The Awakening of a Blazar at Redshift 2.7 Temporally Coincident with the Arrival of Cospatial Neutrino Event IceCube-201221A
Jiang Xiong, Neng-Hui Liao, Yibo Wang, Rui Xue, Ning Jiang, Tinggui Wang
Abstract
Abstract We report on multiwavelength studies of a blazar NVSS J171822+423948, which is identified as the low-energy counterpart of 4FGL J1718.5+4237, the unique γ -ray source known to be cospatial with the IceCube neutrino event IC-201221A. After a 12 yr long quiescent period undetected by Fermi-LAT, γ -ray activities with a tenfold flux increase emerge soon (a few tens of days) after the arrival of the neutrino. Associated optical flares in the Zwicky Transient Facility g , r , and i bands are observed together with elevated Wide-field Infrared Survey Explorer infrared fluxes. Synchronized variations suggest that both the γ -ray emission and the neutrino event are connected to the blazar. Furthermore, the optical spectrum reveals emission lines at a redshift z = 2.68 ± 0.01. Thus, it is the first candidate for a neutrino-emitting blazar at the redshift above 2. Discussions of theoretical constraints of neutrino production and comparisons with other candidates are presented.