The (missing) gamma-ray counterparts to Einstein Probe events. I: Search for keV-GeV counterparts of Einstein Probe fast X-ray transients
Year: 2026
Type: article
Abstract: While the connection of some fast X-ray transients (FXTs) detected by (EP) with gamma-ray bursts (GRBs) is increasingly compelling, most lack γ-ray counterparts, leaving open the question of whether and how they are related to the GRB phenomenon. Einstein Probe We performed a systematic broadband search for γ-ray counterparts to the 72 transients detected by EP during its first year of operations, using dedicated targeted searches in /GBM and /LAT data complemented by information from SVOM/GRM, SVOM/ECLAIRs, Swift/BAT, and Konus-Wind. For the visible events without a γ-ray counterpart, we derived simultaneous 3σ flux upper limits during the full duration of the event and its peak time in the 10--1000,keV and the 0.1--1, GeV band. Fermi Fermi We found that 23 events (32%) have a γ-ray counterpart (seven sub-threshold events), while 49 events (68%) have no detected γ-ray counterpart, despite most being visible. For the events with a γ-ray counterpart, when the X-ray flux is measured over the γ-ray T_̊m 90, it is systematically higher than the flux measured over the full X-ray duration, with flux ratios of 1 łesssim F_ ̊m 10-1000,keV /F_ ̊m 0.5-4,keV łesssim 100, suggesting that γ-rays are preferentially detected during the brightest X-ray phases. For events without a γ-ray counterpart, the most stringent /GBM upper limits imply spectral peak energies of E_ Fermi ̊m peak łesssim 30--100,keV, and for the 14 events with measured redshifts, the isotropic-equivalent luminosities are of ̊m iso łesssim1 -9 erg s 48 51 -1 . Assuming that the same correlations established for classical GRBs hold in this regime, these lower energetics imply lower bulk Lorentz factors (Γ_0 łesssim 100) for EP FXTs compared to those of classical GRBs, as independently confirmed in a few cases by afterglow constraints. No significant GeV emission was detected by /LAT for any visible EP transient, yielding 1 łesssim F_ Fermi 0.1 - ̊m GeV /F_ 0.5 - ̊m keV łesssim 10, which places the first observational constraints on a potential synchrotron self-Compton component for this new population of X-ray transients. While the origin of individual EP transients without γ-ray counterparts remains uncertain, these high-energy constraints suggest that many of them could be compatible with the GRB population, extending it towards a lower E_̊m peak, lower luminosities, and lower bulk Lorentz factors. The EP mission is finally providing the opportunity to systematically characterize these unexplored regimes of jet physics.
Source: Astronomy and Astrophysics
Authors: M. E. Ravasio, A. Chopra, P. G. Jonker, Q. Wu, E. Burns, B. Banerjee, G. Oganesyan, S. Ascenzi, J. T. Palmerio, F. Daigne, J. Wood, R. Hamburg, A. J. Levan, J. A. Quirola Vasquez, J. Sánchez-Sierras, J. N. D. van Dalen, A. P. C. van Hoof, H. Sun, W. Yuan, N. Sarin, A. Martin-Carrillo, D. Adrien, C. Cai, Y. Zhang, S. Ronchini, M. Branchesi
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: “la Caixa” Foundation, China National Space Administration, European Commission, Chinese Academy of Sciences, Centre National d’Etudes Spatiales, Nuclear Safety and Security Commission, Agenzia Spaziale Italiana, Centre National de la Recherche Scientifique, Gran Sasso Science Institute, HORIZON EUROPE Framework Programme, European Research Council, Agencia Estatal de Investigación, Xi'an Institute of Optics and Precision Mechanics, Institute of High Energy Physics, National Astronomical Observatories, Chinese Academy of Sciences
OpenAlex ID: W7223333114