Triggering the Untriggered: The First Einstein Probe-detected Gamma-Ray Burst 240219A and Its Implications
Yin, Yi-Han Iris;
Zhang, Bin-Bin;
Yang, Jun;
Sun, Hui;
Zhang, Chen;
Shao, Yi-Xuan;
Hu, You-Dong;
Zhu, Zi-Pei;
Xu, Dong;
An, Li;
Gao, He;
Wu, Xue-Feng;
Zhang, Bing;
Castro-Tirado, Alberto Javier;
Pandey, Shashi B.;
Rau, Arne;
Lei, Weihua;
Xie, Wei;
Ghirlanda, Giancarlo;
Piro, Luigi;
O'Brien, Paul;
Troja, Eleonora;
Jonker, Peter;
Yu, Yun-Wei;
An, Jie;
Chen, Run-Chao;
Chen, Yi-Jing;
Dong, Xiao-Fei;
Eyles-Ferris, Rob;
Fan, Zhou;
Fu, Shao-Yu;
Fynbo, Johan P. U.;
Gao, Xing;
Huang, Yong-Feng;
Jiang, Shuai-Qing;
Jiang, Ya-Hui;
Julakanti, Yashaswi;
Kuulkers, Erik;
Lao, Qing-Hui;
Li, Dongyue;
Ling, Zhi-Xing;
Liu, Xing;
Liu, Yuan;
Mou, Jia-Yu;
Pan, Xin;
Wei, Daming;
Wu, Qinyu;
Yadav, Muskan;
Yang, Yu-Han;
Yuan, Weimin;
Zhang, Shuang-Nan
November 2024, The Astrophysical Journal Letters, Volume 975, Issue 2, id.L27, 12 pp.
The Einstein Probe (EP) achieved its first detection and localization of a bright X-ray flare, EP240219a, on 2024 February 19, during its commissioning phase. Subsequent targeted searches triggered by the EP240219a alert identified a faint, untriggered gamma-ray burst (GRB) in the archived data of Fermi Gamma-ray Burst Monitor (GBM), Swift Burst Alert Telescope (BAT), and Insight-HXMT/HE. The EP Wide-field X-ray Telescope (WXT) light curve reveals a long duration of approximately 160 s with a slow decay, whereas the Fermi/GBM light curve shows a total duration of approximately 70 s. The peak in the Fermi/GBM light curve occurs slightly later with respect to the peak seen in the EP/WXT light curve. Our spectral analysis shows that a single cutoff power-law (PL) model effectively describes the joint EP/WXT–Fermi/GBM spectra in general, indicating coherent broad emission typical of GRBs. The model yielded a photon index of ∼–1.70 ± 0.05 and a peak energy of ∼257 ± 134 keV. After detection of GRB 240219A, long-term observations identified several candidates in optical and radio wavelengths, none of which was confirmed as the afterglow counterpart during subsequent optical and near-infrared follow-ups. The analysis of GRB 240219A classifies it as an X-ray-rich GRB (XRR) with a high peak energy, presenting both challenges and opportunities for studying the physical origins of X-ray flashes, XRRs, and classical GRBs. Furthermore, linking the cutoff PL component to nonthermal synchrotron radiation suggests that the burst is driven by a Poynting flux-dominated outflow.
