Contemporaneous X-ray Observations of 30 Bright Radio Bursts from the Prolific Fast Radio Burst Source FRB 20220912A

Abstract

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 $\hat{I}{\cdot }_{\text{ x/r}}=F_{\text{X-ray}}/F_{\text{radio}}$, this corresponds to $\hat{I}{\cdot }_{\text{ x/r}}<7\times 10^6$. For persistent emission from the location of FRB 20220912A, we derive a 99.7% $0.5-10.0$ keV isotropic flux limit of $8.8\times 10^{-15}$ erg cm${}^{-2}$ s${}^{-1}$ (unabsorbed) or an isotropic luminosity limit of 1.4$\times 10^{41}$ 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 $\hat{I}{\cdot }_{\text{ x/r}}$ to date, $\hat{I}{\cdot }_{\text{ x/r}}<2\times 10^6$, 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 $\hat{I}{\cdot }_{\text{ x/r}}$ of $8\times 10^5$, which is only 3 times the observed value of $\hat{I}{\cdot }_{\text{ x/r}}$ for SGR 1935+2154.

23 pages, 3 figures. ApJ in press (accepted after resubmission July 19th, 2024)