The (missing) gamma-ray counterparts to EP events

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 times10 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.