The (missing) gamma-ray counterparts to EP events
Abstract
While the connection of Fast X-ray Transients (FXTs) detected by Einstein Probe (EP) with Gamma-ray Bursts (GRBs) is increasingly compelling, most lack gamma-ray counterparts, leaving their relation to GRBs unclear. We performed a systematic search for gamma-ray counterparts to the 72 transients detected by EP in its first year, using targeted searches in Fermi/GBM and Fermi/LAT data, complemented by SVOM/GRM, SVOM/ECLAIRs, Swift/BAT and Konus-Wind. For the visible events without a gamma-ray counterpart, we derived simultaneous 3σ flux upper limits in the 10-1000 keV and the 0.1-1 GeV band. We find that 23 events (32%) have a gamma-ray counterpart (seven sub-threshold), while 49 events (68%) have none, despite most being visible. For events with a gamma-ray counterpart, the X-ray flux over the gamma-ray T90 is systematically higher than over the full X-ray duration, with flux ratios 1 ≲ F[10−1000 keV]/F[0.5−4 keV] ≲ 100, suggesting that gamma-rays are preferentially detected during the brightest X-ray phases. For events without gamma-ray counterpart, the most stringent Fermi/GBM upper limits imply Epeak ≲ 30-100 keV and, for the 14 events with measured redshifts, Liso≲ 1048−9×1051 erg/s. If classical GRB correlations hold in this regime, these lower energetics imply lower bulk Lorentz factors (Γ0 ≲ 100), as confirmed in a few cases by afterglow constraints. No significant GeV emission was detected by Fermi/LAT, yielding 1 ≲ F0.1−1 GeV/F0.5−4 keV ≲ 10, placing the first constraints on a potential synchrotron self-Compton component for this new population. Although individual origins remain uncertain, many EP FXTs could be compatible with the GRB population, extending it toward lower Epeak, luminosities, and bulk Lorentz factors.
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BibTeX
@misc{ravasio2026missing,
title = {{The (missing) gamma-ray counterparts to EP events}},
author = {Maria Edvige Ravasio and Ansh Chopra and Peter G. Jonker and Qinyu Wu and Eric Burns and Biswajit Banerjee and Gor Oganesyan and Stefano Ascenzi and Jesse T. Palmerio and Frederic Daigne and Joshua Wood and Rachel Hamburg and Andrew J. Levan and Jonathan A. Quirola-Vasquez and Javier Sanchez-Sierras and Joyce N. D. van Dalen and Agnes P. C. van Hoof and Hui Sun and Weimin Yuan and Nikhil Sarin and Antonio Martin-Carrillo and Dylan Adrien and Ce Cai and YanQiu Zhang and Samuele Ronchini and Marica Branchesi},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.28734},
eprint = {2610.28734},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.28734},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - The (missing) gamma-ray counterparts to EP events
AU - Maria Edvige Ravasio
AU - Ansh Chopra
AU - Peter G. Jonker
AU - Qinyu Wu
AU - Eric Burns
AU - Biswajit Banerjee
AU - Gor Oganesyan
AU - Stefano Ascenzi
AU - Jesse T. Palmerio
AU - Frederic Daigne
AU - Joshua Wood
AU - Rachel Hamburg
AU - Andrew J. Levan
AU - Jonathan A. Quirola-Vasquez
AU - Javier Sanchez-Sierras
AU - Joyce N. D. van Dalen
AU - Agnes P. C. van Hoof
AU - Hui Sun
AU - Weimin Yuan
AU - Nikhil Sarin
AU - Antonio Martin-Carrillo
AU - Dylan Adrien
AU - Ce Cai
AU - YanQiu Zhang
AU - Samuele Ronchini
AU - Marica Branchesi
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2610.28734
UR - https://arxiv.org/abs/2610.28734
AB - While the connection of Fast X-ray Transients (FXTs) detected by Einstein Probe (EP) with Gamma-ray Bursts (GRBs) is increasingly compelling, most lack gamma-ray counterparts, leaving their relation to GRBs unclear. We performed a systematic search for gamma-ray counterparts to the 72 transients detected by EP in its first year, using targeted searches in Fermi/GBM and Fermi/LAT data, complemented by SVOM/GRM, SVOM/ECLAIRs, Swift/BAT and Konus-Wind. For the visible events without a gamma-ray counterpart, we derived simultaneous 3σ flux upper limits in the 10-1000 keV and the 0.1-1 GeV band. We find that 23 events (32%) have a gamma-ray counterpart (seven sub-threshold), while 49 events (68%) have none, despite most being visible. For events with a gamma-ray counterpart, the X-ray flux over the gamma-ray T_90 is systematically higher than over the full X-ray duration, with flux ratios 1 ≲ F_[10−1000 keV]/F_[0.5−4 keV] ≲ 100, suggesting that gamma-rays are preferentially detected during the brightest X-ray phases. For events without gamma-ray counterpart, the most stringent Fermi/GBM upper limits imply E_peak ≲ 30-100 keV and, for the 14 events with measured redshifts, L_iso≲ 10^48−9×10^51 erg/s. If classical GRB correlations hold in this regime, these lower energetics imply lower bulk Lorentz factors (Γ_0 ≲ 100), as confirmed in a few cases by afterglow constraints. No significant GeV emission was detected by Fermi/LAT, yielding 1 ≲ F_0.1−1 GeV/F_0.5−4 keV ≲ 10, placing the first constraints on a potential synchrotron self-Compton component for this new population. Although individual origins remain uncertain, many EP FXTs could be compatible with the GRB population, extending it toward lower E_peak, luminosities, and bulk Lorentz factors.
ER -