An extremely soft and weak fast X-ray transient associated with a luminous supernova
astro-ph.HE
/ Authors
W. -X. Li, Z. -P. Zhu, X. -Z. Zou, J. -J. Geng, L. -D. Liu, Y. -H. Wang, R. -Z. Li, D. Xu, H. Sun, X. -F. Wang
and 95 more authors
Y. -W. Yu, B. Zhang, X. -F. Wu, Y. Yang, A. V. Filippenko, X. -W. Liu, W. -M. Yuan, D. Aguado, J. An, T. An, D. A. H. Buckley, A. J. Castro-Tirado, S. -Y. Fu, J. P. U. Fynbo, D. A. Howell, J. -W. Hu, S. -Q. Jiang, A. Kumar, J. -R. Mao, J. R. Maund, X. Liu, B. Mockler, A. Moskvitin, M. Andrews
/ Abstract
Long gamma-ray bursts (LGRBs), including their subclasses of low-luminosity GRBs (LL-GRBs) and X-ray flashes (XRFs) characterized by low spectral peak energies, are known to be associated with broad-lined Type Ic supernovae (SNe Ic-BL), which result from the core collapse of massive stars that lose their outer hydrogen and helium envelopes. However, the soft and weak end of the GRB/XRF population remains largely unexplored, due to the limited sensitivity to soft X-ray emission. Here we report the discovery of a fast X-ray transient, EP250108a, detected by the Einstein Probe (EP) in the soft X-ray band at redshift $z = 0.176$, which was followed up by extensive multiband observations. EP250108a shares similar X-ray luminosity as XRF\,060218, the prototype of XRFs, but it extends GRBs/XRFs down to the unprecedentedly soft and weak regimes, with its $E_{\rm peak} \lesssim 1.8\,\mathrm{keV}$ and $E_{\rm iso} \lesssim 10^{49}\, \mathrm{erg}$, respectively. Meanwhile, EP250108a is found to be associated with SN\,2025kg, one of the most luminous and possibly magnetar-powered SNe Ic-BL detected so far. Modeling of the well-sampled optical light curves favors a mildly relativistic outflow as the origin of this event. This discovery demonstrates that EP, with its unique capability, is opening a new observational window into the diverse outcomes of death of massive stars.