的能譜中可能存在著由光子和類軸子相互轉化而形成的振蕩結構. 首先我們選取了3個在銀盤上且非常明亮的超新星遺跡作為目標源(分別是IC443、W44和W51C), 利用Fermi-LAT對這3個超新星遺跡的觀測來尋找光子-類軸子振蕩信號. 在IC443的能譜中, 我們找到了疑似的振蕩結構, 但是其對應的類軸子參數(shù)空間已經(jīng)被太陽軸子望遠鏡CAST (CERN (European Centre for Nuclear Research) Axion Solar Telescope)排除. 我們猜測, 由于IC443是個空間延展的源, 其能譜中出現(xiàn)的疑似的振蕩結構可能是來自不同區(qū)域伽馬射線輻射疊加的結果. 然后我們選取了10個明亮的位于銀盤上的TeV源, 利用H.E.S.S.發(fā)表的能譜數(shù)據(jù)繼續(xù)搜尋類軸子. 然而我們并沒有找到明顯的光子-類軸子振蕩信號, 隨后計算出了對類軸子參數(shù)空間的限制.這是首次利用天文觀測數(shù)據(jù)在高質(zhì)量區(qū)域(100 neV)對解釋河外TeV伽馬射線反常弱吸收的類軸子模型參數(shù)空間進行排除.

我們還利用Fermi-LAT伽馬射線觀測, 搜尋了來自暗物質(zhì)子暈結構的弱相互作用大質(zhì)量粒子湮滅信號. 目前有大量數(shù)值模擬的結果顯示, 像銀河系這樣的星系中存在大量的暗物質(zhì)子暈結構. 暗物質(zhì)粒子可以湮滅或者衰變產(chǎn)生伽馬射線. 因此質(zhì)量足夠大且距我們足夠近的暗物質(zhì)子暈可能會以穩(wěn)定延展伽馬射線源的形式出現(xiàn), 同時沒有其他波段的對應天體. 以此為標準, 我們找到了一個可能的暗物質(zhì)子暈候選體3FGL J1924.8?1034, 但是由于Fermi-LAT角分辨率的局限, 我們不能排除它是由兩個(及以上)鄰近點源組成的可能.

由于高的質(zhì)光比, 矮橢球星系一直被認為是暗物質(zhì)間接探測的理想目標. 我們搜尋了銀河系附近矮橢球星系的伽馬射線輻射, 來探測弱相互作用大質(zhì)量粒子的信號. 分析發(fā)現(xiàn)來自Reticulum II方向的伽馬射線信號是隨時間穩(wěn)步增長的. 隨后我們對所有目標源進行了聯(lián)合分析, 得到的聯(lián)合伽馬射線信號已經(jīng)超過了4σ的局域置信度. 在暗物質(zhì)間接探測中, 主要困難在于如何把暗物質(zhì)湮滅或衰變產(chǎn)物的信號從天體物理背景中分離出來. 如果是搜尋具有某些獨特特征的能譜, 如線譜和箱型能譜, 在這方面遇到的困難就要小一些, 因為通常的天體物理輻射過程難以出現(xiàn)這種特殊結構的能譜. 在本文的工作中, 我們還利用了Fermi-LAT數(shù)據(jù)來搜尋暗物質(zhì)粒子可能產(chǎn)生的特征能譜(包括線譜和箱型能譜)信號.我們分別在銀河系衛(wèi)星星系和銀河系內(nèi)的暗物質(zhì)子暈結構(通過N體模擬)尋找潛在的線譜信號. 由于沒有發(fā)現(xiàn)明顯信號, 我們隨后計算出了暗物質(zhì)湮滅成兩個光子的湮滅截面的相應上限. 隨后我們還在矮橢球星系中, 研究了由暗物質(zhì)湮滅或衰變所產(chǎn)生的中間粒子衰變發(fā)出的箱型伽馬射線能譜信號.

The presence of a large amount of dark matter (DM) in the Universe has already been convincingly established. DM is believed to make up a quarter of the energy density of the current Universe. Thanks to high-resolution numerical simulations made possible by modern supercomputers and the gravitational lensing observations, the distribution of DM in structures ranging from dwarf galaxies to clusters of galaxies has been understood better than before. But the nature of DM remains unknown.

Various hypothetical particles have been proposed, such as weakly-interacting massive particles (WIMPs), axion, axion-like particles (ALPs), sterile neutrino and gravitino.WIMPs may be able to annihilate with each other (or alternatively decay) into stable highenergy particle pairs, including gamma-rays, charged particles and neutrinos. ALPs and photons can convert to each other in electromagnetic fields through the Primakoff process,which could result in the detectable spectral oscillation phenomena in the gamma-rays observation. My research mainly focused on the indirect detection of dark matter, such as ALPs and WIMPs, using publicly available Fermi Large Area Telescope (Fermi-LAT) data and the the published data of High Energy Stereoscopic System (H.E.S.S.) observation.

The conversion between photons and ALPs in the Milky Way magnetic field could result in the detectable oscillation phenomena in the gamma-ray spectra of the Galactic sources.First, we search for such oscillation effects in the spectra of supernova remnants caused by the photon-ALP conversion, using the Fermi LAT data. The inclusion of photon-ALP oscillations yields an improved fit to theγ-ray spectrum of IC443, which gives a statistical significance of 4.2σin favor of such spectral oscillation. However, the best-fit parameters of ALPs are in tension with the CAST(CERN(European Centre for Nuclear Research)Axion Solar Telescope) limits. Secondly, we use the H.E.S.S. observations of some TeV sources in the Galactic plane to exclude the highest ALP mass region (i.e., ALP massma～10?7eV)that accounts for the anomalously weak absorption of TeV gamma-rays for the first time.

A Milky Way-like galaxy is predicted to host tens of thousands of galactic DM subhalos.Annihilation of WIMPs in massive and nearby subhalos could generate detectable gammarays,appearing as unidentified,spatially-extended and stable gamma-ray sources. We search for such sources in the third Fermi Large Area Telescope source List (3FGL) and report the identification of a new candidate, 3FGL J1924.8?1034. 3FGL J1924.8?1034 is found spatially-extended at a high confidence level of 5.4σ. No significant variability has been found and its gamma-ray spectrum is well fitted by the dark matter annihilation intobˉbwith a mass of～43 GeV. All these facts make 3FGL J1924.8?1034 a possible dark matter subhalo candidate. However, due to the limited angular resolution, the possibility that the spatial extension of 3FGL J1924.8?1034 is caused by the contamination from the other un-resolved point source can not be ruled out.

The Milky Way dwarf spheroidal galaxy is considered one of the most ideal targets for indirect detection of dark matter due to their high dark matter density and low astrophysical backgrounds. We search for gamma-ray emission from nearby Milky Way dwarf spheroidal galaxies and candidates with Fermi-LAT data. Intriguingly, the peak TS (Test Statistic)value of the weak emission from Reticulum II rises continually. We also find that the combination of all these nearby sources will result in a more significant (>4σ) gammaray signal. A commonly encountered obstacle in indirect searches for dark matter is how to disentangle possible signals from astrophysical backgrounds. Gamma-ray features, in particular monochromatic gamma-ray lines and boxlike spectral features, provide smoking gun signatures. We analyze the Fermi LAT observation of Milky Way satellites and the local volume dark matter subhalo population(withN-body simulation)to search for potential line signals,respectively. The corresponding upper limits on the cross section of DM annihilation into two photons are derived, without significant signal found. Then we study the boxshaped DM signals, which is generated by the decay of intermediate particles produced by DM annihilation or decay, with Fermi-LAT observations of dwarf spheroidal galaxies.