A repeating fast radio burst source in a globular cluster

Fast radio bursts (FRBs) are exceptionally luminous flashes of unknown physical origin,reaching us from other galaxies1. Most FRBs have only ever been seen once, while othersflash repeatedly, though sporadically2,3. Many models invoke magnetically powered neutronstars (magnetars) as the engines producing FRB emission4,5. Recently, CHIME/FRB announcedthe discovery6 of the repeating FRB 20200120E, coming from the direction of thenearby grand design spiral galaxy M81. Four potential counterparts at other observing wavelengthswere identified6 but no definitive association with these sources, or M81, could bemade. Here we report an extremely precise localisation of FRB 20200120E, which allows us toassociate it with a globular cluster (GC) in the M81 galactic system and to place it 2 pc offsetfrom optical center of light of the GC. This confirms6 that FRB 20200120E is 40 times closerthan any other known extragalactic FRB. Because such GCs host old stellar populations, thisassociation strongly challenges FRB models that invoke young magnetars formed in a corecollapsesupernova as powering FRB emission. We propose, instead, that FRB 20200120Eis a highly magnetised neutron star formed via either accretion-induced collapse of a whitedwarf or via merger of compact stars in a binary system7. Alternative scenarios involvingcompact binary systems, efficiently formed inside globular clusters, could also be responsiblefor the observed bursts.

快速射电暴（Fast Radio Bursts，FRBs）是物理起源不明的极端明亮射电闪暴，源自河外星系[1]。绝大多数FRBs仅被单次观测到，另有部分会重复爆发，但爆发频次极不稳定[2,3]。诸多模型将强磁化中子星（magnetars，磁星）作为产生FRB辐射的能量源[4,5]。近期，CHIME/FRB合作组宣布发现重复快速射电暴FRB 20200120E，其爆发方向位于邻近的宏象螺旋星系M81[6]。此前研究人员在其他观测波段共识别出4个潜在对应天体[6]，但无法确认该FRB与这些天体或M81星系存在确切关联。
本文报道了FRB 20200120E的极高精度定位结果，据此可将其与M81星系系统中的球状星团（globular cluster，GC）相关联，且该暴的位置与该球状星团的光学光度中心偏移量仅为2秒差距（parsec，pc）。这一结果证实[6]，FRB 20200120E的距离比其他所有已知河外FRB近40倍。由于此类球状星团中的恒星族群均为老年恒星，这一关联结果对“以核心坍缩超新星形成的年轻磁星作为FRB辐射能量源”的FRB模型构成了强烈挑战。
对此，我们提出另一种假说：FRB 20200120E的辐射源为一颗强磁化中子星，其形成途径可能为白矮星的吸积诱导坍缩，或是双星系统中致密星的并合过程[7]。另一种涉及“在球状星团内高效形成的致密双星系统”的假说，也可以解释本次观测到的爆发事件。