2025ApJ...984..178S

We report on X-ray limits on observations of FRB 20190520B during simultaneous observations with the Green Bank Telescope and the Chandra X-ray Observatory. FRB 20190520B is an outlier in the fast radio burst (FRB) population due to its excess source-local dispersion, its association with compact persistent radio emission, and its high line-of-sight magnetic field variability. One radio burst was detected during the overlapping observing time, and no X-ray emission was detected. We find line-of-sight density and burst duration limits on FRB 20190520B's X-ray luminosity, interpreting the nondetection in the context of known X-ray populations that might serve as progenitors for FRB 20190520B. We place a direct limit on the presence of a massive black hole (MBH) in this system, limiting mass and the Eddington ratio λ_E to ${M}_{\mathrm{BH}}\lt 4.3\times 1{0}^{3}({\lambda }_{{\rm{E}}}^{-1})\,{M}_{\odot }$ (in a low-density line-of-sight limit) and ${M}_{\mathrm{BH}}\lt 1.8\times 1{0}^{4}({\lambda }_{{\rm{E}}}^{-1})\,{M}_{\odot }$ (high-density limit). These limits are both inconsistent with the scenario derived by Anna-Thomas et al. in which the FRB is viewed through a MBH wind; however, the model is still allowed if the wind luminosity has an X-ray emission fraction of less than 8%. Our observations would have been sensitive to the brightest ∼15% of the ultraluminous X-ray source (ULX) population and the brightest flares from soft gamma-ray repeaters, but the nondetection is consistent with expectations for SGR 1806-20-like flares, most ULX hypernebula models, and X-ray binaries.