Bursts before Burst: A Comparative Study on FRB 200428-associated and FRB-absent X-ray Bursts from SGR J1935+2154

Abstract

Accompanied by an X-ray burst, the fast radio burst FRB 200428 was recently confirmed to originate from the Galactic magnetar SGR J1935+2154. Just before FRB 200428 was detected, the Fivehundred-meter Aperture Spherical radio Telescope (FAST) had been monitoring SGR J1935+2154 for eight hours. From UTC 2020-04-27 23:55:00 to 2020-04-28 00:50:37, FAST detected no pulsed radio from SGR J1935+2154, while Fermi/GBM registered 34 SGR bursts in the X/soft γ-ray band, forming a unique sample of X-ray bursts in the absence of FRBs. After a comprehensive analysis on light curves, time-integrated and time-resolved spectral properties of these FRB-absent X-ray bursts, we compare this sample with the FRB-associated X-ray burst detected by Insight-HXMT. The FRB-associated burst distinguishes itself from other X-ray bursts by its non-thermal spectrum and a higher spectral peak energy, but is otherwise not atypical in many other aspects. We also compare the cumulative energy distribution of our X-ray burst sample with the cumulative radio energy distribution of first repeating FRB source, FRB 121102, with the calibration of FRB 200428-X-ray burst association. We find a similarity between the two, offering indirect support of the magnetar-origin of cosmological FRBs. The event rate density of magnetar bursts is about ∼ 150 times higher than the FRB event rate density at the energy of FRB 200428. This again suggests that only a small fraction of X-ray bursts are associated with FRBs if all FRBs originate from magnetars.

"Slow" Radio Bursts from Galactic Magnetars?

“…Such models, on the other hand, face the challenge of interpreting diverse polarization angle variations from some repeating FRBs (Luo et al 2020;. Alternatively, if FRB emission indeed originates from narrow beams within magnetar magnetospheres, the lack of detection of abundant SRBs would suggest that the rarity of FRB-SGR-burst associations is likely intrinsic, i.e., the SGR-burst that makes FRB 200428 was physically distinct from other SGR bursts (Li et al 2020;Younes et al 2020;Yang et al 2020a).…”

“…Whereas whether the FRB-associated X-ray burst is physically special is still subject to debate (e.g. Li et al 2020;Younes et al 2020;Yang et al 2020a), one plausible possibility is that the FRB emission is much more narrowly beamed than the SGR burst emission (Lin et al 2020).…”