柳若愚

(南京大學(xué)天文與空間科學(xué)學(xué)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(lǐng)域的熱點(diǎn)問題.近年隨著一些大型探測(cè)器(如Pierre Auger天文臺(tái))的運(yùn)行,極高能宇宙線的研究取得很大進(jìn)展.然而由于極高能宇宙線事例相對(duì)較少及其在從源到地球傳播過程中的復(fù)雜性(如與宇宙微波背景輻射以及磁場(chǎng)的作用),需要通過觀測(cè)這些宇宙線在強(qiáng)子反應(yīng)中產(chǎn)生的次級(jí)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(tái)探測(cè)到了54個(gè)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(xué)的新時(shí)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(cè)流量,推斷出極高能宇宙線源的能量產(chǎn)生率與強(qiáng)子反應(yīng)效率需要滿足一定的關(guān)系才能夠同時(shí)產(chǎn)生高能中微子.根據(jù)該關(guān)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(duì)論性駭新星遺跡可以同時(shí)作為IceCube高能中微子的源與極高能宇宙線的源,并計(jì)算了在該情況下預(yù)期的中微子能譜.同時(shí),預(yù)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(cè)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(cè)到的高能中微子的源.我們首先修正了IceCube團(tuán)隊(duì)早期工作中對(duì)伽瑪射線暴中微子流量的過高估計(jì).然后使用蒙特卡洛模擬,基于觀測(cè)的伽瑪暴參數(shù)的統(tǒng)計(jì)分布以及經(jīng)驗(yàn)關(guān)系,模擬出了1組伽瑪暴樣本,包括能觸發(fā)儀器響應(yīng)的“亮暴”與無法觸發(fā)儀器響應(yīng)的“暗暴”.在確保“亮暴”符合觀測(cè)的基礎(chǔ)上,得出了“暗暴”也無法產(chǎn)生IceCube觀測(cè)到的高能中微子的結(jié)論,進(jìn)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(chǎng)和磁場(chǎng)中的傳播.對(duì)于前者,著重計(jì)算了極高能宇宙線在星系際空間傳播時(shí)與背景光子場(chǎng)的光致蛻變、光介子反應(yīng)和Bethe-Heitler反應(yīng),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(xué)組成可以與觀測(cè)相符合.對(duì)于后者,針對(duì)Pierre Auger天文臺(tái)發(fā)現(xiàn)的能量大于5.7×1019eV的宇宙線到達(dá)方向與鄰近宇宙的質(zhì)量分布有成協(xié)的趨勢(shì),通過模擬不同種類的宇宙線在銀河系磁場(chǎng)中受到的偏折,并利用帶電粒子在磁場(chǎng)中的路徑只依賴于磁剛度的特性,根據(jù)Pierre Auger天文臺(tái)對(duì)于極高能宇宙線化學(xué)成份的觀測(cè),對(duì)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(xué)演化與電子的輻射機(jī)制,我們對(duì)伽瑪暴GRB 090902B與GRB 130427A多波段余輝輻射的光變曲線進(jìn)行擬合,研究了這兩個(gè)伽瑪暴的瞬時(shí)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.