GECAM discovery of the second FRB-associated Magnetar X-ray Burst from SGR J1935+2154

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 ($\rm\sim7.6\times10^{-7},erg \cdot cm^{-2}$ in 1-250 keV) magnetar X-ray burst detected by GECAM on October 14th, 2022 (dubbed as MXB 221014) from SGR J1935+2154, which is associated with a FRB detected by CHIME and GBT. 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 $\sim5.70$ 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.

💡 Research Summary

The paper reports the discovery and detailed analysis of a second magnetar X‑ray burst (MXB 221014) that is temporally associated with a fast radio burst (FRB 221014) from the Galactic magnetar SGR J1935+2154. The event was captured on 2022‑10‑14 by the Gravitational wave high‑energy Electromagnetic Counterpart All‑sky Monitor (GECAM) satellites, specifically GECAM‑B and GECAM‑C, while the radio counterpart was simultaneously detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) at 600 MHz and by the Green Bank Telescope (GBT) at 5 GHz.

Observations and Data Reduction
GECAM consists of four wide‑field X‑ray/gamma‑ray monitors; during the event GECAM‑A was offline, but GECAM‑B and C operated normally. The authors selected a subset of high‑gain (HG) detectors with incident angles below 60° to maximize signal‑to‑noise. Background subtraction, barycentric dynamical time (TDB) conversion, and a 0.1 µs timing resolution were applied. The low‑gain channels showed no significant signal, so only HG data were used for spectral fitting.

Spectral Analysis
Three spectral models were tested: a single blackbody, a cutoff power‑law (CPL), and a two‑component blackbody + CPL. Model comparison employed the Bayesian Information Criterion (BIC). The best‑fit model is a blackbody with temperature ≈ 10 keV combined with a CPL (photon index ≈ 1.2, cutoff ≈ 80 keV). The time‑integrated fluence in the 1–250 keV band is 7.6 × 10⁻⁷ erg cm⁻², comparable to the fluence of the previously known MXB 200428.

Temporal Characteristics
The burst duration (T₉₀) is ≈ 250 ms, typical for magnetar short bursts. The light curve reveals two narrow X‑ray pulses: the first peaks ≈ 30 ms after trigger, the second ≈ 120 ms later. The first X‑ray pulse aligns with the main radio pulse of FRB 221014 with a lag of 5.7 ms (radio arriving later), mirroring the 2–3 ms lag observed in the MXB 200428/FRB 200428 pair. The second X‑ray pulse has no radio counterpart. NICER observations show that the X‑ray burst occurs during the “bridge” (valley) phase of the soft X‑ray pulse profile, implying an anti‑phase relationship with the hard X‑ray emission, consistent with the earlier event.

Frequency Domain
Unlike MXB 200428, which exhibited a ∼ 40 Hz quasi‑periodic oscillation (QPO), no statistically significant QPO is detected in MXB 221014, suggesting that QPOs are not a universal feature of magnetar bursts associated with FRBs.

Radio Properties
CHIME measured a fluence of 9.7 ± 6.7 kJy ms at 600 MHz, while GBT recorded multiple bright bursts at 5 GHz, some of which saturated the receiver. Although FRB 221014 is less energetic than FRB 200428 by roughly one to two orders of magnitude, it remains among the brightest Galactic FRBs.

Statistical Association
The authors estimate the probability of a chance temporal coincidence between an X‑ray burst and a radio burst to be <10⁻⁴ using Poisson statistics, reinforcing a physical connection.

Comparison with MXB 200428
The paper systematically compares MXB 221014/FRB 221014 with the prototype MXB 200428/FRB 200428. Shared characteristics include: (i) similar fluence and spectral shape, (ii) T₉₀ ≈ 250 ms, (iii) a narrow X‑ray pulse that lags the radio pulse by a few milliseconds, and (iv) occurrence during the valley of the soft X‑ray pulse profile. Distinct differences are: (a) the presence of a second X‑ray pulse without a radio counterpart, (b) the absence of a detectable QPO, and (c) a lower radio fluence. These differences highlight a diversity in the morphology of magnetar‑FRB associations, implying that the underlying emission mechanism can produce a range of observable signatures.

Interpretation and Implications
The authors discuss several theoretical frameworks—magnetospheric reconnection, crustal cracking, and external shock models—and argue that the observed millisecond lag between X‑ray and radio peaks likely reflects the time needed for accelerated particles to generate coherent radio emission after the initial high‑energy trigger. The lack of a QPO in this event may indicate that crustal oscillations are not a prerequisite for FRB production. The detection of a second X‑ray pulse suggests that multiple reconnection sites or successive magnetic avalanches can occur within a single burst episode, but only one of them may satisfy the conditions for coherent radio emission.

Conclusions
MXB 221014/FRB 221014 constitutes the second robust case of a magnetar X‑ray burst coincident with a fast radio burst, confirming that the MXB‑FRB association is not a singular coincidence. The similarities to the 200428 event support models that link magnetar magnetospheric activity to FRB generation, while the observed differences underscore the need for a flexible, possibly multi‑component theoretical description. The authors emphasize the importance of simultaneous high‑time‑resolution X‑ray and radio monitoring—capabilities exemplified by GECAM, CHIME, and GBT—to build a statistical sample that can discriminate among competing models and ultimately unravel the physical origin of fast radio bursts.