An energetic dirty fireball detected in soft X-rays

/ Authors

Cankun Dai, J. Quirola-V'asquez, Y. Wang, H. Li, J. Yang, X. Chen, Alex L. Wang, H. Sun, X.-Y. Wang, B. Zhang

and 69 more authors

P. G. Jonker, Y. Liu, W. Yuan, D. Xu, Zhiliang Dai, M. Ravasio, L. Piro, P. O’Brien, D. Stern, H. Zhang, Y.-P. Yang, T. An, Y. Qiu, L. Xin, W. Li, R. Liu, X. Wu, C.Y. Wang, D. Wei, Y. Huang, F. Bauer, Wen Lei, B.-B. Zhang, Nanxun Sun, H. Gao, V. S. Dhillon, J. An, Chengdong Bai, A. Martin-Carrillo, H. Cheng, J. Chavez, Y. Chen, G. Du, J. V. Dalen, A. Esamdin, Y. Fan, X. Gao, F. Harrison, J. Hu, M.Q. Huang, Shi Jia, A. Levan, C.-K. Li, D.Y. Li, E. Liang, S. Littlefair, X.W. Liu, Z. Liu, Zijin Ling, D. Malesani, H. Pan, A. Rodriguez, A. Rossi, D. M. S'anchez, J. S'anchez-Sierras, X. Sun, M. Torres, A. V. Hoof, X. Wang, Q. Wu, X.P. Xu, Y. Xu, Y.W. Yu, C. Zhang, M.H. Zhang, S. Zhang, Y. Zhang, Y.H. Zhang, Z.-P. Zhu

/ Abstract

The collapse of massive stars drives explosions that power relativistic fireballs. If only a small amount of matter is entrained, such clean fireballs can expand with Lorentz factors $\Gamma>100$, accounting for gamma-ray bursts (GRBs). It has been hypothesized that energetic explosions with more baryon contamination, dubbed ``dirty fireballs'', may exist in nature, but they have not been observed. Here we report the observation of an extragalactic fast X-ray transient, EP241113a, detected by Einstein Probe. Compared to GRBs, it has a similar isotropic energy of $1.4\times 10^{51}$ erg, but significantly lower spectral peak energy. Theoretical modeling of its early X-ray afterglow suggests a relativistic jet with a low Lorentz factor of $\Gamma \sim 20$ aligned close to the line-of-sight, signifying the prototype of a dirty fireball.