Fig. 2: Limits on the energy of X-ray bursts between 0.5 and 10 keV at the times of radio bursts from FRB 20200120E, FRB 20121102A and FRB 20180916B.
The red curve shows the 3σ limits derived from NICER observations of FRB 20200120E at the time of burst B4, after incorporating the spectral response of the NICER XTI. The black curve shows the 3σ limits derived from XMM-Newton observations of FRB 20200120E at the time of burst B9, after including the spectral response of the XMM-Newton EPIC/pn camera. The light-blue curves correspond to previous 3σ limits derived from Chandra and XMM-Newton observations at the times of multiple radio bursts from FRB 20121102A23. Stacked 3σ limits for FRB 20121102A are shown in dark blue. The magenta curve shows previous 3σ limits derived from a Chandra observation at the time of a radio burst from FRB 20180916B19. Several fiducial spectral models are shown for comparison. The green curves correspond to exponentially cut-off power-law spectra with Γ = 1.5 and Ecut = 85 keV, similar to the X-ray counterpart associated with the FRB-like radio burst detected from SGR 1935+2154 on 28 April 202011,12,13,14. The orange curves represent blackbody spectra with kT = 10 keV, which have been observed from some magnetar hard X-ray bursts119,120. The purple curves show exponentially cut-off power-law spectra with Γ = −0.5 and Ecut = 500 keV, similar to the spectrum observed during SGR 1806–20’s giant flare31,121. The dashed spectra were calculated using a hydrogen column density of NH = 6.73 × 1020 cm−2 towards FRB 20200120E24, and the dotted spectra were calculated using a hydrogen column density of NH = 1023 cm−2 to illustrate the effects of high absorption local to the source. Larger hydrogen column densities yield harder X-ray spectra, with increased absorption at lower X-ray energies. Although high absorption towards FRB 20200120E is not expected, this scenario is shown for comparison. Each fiducial X-ray spectrum was normalized such that the integrated X-ray burst energy in the 0.5–10 keV energy range is equal to the NICER 0.5–10 keV band-averaged energy limit from FRB 20200120E.
