2024ApJ...974..170C

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) × 10^–10 erg cm^–2 (99.7% credible interval, unabsorbed) on a timescale of 100 ms. Translated into a fluence ratio η_x/r = F_X–ray/F_radio, this corresponds to η_x/r < 7 × 10⁶. 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 × 10^–15 erg cm^–2 s^–1 (unabsorbed) or an isotropic luminosity limit of 1.4 × 10⁴¹ 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 η_x/r to date, η_x/r < 2 × 10⁶, 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 η_x/r of 8 × 10⁵, which is only 3 times the observed value of η_x/r for SGR 1935+2154.