Radio Burst Phenomenology of AD Leonis and Associated Signatures of Propagation Effects

TL;DR

The paper analyzes high-time-resolution FAST observations of AD Leonis in the 1.0–1.5 GHz range, uncovering three distinct spectro-temporal patterns: broadband modulation lanes, S-burst envelopes, and fine S-burst striae. Through 2D Fourier secondary spectra and auto-correlation analyses, the authors show that the modulation lanes are consistent with propagation and caustics formed by a 1D sinusoidal plasma screen in the star’s magnetosphere, enabling constraints on source-screen geometry and local density fluctuations. S-burst envelopes are treated as intrinsic ECM-driven emissions from fast electrons along magnetic field lines, with drift framed by the parallel magnetic gradient; striae remain enigmatic, potentially requiring either more complex propagation effects or an as-yet-unknown intrinsic mechanism. The findings demonstrate that propagation effects can probe kilometer-scale plasma structures in stellar magnetospheres and motivate future high-time-resolution observations with FAST and next-generation facilities to map magnetospheric density inhomogeneities.

Abstract

We present the high-resolution radio dynamic spectra of AD Leonis (AD Leo) between 1.0 and 1.5 GHz taken by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) on Dec. 1st, 2023. Over a 15-minute period, we identify complex, superimposed spectro-temporal structures, including: (1) broadband, second-long modulation lanes with downward frequency drifts, (2) narrowband ($\approx$ 50 MHz), short-duration S-burst envelopes with upward drifts, and (3) even narrower ($\approx$ 10 MHz), millisecond-scale S-burst striae within these envelopes. Using the discrete Fourier transform and auto-correlation function, we identify two dominant periodic emission patterns, corresponding to the periodicities of the S-bursts ($\approx0.1$ s) and the striae ($\approx0.01$ s). The complex superposition of diverse time-frequency structures poses a challenge to interpreting all the emission variability as intrinsic to the source. We propose that the modulation lanes could be a propagation effect as the radio waves traverse an inhomogeneous, regularly structured plasma region in the AD Leo's magnetosphere. By modelling a plasma screen with sinusoidal phase variation in one dimension, we show that we could qualitatively reconstruct the observed modulation lanes. The origin of the finest structures, the striae, remains unclear. Our work highlights that propagation effects in the stellar magnetosphere can potentially probe kilometre-scale structures in the emission regions and provide novel constraints on density inhomogeneities caused by magnetohydrodynamic waves that are difficult to access by other means.

Figures (13)

• Figure 1: (a) Stokes V radio dynamic spectrum of the 15-minute observation analyzed in this work. Positive values represent left-hand circular polarized emission. The red arrows mark the times of the dynamic spectra in Figure \ref{['fig:figure3']} (a, c, e) and the secondary spectra in Figure \ref{['fig:figure5']} (a, d, g). The orange dashed bars mark the time spans of two major clusters of bursts. (b, c) Zoomed-in dynamic spectra of the two clusters of bursts. The orange bars in panel (c) mark the time spans of the dynamic spectra in Figure \ref{['fig:figure2']} (a, c, e). The blank horizontal gaps represent the corrupted channels with strong RFIs. Examples of the modulation lanes are indicated in panel (b, c).
• Figure 2: (a, c, e) Examples of the modulation lanes in the dynamic spectra. The time and frequency resolutions of the dynamic spectra are $0.39\;\mathrm{ms}$ and $0.49\;\mathrm{MHz}$, respectively. The rectangular diffuse pattern marked by the white horizontal line in panel (a) is an instrumental artifact caused by the 1 s long noise diode signal used for calibration, which overlaps in time with two sweeping modulation lanes. The overall frequency sweep of the modulation lanes is indicated by a white dashed line. (b, d, f) Corresponding smoothed dynamic spectra after applying a low-pass filter. The low-pass filter smooths out structures smaller than 0.03 s in time or 20 MHz in frequency. Details of the low-pass filter are provided in Appendix \ref{['sec:low-pass']}. In panel (b), the artifact related to the noise diode signal is removed after the filter.
• Figure 3: (a, c, e) Examples of S-burst envelopes and striae in the dynamic spectra. The time and frequency resolutions are the same with Figure \ref{['fig:figure2']}. (b, d, f) Corresponding filtered dynamic spectra after applying the same low pass filter with Figure \ref{['fig:figure2']}, which reveal the S-burst envelopes. Panel (g, h) is an example of a series of simple S-bursts, occurring around 2 hours before the events in focus.
• Figure 4: An example of the superposition of modulation lanes, S-bursts and striae in the dynamic spectra. Panels (a) and (b) are the original and low-pass filtered dynamic spectra. The white dashed lines delineate the modulation lanes and the red arrows mark the locations of S-burst envelopes that cross two adjacent modulation lanes. Panel (c) illustrates the topological relationship between the modulation lanes, S-burst envelopes and striae.
• Figure 5: (a, d, g, j) Examples of the secondary spectra based on 8 s long data. The first 1 s long dynamic spectra of the data have been shown in Figure \ref{['fig:figure3']}(a, c, e, g). The grey scale is the signal-to-noise ratio (SNR) of power in linear scale. In panel (a), the red dashed ellipse marks the size of the window of the low-pass filter. The sub-panels are the zoom-in core regions, ranging from -50 Hz to 50 Hz and from -0.03 $\mu$s to 0.03 $\mu$s. The blue (green) cross-hairs indicate the strongest low frequency (high frequency) knots in the Fourier plane. (b, e, h, k) ACF of the corresponding original dynamic spectra. The green dashed lines indicate the high frequency fringe mode, with the period and the drift rate marked in the bottom right corner. These properties are summarized in Table \ref{['tab:table1']}. (c, f, i, l) ACF of the filtered dynamic spectra. The blue dashed lines indicate the low frequency fringe mode.