FRB 20121102A enters a new active epoch with a lower DM as revealed by FAST

ATel #17156; Junshuo Zhang(NAOC), Pei Wang(NAOC), Yongkun Zhang(NAOC), Di Li(THU, NAOC), Weiwei Zhu(NAOC), Bing Zhang(UNLV), Jinhuang Cao(NAOC), Yi Feng(Zhejiang Lab), Jinlin Han(NAOC), Wanjin Lu(NAOC), Yidan Wang(NAOC), Dengke Zhou(Zhejiang Lab), Yuhao Zhu(NAOC), and FAST FRB Key Science Project
on 21 Apr 2025; 16:06 UT
Credential Certification: Pei Wang (wangpei@nao.cas.cn)

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FAST has been monitoring FRB 20121102A for 20 minutes monthly since February of 2023, with the central beam of the FAST 19-beam L-band receiver pointing to RA = 05h31m58.70s, DEC = +33d08m52.5s (Chatterjee et al. 2017; Marcote et al. 2017), taking pulsar-search data with a 49.152 microsecond sampling and 4096 frequency channels in between 1.0 GHz and 1.5 GHz.

Following non-detections since 28 December 2023, FAST detected 61 bursts from FRB 20121102A in 20 minutes during UTC 2025-04-14 07:29:00 - 07:49:00, with the brightest burst at the topocentric coordinate time of MJD 60779.320601234. The burst rate reaches ~180/hr above the fluence threshold of 0.015 Jy ms, indicating that this repeater is likely entering another active episode.

This recent activation of FRB 20121102A is generally consistent with the proposed periodicity around 160 days (Rajwade et al. 2020; Cruces et al. 2020; Braga et al. 2025; Wang et al. 2025), even though in several previous projected active windows no burst was detected by FAST.
The dispersion measure (DM) of the bright burst is optimized to 543.5 +/- 0.1 pc cm^-3 using the DM-power package (Lin et al. 2023), decreasing by about 9 pc cm^-3 compared to previous detections in Wang et al. (2025) and ATel#15619, suggesting a consistent temporal DM decrease trend of ~ -4.5 pc cm^-3 yr^-1.

We encourage more follow-up monitoring 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.