Four January 2023 GBM frequencies toward GRO J2058+42 lie within 0.8 microhertz of a nominal RX J0440.9+4431 sideband

Four public Fermi/GBM frequency records assigned to GRO J2058+42 cover 2023 January 24 through 31. The GBM team already warns that NICER did not verify this event. We compare these records with RX J0440.9+4431, which was bright at that time. We remove the RX binary timing correction and put both source tracks on the same clock. We measure Fermi orbit periods from public spacecraft positions. After we add twice the orbit frequency to the RX track, the four offsets are -0.061, -0.482, -0.218, and -0.713 microhertz. No offset or scale was fitted. Linear interpolation changes each prediction by less than 0.0067 microhertz. BAT and MAXI give mean fluxes compatible with zero at the GRO position during the event. Comparable GBM pulse fluxes in 2019 came with a clear BAT signal. The close timing relation gives a specific candidate cause for the disputed records. The nominal curve does not explain all offsets within the quoted GRO errors. The source pair and sideband order were selected after data inspection. We did not recover the detector sampling window or prove the source of the photons. The package contains original data bytes, source URLs, code, figures, output tables, and independent numerical checks.

GRO J2058+42 · RX J0440.9+4431 · Fermi GBM · source confusion · spacecraft sideband · pulsar timing · Swift BAT · MAXI · reproducible research