Detection of high activity from repeating FRB 20121102A at L-band with FAST
ATel #17642; Junshuo Zhang(NAOC), Yongkun Zhang(NAOC), Yi Feng(Zhejiang Lab), Chengwei Liang(GZHU), Shuo Cao(NAOC), Jiaheng Zhang(CCNU), Pei Wang(NAOC), Tiancong Wang(BNU), Dejiang Zhou(NAOC), Jiarui Niu(NAOC), Di Li(THU, NAOC), Weiwei Zhu(NAOC), Bing Zhang(HKU), Jinlin Han(NAOC), Yuanchuan Zou(HUST), Rui Luo(GZHU), Chenhui Niu(CCNU), Ye Li(PMO), Wenfei Yu(SHAO), Songbo Zhang(PMO), Wanjin Lu(NAOC), Jinhuang Cao(NAOC), Yidan Wang(NAOC), Dengke Zhou(Zhejiang Lab), Yuhao Zhu(NAOC), and FAST FRB Key Science Project
on 1 Feb 2026; 12:04 UT
Credential Certification: Pei Wang (wangpei@nao.cas.cn)
Subjects: Radio, Transient, Fast Radio Burst
FAST has been monitoring FRB 20121102A at a 15-day cadence since August 2025, with the 19-beam L-band receiver covering a frequency bandwidth from 1.0-1.5 GHz.
Here we report a series of burst detections in four observations as part of regular monitoring and follow-up of ATel #17633. Following a period of non-detections since 1 May 2025, FAST detected a few bursts in the first two sessions and followed by a series of bursts in the subsequent two sessions above the fluence threshold of 0.015 Jy ms, suggesting that this repeater is likely in another highly active episode at L-band. The configuration and burst rates of the four observations are as follows:
| Session | Start time (UTC) | Duration (s) | Time resolution (μs) | Burst rate (hr-1) |
|---|
| 14 sessions | 2025-05-01 to 2026-01-07 | 1200 per session | 49.152 | Non-detection |
| 1 | 2026-01-07 13:28:00 | 1200 | 49.152 | 3 |
| 2 | 2026-01-22 12:11:00 | 1200 | 49.152 | 9 |
| 3 | 2026-01-29 14:49:20 | 3600 | 49.152 | 105 |
| 4 | 2026-01-30 12:32:00 | 7200 | 98.306 | 102 |
Recent observations indicate that the recent activation of FRB 20121102A at L-band appears to align with the proposed periodicity of ~160 days (Rajwade et al. 2020; Cruces et al. 2020; Braga et al. 2025; Wang et al. 2025), despite the absence of pulse detections in several previously projected active windows by FAST during the 2024-2025 period.
One of the brightest bursts was detected at the topocentric arrival time of MJD 61069.632513757, with a S/N of 52.9.
The dispersion measure (DM) of this burst was optimized to 539.1 ± 0.1 pc cm-3 using the DM-power package (Lin et al. 2023). This is consistent with the value reported by Effelsberg, indicating that the DM of FRB 20121102A has decreased by about 4 pc cm-3 compared to our previous detections in ATel #17156, following the temporal DM decrease trend described in Wang et al. 2025.
The rotation measure (RM) of this burst was detected to 23,023 ± 15 rad m-2 with a 15% linear polarization fraction in L-band, represents a continuing DM's and RM's trends of long-term persistent dropping.
We encourage more follow-up efforts with multi-band facilities.
FAST is a Chinese national mega-science facility, built and operated by the National Astronomical Observatories, Chinese Academy of Sciences (NAOC). We appreciate all the members in the FAST FRB key science project collaboration for their support and assistance during the observations.
Rajwade, K. M.; Mickaliger, M. B.; Stappers, B. W. et al., 2020, MNRAS, 495, 3551
Cruces, M.; Spitler, L. G.; Scholz, P. et al., 2021, MNRAS, 500, 448
Braga, C. A.; Cruces, M.; Cassanelli, T.; Espinoza-Dupouy, M. C. et al., 2025, A&A, 693, A40
Wang, P.; Zhang, J.-S.; Yang, Y.-P. et al., 2025, ArXiv, 2507.15790
Lin, H.-H.; Main, R.; Pen, U.-L. et al., 2023, ArXiv, 2208.13677
