TITLE: GCN CIRCULAR NUMBER: 36347 SUBJECT: LIGO/Virgo/KAGRA S240422ed: SRG/eROSITA archival upper limit for EP240426a DATE: 24/04/29 18:03:12 GMT FROM: Arne Rau at MPE arau@mpe.mpg.de

Zhu Liu, Arne Rau, Chandreyee Maitra, Pietro Baldini, Andrea Merloni (all MPE), Lorenzo Ducci (IAAT), Mirko Krumpe, Axel Schwope (both AIP):

We report constraints on the historical X-ray emission based on archival X-ray observations from the eROSITA X-ray telescope (Predehl et al. A&A 647, A1, 2021) on board the SRG mission for the potential electromagnetic counterpart of the LIGO/Virgo/KAGRA candidate NSBH event S240422ed (GCN 36236) detected by EP/FXT (EP240426a, GCN 36313, GCN 36332).

EP240426a was within the Field-of-View of eROSITA during each of the first four eROSITA all-sky surveys (eRASS) conducted between 2019 December and 2021 December. It was not detected in the combined eRASS data (hereafter referred to as eRASS:4). Assuming a power-law model with a photon index of 2.0 and a Galactic column density of 4.33E21 cm^-2 (HI4PI map, N. Ben Bekhti et al., 2016, A&A 594, A116), we derived a 3-sigma X-ray flux upper limit of 1.2E-13 (8.4e-14) erg/s/cm^2 in the 0.5-10.0 (0.2-5.0)keV band, using the eRASS:4 data (total exposure uncorrected for vignetting is approximately 894s). EP240426a was not detected in all individual eRASS scans except for eRASS2, during which a faint source was detected (albeit with a low detection likelihood, e.g. ML_DET = 6.3) with an estimated 0.5-10.0 keV absorbed flux comparable to the derived upper limits.

The 3-sigma 0.5-10.0keV flux upper limit derived from eROSITA data is much lower than the flux measured from EP/FXT (absorption-corrected 0.5-10 keV flux of around 9.2E-13 erg/s/cm2, corresponding to an absorbed 0.5-10keV flux of around 6.3E-13 erg/s/cm^2), suggesting that EP240426a showed significant X-ray flux increase.

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