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守得“云”開見磁星

文章來源:科技日報   發布時間:2019-04-18  【字號:     】  

王國燕 何聰 制圖

  “一鉤新月天如水”,晴朗的夜空在人們的想象中,往往是寂靜祥和的。然而,在天文學家眼里,黑暗的宇宙卻到處埋藏著強力的定時炸彈,隨時隨地會猛然爆發。尤其是在人們開始在X射線、伽馬射線等極高能量的電磁波段觀察宇宙時,眾多的爆發事件就像四處綻放的煙火一樣熱鬧而迷人。這些爆發事件到底是什么?這個問題從上世紀60年代起就吸引了天文學家的好奇心。

  近日,在國際著名期刊《自然》雜志上,一項由中國科學技術大學研究人員主導的研究,就介紹了一個極有可能是由雙中子星并合產生的有趣的X射線爆發事件,并證實了其并合產物是幾十年前科學家預言的磁星。

  存在于理論中的強磁場天體

  中子星是比太陽還要重八倍以上的恒星死亡后留下的致密星體,它們體積超小,密度卻極高。由于宇宙中的恒星喜歡扎堆出現,兩個中子星攜手相伴的情況也時有發生。相互繞轉的雙中子星產生的引力場快速變化,就像是在不斷地攪拌原本平靜的時空,向外泛起漣漪。隨著能量的消耗,雙中子星會慢慢越靠越近,最終在劇烈碰撞、釋放引力波暴之后合二為一,形成一個新的質量更大的致密天體——2017年8月17日激光干涉引力波天文臺(LIGO)等就首次探測到了雙中子星并合引力波事件的信號。如果這個新天體的質量超過中子星的質量上限,那么天體內部物質的壓力將難以抗衡星體自身的引力,使得星體直接塌縮成為一個黑洞。但天文學家認為,在塌縮之前它很有可能會先形成一個更加有趣的天體——磁星。

  磁星,顧名思義,是一個具有超強磁場的中子星,其表面磁場比目前人類實驗室能制造出來的最強磁場還強上億倍。這樣的磁星可以擁有超高的自轉速度,每秒可自轉幾百上千周。由于高速自轉,即使磁星的質量超過中子星質量上限,其離心力也能幫助它短期抗衡強大的引力而不會進一步塌縮成黑洞。

  除了留下中心的磁星,并合事件還會在周邊留下大量的拋出物質。磁星具有強引力,快速吸入拋出物質,并在兩極方向產生一個只持續幾秒的超高速噴流。如果噴流方向恰好對著地球,就如同它向我們開了一炮,可以讓我們在短時間內探測到大量的高能量伽馬射線,這就是被稱為短伽馬暴的高能天體物理事件。

  早在上個世紀90年代初,科學家就提出了磁星的設想。之后它逐漸被學界廣泛接受,并用于解釋一些特殊類型的中子星。然而時至今日,科學家仍未在雙中子星并合之后探測到它的“真身”。

  “烏云”遮蔽新天體輻射

  由于雙中子星并合既能產生強烈的引力波暴,也能產生短伽馬暴,所以天文學家非常希望在探測到引力波輻射以后,還能在對應的方向看到一個短伽馬暴,從而進一步研究并合事件中的各種物理過程,找到磁星。然而,由于噴流“開炮”的方向是隨機的,它恰好對準地球的可能性其實很小,所以即使引力波探測器已經探測到不少的引力波事件,想觀測到一個對應的短伽馬暴卻很難。

  但天文學家找到了另一種可能性。在理論預言中,磁星有一個重要的特點:較差自轉,即磁星的內部與外部的自轉速度并不一致,靠近中心區域的自轉速度更快。這就會不斷扭曲穿過磁星內部的磁力線,使其最終浮出磁星表面并斷裂,釋放出大量能量從而減緩磁星自轉,而還在磁星內部的磁力線則會重新連接并開始新一輪的扭轉。這種磁重聯過程能向四面八方發出大量的X射線波段至可見光波段的輻射。這樣,即使沒有看到短伽馬暴,科學家仍然有可能看到一個強烈的爆發事件,只不過是在X射線、可見光等波段。

  既然這樣的輻射是朝向四面八方發射的,那么是不是就應該很容易被觀測到呢?答案也是否定的。此前,人們還沒有發現過任何有類似特征的孤立X射線爆發。這是因為雙中子星并合中產生的拋出物質會形成一個圍繞著新天體的包層,擋住了來自中心的大部分輻射。看來,要想不通過短伽馬暴而僅靠探測X射線爆發來發現雙中子星并合事件、尋找磁星,還真是困難重重啊。

  不過,理論和數值模擬告訴我們,在中子星并合形成的磁星吸收周邊物質的時候,劇烈的吸積過程除了形成噴流以外還會產生高速的彌漫星風,而磁星周邊強大的磁場會成為這些星風的向導,將它們導向兩極。于是這些高速星風在磁星形成之初就會快速地吹走磁星兩極附近的遮擋物質,打開一個較大的通道,后續的X射線輻射就能借此沖破障礙。當我們的視線對著這個通道時,就有可能看到預言中的沒有對應短伽馬暴的孤立的X射線爆發,找到磁星。

  七百萬秒曝光中搜尋那次閃光

  中科大研究團隊4月11日在線發表于《自然》上的研究,正是報導了首例這樣的事件。

  錢德拉空間望遠鏡是目前世界上最先進的X射線探測器之一。在1999年到2016年間,它陸續對天空中精心選出的一小片僅有約1/4滿月大小的區域拍攝了102次,總計曝光7百萬秒。中科大研究團隊在研究該天區中天體的光變特性時,發現了2015年3月一個天體的X射線輻射從無到有,在短時間內增亮了上千倍。他們將這個爆發事件命名為CDF-S XT2(簡稱XT2)。而在隨后的數據搜尋中,他們發現在XT2爆發前后,世界上的伽馬射線探測器都沒有記錄到對應的伽馬暴。

  通過分析XT2的X射線數據,研究人員發現,它在爆發后約2000秒內亮度基本不變,隨后則快速變暗直至消失。而得益于錢德拉空間望遠鏡的超高角分辨率,研究人員將XT2定位于距離地球約66億光年的一個星系的外圍部分。這些特征都符合科學家對磁星的理論預期。研究人員估計了觀測到類似XT2爆發事件的頻率,發現與此前根據引力波探測結果可靠估算得到的雙中子星并合事件的發生頻率吻合得很好,從而再次證明了XT2源于雙中子星并合的合理性。

  根據上述分析,研究人員相信它就是一直被苦苦追尋的大質量毫秒磁星X射線爆發。這讓天文學家興奮不已。長久以來只存在于理論預言中的現象,如今終于出現。XT2的發現證明了雙中子星并合能夠產生磁星的理論正確性,同時幫助天文學家更為深入地了解了中子星的內部物質狀態。而且,這一發現有助于科學家在未來的引力波事件探測中,擺脫對伽馬暴的依賴,通過X射線觀測直接獲知其來源天體的物理信息。

  略為遺憾的是,因為發現時間較晚,研究人員錯過了在XT2爆發之后立刻調用其他望遠鏡對它進行后續觀測的機會,所以無法得到爆發過程中更多的其他信息。但可以期待的是,在未來的X射線巡天望遠鏡的監測中,科學家將有機會看到更多的類似爆發事件。XT2作為宇宙深處磁星X射線輻射沖過層層云靄到達我們的曙光,將給未來的引力波探測、中子星研究帶來新的希望。

  (作者:鄭學琛、薛永泉,分別系荷蘭萊頓大學博士生、中國科學技術大學天文系教授)




(責任編輯:侯茜)

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