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 $T_{\rm 90}$ is systematically higher than over the full X-ray duration, with flux ratios $1 \lesssim F_{\rm [10-1000\,keV]}/F_{\rm [0.5-4\,keV]} \lesssim 100$, 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 $E_{\rm peak} \lesssim 30$-100 keV and, for the 14 events with measured redshifts, $L_{\rm iso}\lesssim 10^{48}-9\times10^{51}$ erg/s. If classical GRB correlations hold in this regime, these lower energetics imply lower bulk Lorentz factors ($Γ_0 \lesssim 100$), as confirmed in a few cases by afterglow constraints. No significant GeV emission was detected by Fermi/LAT, yielding $1 \lesssim F_{0.1-1\,{\rm GeV}}/F_{0.5-4\,{\rm keV}} \lesssim 10$, 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 $E_{\rm peak}$, luminosities, and bulk Lorentz factors.