柳若愚

(南京大學天文與空間科學學院 南京 210023)

極高能宇宙線一般指來自地外的能量高于1018電子伏特(eV)的高能質(zhì)子與原子核,其起源的研究一直是高能天體物理和粒子天體物理領域的熱點問題.近年隨著一些大型探測器(如Pierre Auger天文臺)的運行,極高能宇宙線的研究取得很大進展.然而由于極高能宇宙線事例相對較少及其在從源到地球傳播過程中的復雜性(如與宇宙微波背景輻射以及磁場的作用),需要通過觀測這些宇宙線在強子反應中產(chǎn)生的次級粒子(如中微子)來獲得其起源的額外信息.最近,位于南極的IceCube中微子天文臺探測到了54個能量分布在60 TeV–3 PeV內(nèi)的中微子事例,開啟了高能中微子天文學的新時代.在本文中,我們研究了高能中微子、極高能宇宙線的天體物理起源以及它們之間可能的聯(lián)系.

首先,我們討論了高能中微子與極高能宇宙線擁有共同起源的可能性.根據(jù)兩者的觀測流量,推斷出極高能宇宙線源的能量產(chǎn)生率與強子反應效率需要滿足一定的關系才能夠同時產(chǎn)生高能中微子.根據(jù)該關系可以初步排除一些模型.然后,提出了恒星形成星系中的中等相對論性駭新星遺跡可以同時作為IceCube高能中微子的源與極高能宇宙線的源,并計算了在該情況下預期的中微子能譜.同時,預言如果高能中微子與極高能宇宙線有共同起源,IceCube將在10–20 yr內(nèi)觀測到10 PeV的中微子.

接著,我們深入討論了伽瑪射線暴是否可以作為觀測到的高能中微子的源.我們首先修正了IceCube團隊早期工作中對伽瑪射線暴中微子流量的過高估計.然后使用蒙特卡洛模擬,基于觀測的伽瑪暴參數(shù)的統(tǒng)計分布以及經(jīng)驗關系,模擬出了1組伽瑪暴樣本,包括能觸發(fā)儀器響應的“亮暴”與無法觸發(fā)儀器響應的“暗暴”.在確保“亮暴”符合觀測的基礎上,得出了“暗暴”也無法產(chǎn)生IceCube觀測到的高能中微子的結論,進一步排除了伽瑪暴作為中微子起源的可能性.

此外,我們研究了極高能宇宙線在光子場和磁場中的傳播.對于前者,著重計算了極高能宇宙線在星系際空間傳播時與背景光子場的光致蛻變、光介子反應和Bethe-Heitler反應,并發(fā)現(xiàn)從駭新星遺跡產(chǎn)生的極高能宇宙線經(jīng)傳播后的能譜及化學組成可以與觀測相符合.對于后者,針對Pierre Auger天文臺發(fā)現(xiàn)的能量大于5.7×1019eV的宇宙線到達方向與鄰近宇宙的質(zhì)量分布有成協(xié)的趨勢,通過模擬不同種類的宇宙線在銀河系磁場中受到的偏折,并利用帶電粒子在磁場中的路徑只依賴于磁剛度的特性,根據(jù)Pierre Auger天文臺對于極高能宇宙線化學成份的觀測,對這些極高能宇宙線源的距離以及金屬豐度做出了限制.

最后,根據(jù)激波的流體力學演化與電子的輻射機制,我們對伽瑪暴GRB 090902B與GRB 130427A多波段余輝輻射的光變曲線進行擬合,研究了這兩個伽瑪暴的瞬時輻射及晚期高能光子的起源.在此研究中,我們得到這些伽瑪暴產(chǎn)生的激波參數(shù),從而獲知它們加速粒子的能力.

Ultrahigh energy cosmic rays(UHECRs)are the extraterrestrial energetic particles with energy above 1018eV.Thanks to the operation of large UHECR detectors such as the Pierre Auger Observatory(PAO),our understanding on UHECRs has been significantly improved in the past decade.However,due to the rarity of events and the complexity in their propagation to Earth,there are still lots of puzzles on UHECRs to be studied.Hence,it is useful to extract some additional information from the observation on the secondary particles(such as neutrinos)which are generated in the propagation of UHECRs.Recently,IceCube neutrino observatory located in the Antarctic Pole has discovered 54 neutrino events with energy from 60 TeV to 3 PeV,opening a new era of neutrino astronomy.In this paper,we investigate the astrophysical origin of high-energy neutrinos and UHECRs,and the possible link between them.

We find that the high-energy neutrinos may possibly originate from the same sources of the UHECRs,and deduce that the energy production rate of the source and the neutrino production efficiency satisfy a certain relation.Based on the relation,some sources can be excluded.We propose semi-relativistic hypernova remnants in star-forming galaxies as the common sources of the UHECRs and the high-energy neutrinos detected by IceCube,and predict that the IceCube is able to observe one～10 PeV neutrino event in 10–20 yr.

Gamma-ray burst(GRB)has been widely studied as the possible sources of high-energy neutrinos.We have first corrected the overestimation on the expected neutrino flux from GRBs obtained by the IceCube Collaboration.Furthermore,we perform a Monte-Carlo simulation to generate a complete sample of GRBs,consisting of both bright GRBs that can trigger the detector and dim GRBs that cannot.On the premise that the bright GRBs can reproduce the properties of observed GRBs,we find that the dim GRBs cannot explain the observed neutrino events,and exclude GRBs as the sources of high-energy neutrinos.

Next,we study the propagation of UHECRs,including the propagation in photon field and in magnetic field.By considering the photodisintegration,the photopion production,and the Bethe-Heitler process of UHECRs in their propagation in the intergalactic space,we obtain the spectrum of UHECRs coming from the hypernova remnants throughout the whole universe.The obtained spectrum and mass composition are consistent with observations.On the other hand,by simulating the propagation of UHECRs in the Galactic magnetic fields,and utilizing the fact that the trajectories of cosmic rays in magnetic field are only rigidity-dependent,we put constraints on the UHECR source distance and metallicity after jointly taking into account the chemical composition and distribution of arrival directions of the UHECRs observed by PAO.

Lastly,we model the multi-wavelength light curves of afterglows of GRB 090902B and GRB 130427A by computing the dynamic evolution of GRB outflows and radiation of accelerated electrons.The origin of high-energy prompt and extended emission is studied.