Evidence for a Damped Millisecond Quasi-Periodic Structure in a Fast Radio Burst

TL;DR

This work reports the first strong indication of an exponentially damped millisecond quasi-periodic oscillation in a non-repeating FRB, FRB 20190122C, using CHIME/FRB baseband data. Through Gaussian pulse modeling, precise peak-timing, and Monte Carlo significance tests, the burst exhibits eight sub-pulses separated by $P_{QPO} \,\approx\, 0.994$ ms and an exponential amplitude decay with scale $\tau_{amp} \,\approx\ 2.24$ (in pulses), yielding a QPO near $f_{QPO} \,\approx\ 10^{3}$ Hz with $Q \approx 7$. Interpreting this as damped magnetospheric Alfvén-mode oscillations in a magnetar-like neutron star suggests $B \approx 2 \times 10^{12}$ G and a potential spin period of $P_{rot} \approx 1$ s, while merger-driven scenarios are disfavored by the lack of frequency drift and the decaying envelope. The result supports magnetar- or young neutron-star-based models for at least some non-repeating FRBs and demonstrates that coherent oscillatory dynamics can imprint on FRB radio signals.

Abstract

Fast radio bursts (FRBs) are millisecond-duration transients of unknown origin, likely associated with compact astrophysical objects. We report evidence for a damped millisecond quasi-periodic structure in a non-repeat FRB~20190122C. The burst consists of eight closely spaced radio pulses separated by $\sim$1 ms, with pulse amplitudes exhibiting an exponential decay starting from the brightest component. Combined Gaussian fitting and time-series analysis reveal a quasi-periodic oscillation (QPO) at $\sim$1 kHz. The observed QPO is consistent with damped magnetospheric oscillations. Assuming an Alfvén wave origin, we estimate a surface magnetic field of $\sim 10^{12}$ G and a characteristic spin period of $\sim$1 s, favoring a low-field magnetar or young neutron star scenario. The absence of frequency drift and the presence of exponential damping disfavor a merger-driven origin. These results suggest the first detection of an exponentially decaying QPO in any FRB, marking a rare detection of coherent oscillatory behavior in FRBs.

Figures (7)

• Figure Figure. 1: Temporal profile of FRB 20190122C (black points) with Gaussian component fitting (red solid line). The burst is modeled as a sum of multiple Gaussian pulses, each represented by a red-dotted curve. Vertical dashed black lines indicate the location of individual pulse peaks.
• Figure Figure. 2: Linear fit to the arrival times of the eight pulse components in the burst. The best-fit model is shown in red with the fitted relation $t_n = T_0 + n P_{\mathrm{QPO}}$, where $P_{\mathrm{QPO}} = ({\text{0.994}} \pm {\text{0.014}})$ ms. The lower panel displays the residuals between the observed and fitted times.
• Figure Figure. 3: Exponential decay of pulse amplitudes. The black circles with error bars denote the measured amplitudes of the eight pulses. A subset of the pulses (from the third to the eighth, i.e., $n = 2$ to $n = 7$) is fitted with an exponential decay function $A(n) = A_0 \exp(-n/\tau)$, shown as the red curve. The best-fit parameters are $A_0 = 105.83 \pm 3.80$ and $\tau = 2.24 \pm 0.06$.
• Figure Figure. 4: Reported quasi-periodic structures (or bursts) in fast radio bursts pastor2023fast2021ApJ...919L...6Mchime2020periodicbochenek2020fast2020Natur.587...54Crajwade2020possible. The horizontal axis shows the characteristic period or quasi-period in seconds; the vertical axis indicates the number of resolved pulses or sub-bursts. Marker colour encodes the reported significance of periodicity (redder implies higher), using a linear colour scale. Stars denote FRB 20190122C (this work), hollow circles represent non-repeat FRBs, while solid circles mark repeat ones. FRB 20190122C stands out as the only source exhibiting a quasi-periodic structure with clear exponential damping, featuring eight pulses at millisecond intervals.
• Figure Extended Data Figure. 1: Reduced-$\chi^2$ values for fits with different numbers of pulses. The x-axis represents the number of pulses used in the fit, and the y-axis shows the corresponding reduced $\chi^2$ values on a logarithmic scale. The red dashed vertical line at 8 pulses marks the optimal number of pulses, where further increases in the number of pulses do not significantly improve the fit quality. This suggests that 8 pulses provide a balance between model complexity and fit accuracy.