FRB 20121102A is exhibiting hyperactivity with increased RM value at L-band detected by FAST
ATel #17921; Junshuo Zhang(NAOC), Dejiang Zhou(NAOC), Yi Feng(Zhejiang Lab), Chengwei Liang(GZHU), Chenyuan Xu (Zhejiang Lab), Jiarui Niu(NAOC), Xiaohui Liu(NAOC), Yongkun Zhang(NAOC), Pei Wang(NAOC), Di Li(THU, NAOC), Weiwei Zhu(NAOC), Bing Zhang(HKU), Jinlin Han(NAOC), Wenfei Yu(SHAO), Kejia Lee(PKU\&NAOC), Ye Li(PMO), Rui Luo(GZHU), Jinhuang Cao(NAOC), Wanjin Lu(NAOC) and FAST FRB Key Science Project Collaboration
on 23 Jul 2026; 02:55 UT
Credential Certification: Pei Wang (wangpei@nao.cas.cn)
Subjects: Radio, Request for Observations, Transient, Fast Radio Burst
We report the detection of a burst storm from FRB 20121102A with FAST, with a burst rate of ~490 hr-1 above a fluence threshold of 0.015 Jy ms, suggesting that this repeater has entered another highly active episode at L band. The observation began at MJD 61238.99860 and lasted 1800 s, with pulsar-backend data recorded at a sampling time of 49.152 microseconds using 4096 frequency channels covering 1.0 GHz to 1.5 GHz. This burst storm 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), consistent with our previous report in ATel #17642.
We detected a bright burst with a topocentric arrival time of MJD 61239.014431797 at a reference frequency of 1.5 GHz.
The RM of this burst was 35,645 +/- 10 rad m-2, with a linear polarization fraction of ~10% at L band. This value showed an anomalous increase of about 14,000 rad m-2 compared with that reported in our previous ATel #17642 in February 2026.
The best-fit DM of this burst, obtained through DM-phase package, is 536.9+/-0.1 pc cm-3, roughly 2 pc cm-3 lower than those reported by ATel #17633 and ATel #17642, consistent with the decreasing trend reported in Wang et al. 2025. We encourage further follow-up observations 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 thank all members of 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
