Repeating behaviour of FRB 121102: periodicity, waiting times and energy distribution

Abstract: Detections from the repeating fast radio burst FRB 121102 are clustered in time, noticeable even in the earliest repeat bursts. Recently, it was argued that the source activity is periodic, suggesting that the clustering reflected a not-yet-identified periodicity. We performed an extensive multi-wavelength campaign with the Effelsberg telescope, the Green Bank telescope and the Arecibo Observatory to shadow the Gran Telescope Canaria (optical), NuSTAR (X-ray) and INTEGRAL (gamma-ray). We detected 36 bursts with Effelsberg, one with a pulse width of 39\,ms, the widest burst ever detected from FRB 121102. With one burst detected during simultaneous NuSTAR observations, we place a 5-$\sigma$ upper limit of $5\times10^{47}$ erg on the 3--79\,keV energy of an X-ray burst counterpart. We tested the periodicity hypothesis using 165-hr of Effelsberg observations and find a periodicity of 161$\pm$5 days. We predict the source to be active from 2020-07-09 to 2020-10-14 and subsequently from 2020-12-17 to 2021-03-24. We compare the wait times between consecutive bursts within a single observation to Weibull and Poisson distributions. We conclude that the strong clustering was indeed a consequence of a periodic activity and show that if the few events with millisecond separation are excluded, the arrival times are Poisson distributed. We model the bursts' cumulative energy distribution with energies from ${\sim}10^{38}$-$10^{39}$ erg and find that it is well described by a power-law with slope of $\gamma=-1.1\pm 0.2$. We propose that a single power-law might be a poor descriptor of the data over many orders of magnitude.

Analysis of Repeating Behavior in FRB 121102: Insights into Periodicity, Wait Times, and Energy Distribution

The research paper by Cruces et al. focuses on the repeating fast radio burst (FRB) source, FRB 121102. The authors explore its periodicity, temporal behavior, and energy distribution with significant rigor. Using a multi-wavelength observation campaign involving the Effelsberg and Green Bank Telescopes, along with the Arecibo Observatory, the study provides critical insights into the temporal clustering and energetic characteristics of FRB 121102.

The central hypothesis tests whether the observed temporal clustering of FRBs is indicative of an underlying periodicity. Extensive observational data over a total of 165 hours at Effelsberg led to the detection of 36 bursts. Notably, one of the Effelsberg-detected bursts exhibited a pulse width of 39 ms, marking it as the widest detected burst from FRB 121102. The authors provide a robust analysis of periodicity, hypothesizing a cycle of 161±5 days. This periodicity suggests that the source displays periodic active windows in which bursts are detectable, with predictions made for active periods extending from July to October 2020 and December 2020 to March 2021.

The paper also examines the waiting times between consecutive bursts and fits these intervals to both Poisson and Weibull distributions. Results indicate that excluding events with millisecond-level separations results in arrival times consistent with a Poisson distribution, challenging previous assertions of significant clustering and suggesting that the previously perceived clustering may be an artifact of not accounting for the periodic activity windows.

Moreover, the energy distribution of the detected bursts is modeled with a notable interest in understanding the broader implications for transient radio phenomena. The cumulative energy distribution analysis, focused on energies ranging from approximately 10^38 to 10^39 erg, is described by a power-law distribution with a slope of γ = -1.1±0.2. This result challenges the appropriateness of a single power-law for bursts spanning many orders of magnitude, hinting at the complexity of the FRB emission mechanism.

The paper emphasizes the significant implications of these findings on both theoretical and practical fronts. The identification of periodic activity patterns in FRB 121102 can drastically improve targeting strategies for observing campaigns aimed at these elusive astrophysical objects. Furthermore, the energy distribution characteristics aid in refining models of FRB origins, potentially implicating progenitor systems involving neutron stars and highly magnetized environments.

For future work, the authors suggest closer examination of the interplay between periodicity and emission characteristics across different FRB sources. The study paves the way for leveraging multi-wavelength capabilities in concert with enhanced analysis techniques on periodic phenomena in the cosmic radio landscape.