基于Fermi-LAT数据的暗物质湮灭线谱搜寻及悟空号的伽马射线分析软件开发

梁云峰

(中国科学院紫金山天文台南京210008)

在现代的天体物理观测中,有许多天文现象包括星系旋转曲线、星系团的质量测量、宇宙微波背景辐射的角功率谱等,难以用现有模型进行解释.它们跨越多个尺度—小至星系尺度,大至整个宇宙尺度,需要在模型中额外引入一定的质量才能令观测现象得到很好的解释,而提供这些质量的“物质”却难以用通常的电磁波或中微子手段探测到.这部分物质被称为“暗物质”.根据现有的观测结果,它们应具有正常的引力相互作用,没有强相互作用和电磁相互作用,寿命很长(显著长于宇宙的寿命).对暗物质粒子的成功探测可望引发一场新的物理学革命.目前暗物质探测的最主要方法之一是所谓的间接探测,其目的是探测暗物质湮灭或衰变产生标准模型中的粒子,包括伽马光子、正反中微子、正反电子、正反质子等.在间接探测中,基于伽马光子的探测受到广泛的关注—因为伽马射线可以直接示踪辐射区的具体方位,有助于研究这些辐射与空间暗物质分布的关联性,而且对它们的探测效率显著地高于中微子.但探测过程仍然面临着很多潜在的挑战,其中一个主要的挑战是天体物理过程导致的高能伽马辐射对暗物质信号的污染,因此搜寻具有独特能谱特征的暗物质信号就显得尤为重要.伽马射线线谱就是这样一种信号,因为尚无已知的天体物理机制能产生此类信号,所以它一旦被可靠探测到,就意味着新物理的发现.为此在攻读博士学位期间,我致力于在Fermi-LAT(Large Area Telescope)数据中搜寻(暗物质)线谱信号.

论文的第1章是对一些相关的背景进行介绍,包括暗物质观测证据、暗物质探测方法、暗物质间接探测、Fermi-LAT仪器及数据、暗物质信号搜寻研究进展等.

论文第2章到第4章介绍我们在伽马射线线谱方面的4个工作:(1)利用Fermi-LAT的85个月的观测数据分析了16个近邻星系团方向的伽马射线辐射,我们在∼43 GeV能量处发现了疑似的线谱信号,该信号一旦在将来得到证实,将是暗物质间接探测领域的重要进展.由于该信号仍较弱,我们给出了对暗物质湮灭到线谱速度的平均截面(即⟨σv⟩χχ→γγ)的限制并指出除非星系团普遍具有很高的增强因子(BF,对于一个样本群体的平均值记为,否则星系团给出的限制要显著弱于银心观测给出的限制;(2)针对我们发现的43 GeV疑似线谱信号,在假定暗物质起源的前提下应该有⟨σv⟩∼5×10−28/103)−1cm3·s−1,我们讨论了几种可能的模型,发现要解释观测到的截面通常需要一个较大的耦合参数;(3)我们通过分析Fermi-LAT对银河系卫星星系(包括所有已知的矮星系和大、小麦哲伦星系)长达91个月的伽马射线辐射观测来搜寻暗物质湮灭产生的线谱信号,并给出了相应的线谱湮灭截面的上限,相较于之前基于4 yr Pass 7数据得到的结果,我们的限制显著增强;(4)基于分析Via Lactea IIN体数值模拟结果得到的暗物质子结构的分布和相关关系,我们模拟了一批银河系附近区域的暗物质子结构群体,并计算了它们的J因子,再结合Fermi-LAT的91个月的观测数据,我们给出了对线谱湮灭截面的限制,这是首次结合N体模拟的结果与高能伽马射线卫星的观测来对线谱湮灭截面做出限制.

在第5章中扼要介绍了我们在暗物质湮灭产生的伽马射线连续谱信号搜寻方面的几点研究进展,包括银心GeV超出的物理真实性研究、矮星系Tuc III方向的疑似GeV辐射信号、2个非认证点源的暗物质晕物理起源的可能性研究.

此外,在攻读博士期间我还参加了暗物质粒子探测卫星(悟空号)项目并负责伽马射线分析软件的开发.在第6章我们介绍了悟空号伽马射线分析软件的数学原理、软件结构、研发进展,并展示了一些初步的结果.我们的结果能很好地吻合Fermi-LAT的观测,这表明我们的伽马射线分析软件以及悟空号探测器本身、数据重建、事例筛选、探测器模拟等都是可靠的.

最后我们对基于悟空号以及Fermi-LAT数据的伽马射线线谱搜寻的未来进行了展望.

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.