GECAM discovery of the second FRB-associated magnetar X-ray burst from SGR J1935+2154

ABSTRACT Fast radio burst (FRB) is mysterious phenomenon with millisecond-duration radio pulses observed mostly from cosmological distance. The association between FRB 200428 and a magnetar X-ray burst (MXB) from SGR J1935+2154 has significantly advanced the understanding of FRB and magnetar bursts. However, it is uncertain whether this association between MXB and FRB (i.e. MXB/FRB 200428) is genuine or just coincidental only based on this single event. Here we report the discovery of a bright ($\rm {\sim} 7.6\times 10^{-7}\, erg \cdot cm^{-2}$ in 1–250 keV) magnetar X-ray burst detected by Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) on 2022 October 14 (dubbed as MXB 221014) from SGR J1935+2154, which is associated with an FRB detected by Canadian Hydrogen Intensity Mapping Experiment and Green Bank Telescope. We conducted a detailed temporal and spectral analysis of MXB 221014 with GECAM data and find that it is a bright and typical ($T_{90}\sim$250 ms) X-ray burst from this magnetar. Interestingly, we find two narrow X-ray pulses in the MXB, one of which temporally aligns with the main pulse of the FRB 221014 ${\sim} 5.70$ ms latter than the peak time of FRB 221014), resembling the feature found in MXB/FRB 200428. Furthermore, we did comprehensive comparison between MXB/FRB 221014 and MXB/FRB 200428, and find that while the two events share several common features, they also exhibit distinct differences, highlighting the variety of the MXB–FRB association morphology. This finding not only confirms the association between MXB and FRB but also provides new insights into the mechanism of and the relationship between FRB and MXB.