We present an extensive contemporaneous X-ray and radio campaign performed on the repeating fast radio burst (FRB) source FRB 20220912A for eight 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 $0.5-10.0$ keV X-ray fluence of $(1.5-14.5)\times 10^{ -10}$ erg cm$^{ -2}$ (99.7% credible interval, unabsorbed) on a timescale of 100 ms. Translated into a fluence ratio $\eta_{ \text{ x/r}} = F_{ \text{ X-ray}}/F_{ \text{ radio}}$, this corresponds to ${ \eta}_{ \text{ x/r}} < 7\times10^{ 6}$. For persistent emission from the location of FRB 20220912A, we derive a 99.7% $0.5-10.0$ keV isotropic flux limit of $8.8\times 10^{ -15}$ erg cm$^{ -2}$ s$^{ -1}$ (unabsorbed) or an isotropic luminosity limit of 1.4$\times10^{ 41}$ 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 ${ \eta}_{ \text{ x/r}}$ to date, ${ \eta}_{ \text{ x/r}} < 2\times10^6$, assuming that all thirty 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 Galactic magnetar SGR 1935+2154 detected on 2020 April 28, we derive a 99.7% credible interval upper limit on ${ \eta}_{ \text{ x/r}}$ of $8\times10^5$, which is only 3 times the observed value of ${ \eta}_{ \text{ x/r}}$ for SGR 1935+2154.
Categories: astro-ph.HE