Abstract

We present an extensive contemporaneous X-ray and radio campaign performed on the repeating fast radio burst (FRB) source FRB 20220912A for 8 weeks immediately following the source's detection by CHIME/FRB. This includes X-ray data from XMM-Newton, NICER, and Swift, and radio detections of FRB 20220912A from CHIME/Pulsar and Effelsberg. We detect no significant X-ray emission at the time of 30 radio bursts with upper limits on a 0.5-10.0 keV X-ray fluence of (1.5-14.5) x 10(-10) erg cm(-2) (99.7% credible interval, unabsorbed) on a timescale of 100 ms. Translated into a fluence ratio eta(x/r) = FX-ray/F-radio, this corresponds to eta(x/r) < 7 x 10(6). For persistent emission from the location of FRB 20220912A, we derive a 99.7% 0.5-10.0 keV isotropic flux limit of 8.8 x 10(-15) erg cm(-2) s(-1) (unabsorbed) or an isotropic luminosity limit of 1.4 x 10(41) erg s(-1) at a distance of 362.4 Mpc. We derive a hierarchical extension to the standard Bayesian treatment of low-count and background-contaminated X-ray data, which allows the robust combination of multiple observations. This methodology allows us to place the best (lowest) 99.7% credible interval upper limit on an FRB eta(x/r) to date, eta(x/r) < 2 x 10(6), assuming that all 30 detected radio bursts are associated with X-ray bursts with the same fluence ratio. If we instead adopt an X-ray spectrum similar to the X-ray burst observed contemporaneously with FRB-like emission from the Galactic magnetar SGR 1935+2154 detected on 2020 April 28, we derive a 99.7% credible interval upper limit on eta(x/r) of 8 x 10(5), which is only 3 times the observed value of eta(x/r) for SGR 1935+2154.