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Joint Observation of SGR J1935+2154 with \textit{Insight}-HXMT and KM40m during the active episode of October 2022

During the October 2022 active episode of magnetar SGR J1935+2154, joint observations by \textit{Insight}-HXMT and the KM40m radio telescope revealed 60 X-ray bursts and one radio burst, with a unique temporal association between a specific X-ray burst (MXB 221021) and the radio burst that exhibits distinct morphological properties compared to previous events, offering new insights into magnetar emission mechanisms.

Original authors: Wang-Chen Xue, Wen-Jun Tan, Yu-Xiang Huang, Xiao-Bo Li, Long-Fei Hao, Shao-Lin Xiong, Ce Cai, Chen-Wei Wang, Yue Wang, Ke-Jia Lee, Heng Xu, Peng Zhang, Ming-Yu Ge, Hao-Xuan Guo, Yue Huang, Cheng-Kui L

Published 2026-04-14

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CC BY 4.0

Original authors: Wang-Chen Xue, Wen-Jun Tan, Yu-Xiang Huang, Xiao-Bo Li, Long-Fei Hao, Shao-Lin Xiong, Ce Cai, Chen-Wei Wang, Yue Wang, Ke-Jia Lee, Heng Xu, Peng Zhang, Ming-Yu Ge, Hao-Xuan Guo, Yue Huang, Cheng-Kui Li, Jia-Cong Liu, Yang-Zhao Ren, Shuo Xiao, Sheng-Lun Xie, Shu-Xu Yi, Zheng-Hang Yu, Jin-Peng Zhang, Yan-Qiu Zhang, Chao Zheng, Shi-Jie Zheng, Shu-Mei Jia, Xiang Ma, Jin Wang, Hai-Sheng Zhao, Yong Chen, Cong-Zhan Liu, Yu-Peng Xu, Li-Ming Song, Shuang-Nan Zhang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Detective Story: Catching a Magnetar in the Act

Imagine the universe as a giant, dark ocean. Most stars are like lighthouses, shining steadily. But Magnetars are different. They are like cosmic "tempests"—neutron stars with magnetic fields so incredibly strong (trillions of times stronger than a fridge magnet) that they can crack their own crusts. When they crack, they let out massive bursts of energy, like a cosmic sneeze.

One specific magnetar, named SGR J1935+2154, is famous because it's the only one we know of that can also shoot out Fast Radio Bursts (FRBs). These are like lightning bolts of radio waves that zip across the universe in milliseconds. For years, scientists have been trying to figure out: Do these radio lightning bolts happen at the exact same time as the X-ray "sneezes," or is there a delay?

In October 2022, this magnetar woke up again and started being very active. Two teams of astronomers decided to watch it closely, like setting up two different security cameras on a house.

The Two Cameras

Insight-HXMT (The X-Ray Camera): This is a Chinese space telescope. It looks at the "hot" stuff—X-rays. It watched the magnetar for about 940,000 seconds (roughly 11 days).
KM40m (The Radio Camera): This is a giant 40-meter radio dish on the ground in Kunming, China. It listened for the "radio lightning" (FRBs) for about 1,400 hours.

They watched the same target at the same time, hoping to catch a moment where both cameras saw something happen simultaneously.

The Big Catch: 60 Sneeze, One Lightning Bolt

During their joint watch, the X-ray camera saw 60 bursts (magnetar sneezes). The radio camera, however, only heard one radio burst.

Usually, when a magnetar sneezes, it's quiet on the radio. But this one time, they caught a match! On October 21, 2022, the X-ray camera saw a burst (called MXB 221021), and almost immediately after, the radio dish heard a burst (called RB 221021).

The Mystery: The "Late" Radio Wave

Here is where things get weird and interesting.

In the past, when this magnetar shot out a radio burst (like the famous one in 2020), the radio wave and the X-ray wave happened at the exact same time, down to the millisecond. It was like a drummer hitting the snare and the cymbal simultaneously.

But this time? The radio wave was late.

The X-ray burst happened first. Then, about 1 second later, the radio burst arrived.

The Analogy: Imagine you see a firework explode in the sky (the X-ray burst). You expect to hear the boom instantly. But in this case, you see the explosion, wait a full second, and then hear the boom.
Why is this strange? Light and radio waves travel at the same speed. If they were created at the same spot at the same time, they should arrive together. A 1-second delay is a long time in astronomy. It suggests that the radio wave wasn't created right where the X-ray was, or it took a weird path to get to us.

The "Ghost" Signal

Another surprise was how weak the radio burst was.

The Analogy: Think of the famous 2020 radio burst as a giant thunderclap that you can hear across the country. This new radio burst (RB 221021) was more like a whisper in a library. It was so faint that most radio telescopes would have missed it entirely.
This tells us that magnetars can produce radio bursts of all sizes, from thunderclaps to whispers. This helps scientists understand that these bursts might exist on a continuous spectrum, rather than just being "big" or "small."

What Does This Mean?

This discovery is like finding a new piece of a puzzle that didn't fit the picture we had before.

It's not always instant: The fact that the radio wave arrived a second late challenges the idea that radio and X-rays are always born together in the same instant. Maybe the radio wave is generated in a different part of the magnetar's magnetic field, or it gets "stuck" for a moment before escaping.
It's a spectrum: The fact that the radio burst was so weak suggests that magnetars are constantly chattering in radio waves, but we just need better ears (more sensitive telescopes) to hear the quiet ones.
The "Whisper" is important: Because the X-ray burst was also relatively weak, it would have been missed by all-sky monitors (telescopes that look at the whole sky). This proves that we need to point our telescopes directly at these active magnetars to catch the subtle, quiet events that teach us the most.

The Bottom Line

This paper is a report from two astronomers who were lucky enough to catch a magnetar "sneezing" and "whispering" at the same time. The fact that the whisper came a second after the sneeze is a new clue that helps us understand the complex, chaotic physics happening inside these super-magnetic stars. It shows us that the universe is full of surprises, and sometimes the quietest signals hold the biggest secrets.

1. Problem Statement

Magnetars, specifically Soft Gamma-ray Repeaters (SGRs), are known to produce X-ray bursts and, in rare cases, Fast Radio Bursts (FRBs). The discovery of the association between FRB 200428 and a magnetar X-ray burst (MXB 200428) from SGR J1935+2154 provided the first compelling evidence that magnetars can generate FRBs. However, the physical mechanisms linking X-ray and radio emissions remain debated. Key open questions include:

Are radio and X-ray emissions produced co-temporally, or are there significant time delays?
Do radio-associated bursts exhibit distinct spectral or temporal properties compared to non-associated bursts?
What is the energy partition between radio and X-ray bands across different burst populations?

To address these questions, simultaneous, high-sensitivity multi-wavelength monitoring is required, particularly for weaker bursts that all-sky monitors might miss.

2. Methodology

The authors conducted a joint observation campaign during the October 2022 active episode of SGR J1935+2154 using two instruments:

Insight-HXMT (Hard X-ray Modulation Telescope): Conducted Target-of-Opportunity (ToO) observations from October 13 to November 1, 2022, covering the energy range of 1–250 keV (Low Energy: 1–10 keV; Medium Energy: 10–35 keV; High Energy: 28–250 keV).
KM40m (Kunming 40-meter Radio Telescope): Observed the source at a central frequency of 2.245 GHz with an 110 MHz bandwidth from October 15, 2022, to March 22, 2023.

Data Analysis Techniques:

Burst Detection: A refined burst search algorithm was applied to HXMT data, identifying 516 total MXBs. The authors focused on the 60 bursts occurring during the overlapping observation windows with KM40m.
Temporal Analysis: Bayesian Block algorithms were used to identify change points in count rates to define burst durations and separate background. Coincidence probabilities were calculated assuming a Poisson distribution.
Spectral Analysis: Time-integrated spectra were extracted for LE, ME, and HE (NaI and CsI) detectors. Models tested included Power Law (PL), Blackbody (BB), Optically Thin Thermal Bremsstrahlung (OTTB), CPL (PL with cutoff), and hybrid models (e.g., OTTB+BB). Model selection was based on $W$ -statistics and the Akaike Information Criterion with sample size correction (AICc).
Radio Analysis: The BEAR software package was used to search for radio bursts. The specific event of interest, RB 221021, was analyzed for its sub-burst structure, fluence, and dispersion measure (DM).

3. Key Contributions

Discovery of a Delayed Association: The paper reports the identification of MXB 221021, an X-ray burst temporally associated with a radio burst (RB 221021) detected by KM40m. Crucially, this event exhibits a significant time delay (~1 second) between the X-ray peak and the radio peak, contrasting with the near-simultaneity (millisecond scale) seen in FRB 200428 and FRB 221014.
Comprehensive Population Study: The authors provide a uniform analysis of 60 simultaneous X-ray/radio observation bursts, characterizing their temporal and spectral properties to establish a baseline for radio-associated events.
Energy Partition Analysis: The study calculates the radio-to-X-ray fluence ratio for the associated event, revealing an unusually low energy partition compared to previous associations.

4. Key Results

Event Statistics: During the overlapping observation period, 60 MXBs were detected. Only one radio burst (RB 221021) was detected by KM40m, which coincided with MXB 221021.
Temporal Characteristics of MXB 221021:
- The X-ray burst duration was approximately 1.16 seconds.
- The radio burst (RB 221021) consisted of three sub-bursts (A, B, C) with widths of ~0.35–1.11 ms.
- Time Delay: After correcting for dispersion and converting to geocentric arrival times, the radio peak occurred ~1.15 seconds after the X-ray peak. This corresponds to roughly 1/3 of the magnetar's spin period (~3.24 s).
- The probability of this being a chance coincidence is low ( $1.8 \times 10^{-4}$ , or 3.6- $\sigma$ ), supporting a physical association.
Spectral Properties:
- MXB 221021 is best described by a Power Law (PL) model with a photon index $\alpha = 1.86 \pm 0.09$ , indicating a non-thermal, relatively soft spectrum.
- Compared to the general sample, MXB 221021 lies in the "long-duration" tail of the distribution but has a fluence within the bulk of the sample.
- The spectral hardness (peak energy) is relatively high compared to non-radio-associated bursts, consistent with previous radio-associated events.
Energetics:
- The X-ray fluence ($8-200$ keV) was $\mathcal{F}_X \approx 9 \times 10^{-9}$ erg cm $^{-2}$ .
- The radio fluence was converted to energy units, yielding a radio-to-X-ray fluence ratio of $\mathcal{F}_R / \mathcal{F}_X \sim 10^{-9}$ . This is orders of magnitude lower than typical magnetar radio/X-ray events (usually $10^{-5}$ to $10^{-8}$ ), suggesting an extremely weak radio counterpart relative to the X-ray emission.
Population Trends: The study confirms a positive correlation between burst duration and fluence. It also notes a tentative trend where more energetic bursts tend to have harder spectra, though with significant scatter.

5. Significance

Challenging Simultaneity Models: The ~1-second delay between the X-ray and radio peaks challenges emission models that predict strict co-temporality between prompt X-ray spikes and coherent radio bursts. This suggests that radio and X-ray emissions may originate from distinct regions within the magnetosphere or involve different propagation effects (e.g., geometric delays or scattering).
Continuum of Radio Emission: The detection of a radio burst with extremely low fluence (far below typical FRB levels) associated with an X-ray burst supports the hypothesis of a continuum in magnetar radio burst energies, bridging the gap between ordinary pulsar radio pulses and bright FRBs.
Importance of Sensitive Follow-up: The fact that this X-ray burst was below the typical sensitivity threshold of all-sky monitors (like Fermi/GBM) highlights the critical role of pointed, high-sensitivity observations (like Insight-HXMT) in uncovering the full diversity of magnetar burst populations and their radio counterparts.
Future Directions: The findings necessitate further simultaneous monitoring with higher radio sensitivity and broader frequency coverage to determine if such delayed associations are a generic feature under specific magnetospheric conditions.