Chromatic activity window of periodic FRBs: FRB 20121102A and FRB 20180916B

TL;DR

This work addresses how periodic FRBs exhibit frequency-dependent activity windows by modeling the phase distribution with a Von Mises function and linking peak phase and window width across frequency via power-law relations. Using standardized, cross-telescope data for FRB $20121102A$ ($P=159.3$ d) and FRB $20180916B$ ($P=16.33$ d), the authors fit frequency-dependent parameters $A_\mu$, $B_\mu$, $A_\delta$, and $B_\delta$ to describe how the activity window shifts and broadens or narrows with $ν$. The main findings show both FRBs trigger earlier at higher frequencies, but FRB $20121102A$ broadens with frequency while FRB $20180916B$ narrows, with implications for progenitor models such as magnetar precession, radius-to-frequency mapping, and binary-comb scenarios. The approach provides predictive active windows for future cycles and highlights the importance of homogeneous sampling and bandwidth-based biases in interpreting chromatic FRB activity. Overall, the study advances a quantitative framework for comparing chromatic activity in periodic FRBs and informs constraints on their emission mechanisms and environments.

Abstract

Two fast radio bursts, FRB 20121102A and FRB 20180916B, show periodic activity with cycles of 159.3 and 16.33 days, respectively. These cycles consist of active and inactive windows, with peak activity centered within the active phase. For FRB 20180916B, studies have reported a frequency-dependent-or ``chromatic''-behaviour, where the activity window starts earlier and becomes narrower at higher frequencies. The activity across frequencies is typically modeled with a power law. In this work, we develop a simple model that combines the phase and frequency dependence of the activity windows of FRB 20121102A and FRB 20180916B. Our goal is to perform a chromaticity study for FRB 20121102A, incorporating model improvements to account for the cyclic nature of its activity window, and to compare the chromatic behaviour between both periodic FRBs. We standardise the detections from the 425 observing epochs for FRB 20121102A and the 214 epochs for FRB 20180916B to account for differences in radio telescope sensitivity. To the normalised detection rate phase distribution, we fit a von Mises distribution and extract the peak activity phase and activity width. These quantities, as a function of frequency, are then modelled as power-laws to construct the chromatic model. For both sources, the activity window starts earlier at higher frequencies. However, FRB 20121102A shows an activity window broadening at higher frequencies, while FRB 20180916B broadens at lower frequencies. Interestingly, it appears to remain active during at least 80% of the cycle at C-band. The observed chromatic behaviour of FRB 20180916B is consistent with previous findings. For FRB 20121102A, a chromaticity in its activity window is also seen; however, the source appears to be active for longer at higher frequencies, opposite to the behaviour of FRB 20180916B.

Figures (11)

• Figure 1: Folded burst rate distribution for FRB 20121102A at L-band (top) and for FRB 20180916B at P-band (bottom). Bursts are symbol-coded according to the observing radio telescope. The burst rate has been scaled to a reference 100-m telescope using \ref{['eq:rates']}. A full list of the telescopes contributing to the L-band and P-band data can be found in \ref{['sec:data_compi']}.
• Figure 2: Von Mises CDF fit (purple) to the empirical CDF (red) of FRB 20121102A at 1360MHz. The fit is centred at a phase $\mu = 0.5$ and shaped by $\kappa = 2.8$
• Figure 3: Von Misses CDF fit (purple) to the empirical CDF (red) of FRB 20180916B at 600MHz. The fit is centred at a phase $\mu = 0.48$ and shaped by $\kappa = 6.4$.
• Figure 4: Modelled active window of FRB 20121102A at 1360MHz. The curve is centred at a phase $\mu = 0.47$ and shaped by $\kappa = 4.28$. The dashed-dotted blue limits indicate the FWHM, while the dashed red limits mark the activity window at $99.7%$ confidence level. Observations are shown with blue points with their respective $3\sigma$ uncertainties
• Figure 5: Modelled active window of FRB 20180916B at 600MHz. The curve is centred at a phase $\mu = 0.47$ and shaped by $\kappa = 9.8$. The dashed-dotted blue limits indicate the FWHM, while the dashed red limits mark the activity window at $99.7%$ confidence level. Observations are shown with blue dots with their respective $3\sigma$ uncertainties