EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions

Camila Angulo-Valdez; Asuka Kuwata; Edilberto Aguilar-Ruiz; Rosa L. Becerra; Ramandeep Gill; Miguel Angel Aloy; Antonio de Ugarte Postigo; Alan M. Watson; Chen-Wei Wang; Diego López-Cámara; Daniel A. Perley; Aleksandra Bochenek; Simone Dichiara; Enrique Moreno Méndez; Nathaniel R. Butler; Benjamin Schneider; Jean-Luc Atteia; Stéphane Basa; William H. Lee; Dalya Akl; Sarah Antier; Alexis Coleiro; James DeLaunay; Damien Dornic; Jean-Grégoire Ducoin; Francis Fortin; Dmitry Frederiks; Leonardo García-García; Noémie Globus; Olivier Godet; Jamie Kennea; Gianluca Lombardi; Alexandra L. Lysenko; Francesco Magnani; Tyler Parsotan; Margarita Pereyra; Anna Ridnaia; Samuele Ronchini; Fredd Sánchez-Álvarez; Dmitry Svinkin; Anastasia Tsvetkova; Mikhail Ulanov; Peter Veres; Hui Yang

arXiv · 2609.27127

EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions

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Abstract

Gamma-ray quiet fast X-ray transients (FXTs) provide a new approach for studying relativistic explosions that evade traditional gamma-ray triggers. In this work, we present multi-wavelength observations and analysis of EP260119a, a high-$z$ FXT detected by Einstein Probe/WXT and followed up with COLIBRÍ, the Liverpool Telescope, and other facilities. Spectroscopy yields a redshift of $z = 5.47$, making EP260119a the most distant FXT detected by Einstein Probe to date. Despite its luminous X-ray and optical emission, no prompt gamma-ray counterpart was detected by SVOM/ECLAIRs, SVOM/GRM, Swift/BAT, or Konus/Wind, despite contemporaneous coverage. The broadband afterglow is well described by synchrotron emission from a uniform relativistic jet propagating into a shallowly stratified external medium close to the constant-density limit, indicating a standard relativistic explosion despite the absence of detectable gamma rays. Occupying the extreme high-$z$ end of the growing Einstein Probe FXT sample, EP260119a supports the possibility that soft X-ray surveys are uncovering relativistic transients that remain undetected by current gamma-ray instruments. Our results indicate that some explosions in the early Universe escape conventional gamma-ray surveys and demonstrate the value of combining sensitive soft X-ray discovery with rapid optical follow-up to obtain a more complete view of the high-$z$ transient population.

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Camila Angulo-Valdez, Asuka Kuwata, Edilberto Aguilar-Ruiz, Rosa L. Becerra, Ramandeep Gill, Miguel Angel Aloy, Antonio de Ugarte Postigo, Alan M. Watson, Chen-Wei Wang, Diego López-Cámara, Daniel A. Perley, Aleksandra Bochenek, Simone Dichiara, Enrique Moreno Méndez, Nathaniel R. Butler, Benjamin Schneider, Jean-Luc Atteia, Stéphane Basa, William H. Lee, Dalya Akl, Sarah Antier, Alexis Coleiro, James DeLaunay, Damien Dornic, Jean-Grégoire Ducoin, Francis Fortin, Dmitry Frederiks, Leonardo García-García, Noémie Globus, Olivier Godet, Jamie Kennea, Gianluca Lombardi, Alexandra L. Lysenko, Francesco Magnani, Tyler Parsotan, Margarita Pereyra, Anna Ridnaia, Samuele Ronchini, Fredd Sánchez-Álvarez, Dmitry Svinkin, Anastasia Tsvetkova, Mikhail Ulanov, Peter Veres, Hui Yang. 2026-09-22. EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions. https://arxiv.org/abs/2609.27127

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