TITLE:   GCN CIRCULAR
NUMBER:  36725
SUBJECT: X-ray transient EP240618a: Fermi/GBM non-detection
DATE:    24/06/21 15:56:04 GMT
FROM:    mariaedvige.ravasio@ru.nl

M. E. Ravasio (Radboud University) and P.G. Jonker (Radboud University) report on behalf of a larger collaboration:

Following the detection of the fast X-ray transient EP240618a by the Einstein Probe WXT and FXT (Sun et al., GCN 36690), we inspected the Fermi/GBM daily data for the presence of a possible gamma-ray counterpart. Fermi was operational (it was not in the SAA) and the location of the source was visible (i.e. it was not Earth-occulted).

Inspecting CSPEC data of the 4 best source-angle detectors (n7, n6, nb, b1) in the energy range 8-900 keV (for n7, n6, nb) and 300-40000 keV (for b1), with a time resolution of 4 s, we found no significant gamma-ray source within the time interval [T0; T0+100] s (same duration reported by Sun et al., GCN 36690, where T0 = 2024-06-18T05:43:43). 

Assuming a power-law model with a photon index of 2, the estimated upper limit to the flux in the energy range 10-1000 keV at the 90% confidence level over the time interval mentioned above is 8.5 x 10^-9  erg/cm^2/s.