EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions
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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BibTeX
@misc{angulovaldez2026ep260119a,
title = {{EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions}},
author = {Camila Angulo-Valdez and Asuka Kuwata and Edilberto Aguilar-Ruiz and Rosa L Becerra and Ramandeep Gill and Miguel Angel Aloy and Antonio de Ugarte Postigo and Alan M Watson and Chen-Wei Wang and Diego López-Cámara and Daniel A Perley and Aleksandra Bochenek and Simone Dichiara and Enrique Moreno Méndez and Nathaniel R Butler and Benjamin Schneider and Jean-Luc Atteia and Stéphane Basa and William H Lee and Dalya Akl and Sarah Antier and Alexis Coleiro and James Delaunay and Damien Dornic and Jean-Grégoire Ducoin and Francis Fortin and Dmitry Frederiks and Leonardo García-García and Noémie Globus and Olivier Godet and Jamie Kennea and Gianluca Lombardi and Alexandra L Lysenko and Francesco Magnani and Tyler Parsotan and Margarita Pereyra and Anna Ridnaia and Samuele Ronchini and Fredd Sánchez-Álvarez and Dmitry Svinkin and Anastasia Tsvetkova and Mikhail Ulanov and Peter Veres and Hui Yang},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.27127},
eprint = {2609.27127},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05785800v1},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions
AU - Camila Angulo-Valdez
AU - Asuka Kuwata
AU - Edilberto Aguilar-Ruiz
AU - Rosa L Becerra
AU - Ramandeep Gill
AU - Miguel Angel Aloy
AU - Antonio de Ugarte Postigo
AU - Alan M Watson
AU - Chen-Wei Wang
AU - Diego López-Cámara
AU - Daniel A Perley
AU - Aleksandra Bochenek
AU - Simone Dichiara
AU - Enrique Moreno Méndez
AU - Nathaniel R Butler
AU - Benjamin Schneider
AU - Jean-Luc Atteia
AU - Stéphane Basa
AU - William H Lee
AU - Dalya Akl
AU - Sarah Antier
AU - Alexis Coleiro
AU - James Delaunay
AU - Damien Dornic
AU - Jean-Grégoire Ducoin
AU - Francis Fortin
AU - Dmitry Frederiks
AU - Leonardo García-García
AU - Noémie Globus
AU - Olivier Godet
AU - Jamie Kennea
AU - Gianluca Lombardi
AU - Alexandra L Lysenko
AU - Francesco Magnani
AU - Tyler Parsotan
AU - Margarita Pereyra
AU - Anna Ridnaia
AU - Samuele Ronchini
AU - Fredd Sánchez-Álvarez
AU - Dmitry Svinkin
AU - Anastasia Tsvetkova
AU - Mikhail Ulanov
AU - Peter Veres
AU - Hui Yang
PY - 2026
DA - 2026/09/22
PB - HAL
DO - 10.48550/arxiv.2609.27127
UR - https://hal.science/hal-05785800v1
AB - 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.
ER -