Kavli Affiliate: Kiyoshi W. Masui
| First 5 Authors: Amanda M. Cook, Paul Scholz, Aaron B. Pearlman, Thomas C. Abbott, Marilyn Cruces
| Summary:
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
${eta}_{text{ x/r}} < 7times10^{6}$. For persistent emission from the
location of FRB 20220912A, we derive a 99.7% $0.5-10.0$ keV isotropic flux
limit of $8.8times 10^{-15}$ 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, ${eta}_{text{ x/r}} < 2times10^6$, 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 $8times10^5$, which is only 3 times
the observed value of ${eta}_{text{ x/r}}$ for SGR 1935+2154.
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