FRB 20121102A monitoring: Updated periodicity in the <i>L</i> band

Context. FRB 20121102A was the first fast radio burst to be observed to repeat. Since then, thousands of bursts have been detected by multiple radio telescopes around the world. Previous work has shown an indication of a cyclic activity level with a periodicity of around 160 days. Knowing when the source repeats is essential for planning multi-wavelength monitoring to constrain the emission extent and progenitor source. Aims. We report the monitoring of FRB 20121102A using the 100-m Effelsberg radio telescope in the L band and update the periodicity of the cyclic activity level. Methods. We used the Lomb-Scargle periodogram on a sample of 284 observing epochs, of which 42% correspond to detections and 58% to non-detections. Our dataset is composed of the seven epochs of our monitoring plus publicly available data. We investigated two methods: i) a binary model, describing the observing epochs with 1 if there are detections and with 0 for non-detections, and ii) a normalised rates model that considers the inferred detection rates. Results. We report no detections in 12.5-hour observations down to a fluence of 0.29 Jy ms. The best period we find for the cyclic activity window is 159.3 ± 0.8 days for the binary model and 159.3 ± 0.2 days for the normalised rates model. We show the activity phase to be 53%. The normalised rates show clear Gaussian-like behaviour for the activity level, in that the number of detections peaks at the centre of the activity window. Conclusions. The periodicity found through both methods is consistent for the L - and S -band datasets, implying it is intrinsic to the source. The activity phase in the S band however, shows an indication of it ending before the L -band activity phase, supporting the idea of a chromatic dependence of the activity window. The sample in the C band is not large enough to further confirm this result.