High Frequency Detections of the Repeater FRB 20121102A with the Effelsberg 100-m Radio Telescope

ATel #17633; Lorena Nicotera (MPIfR), Marlon Bause (MPIfR), Pranav Limaye (MPIfR), Ninisha Manaswini (MPIfR), Laura Spitler (MPIfR)
on 27 Jan 2026; 12:36 UT
Credential Certification: Laura Spitler (lspitler@mpifr-bonn.mpg.de)

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We report detections of FRB 20121102A in two observations with the 100-m Effelsberg radio telescope with the Ultra-broadband (UBB) receiver (1.3-6 GHz). Observation 1 began at 18:27:16 UTC on 19 December 2025 (MJD 61028) and lasted 8083 s. Observation 2 began at 00:44:50 UTC on 16 January 2026 (MJD 61056) and lasted 11325 s. Due to an error in data processing, only a single hand of polarization was observed. The estimated system equivalent flux density of the UBB is roughly 20 Jy. The full observing bandwidth is sampled in seven adjacent frequency sub-bands and a coherently de-dispersed PSRFITS file is recorded for each band. These PSRFITS files had a time resolution of 0.128 ms and frequency resolution of 0.585 MHz.

The data were searched for single pulses using the BLISS pipeline, which is a TransientX-based single pulse search pipeline for BLInd Single pulse Searches by searching over a wide DM and time range and applying several RFI mitigation techniques. The search was conducted in the targeted DM mode with a S/N threshold of 6 over a DM range between 460 and 660 pc/cc. Candidates with a S/N >= 8 were classified using FETCH as well as being visually inspected. One burst was identified in Observation 1 at 3.727 to 4.476 GHz (Band5) with a S/N = 11.2 and FETCH probability score of 1.0. Its estimated fluence is 142 mJy ms. Two bursts were detected at 2.227 to 2.976 GHz (Band3) in Observation 2 with S/N = 10 and 9.5 and both with a FETCH probability score of 1.0. Their estimated fluences are 219 and 170 mJy ms, respectively. The average optimal DM of the three bursts as determined by TransientX is 539 pc/cc, roughly 4 pc/cc lower than the value reported from the FAST telescope (ATel #17156). The link below shows the dynamic spectra for these bursts.

These are the first detections of FRB 20121102A at higher frequencies in several years. FRB 20121102A is known to have an activity period of around 158.1 days ( Gouiffes et al 2025 ). Espinoza-Dupouy et al 2025 recently showed that the activity may also be chromatic. These UBB detections are occurring before the expected beginning of the active window at 1.4 GHz. Moreover, the detection of a burst first at 4.4 GHz in December 2025, followed by two bursts at around 2.6 GHz one month later, would be consistent with a shift in the active window from higher to lower frequency with time. Therefore, we urge wideband observations of this source in the coming months, as these detections may portend a new active phase for this source.

FRB20121102A UBB dynamic spectra