Starspots as the origin of ultrafast drifting radio bursts from an active M dwarf

Abstract

Detecting coherent radio bursts from nearby M dwarfs provides opportunities for exploring their magnetic activity and interaction with orbiting exoplanets. However, it remains uncertain if the emission is related to flare-like activity similar to the Sun or magnetospheric process akin to magnetized planets. Using observations (1.0 - 1.5 GHz) taken by the Five-hundred-meter Aperture Spherical radio Telescope, we found a type of millisecond-scale radio bursts with exceptionally high frequency drift rates ($\sim 8\;\rm{GHz\;s^{-1}}$) from an active M dwarf, AD Leo. The ultrafast drift rates point to a source region with a notably low magnetic scale height ($<0.15\; r_\star$, $r_\star$ as the stellar radius), a feature not expected in a commonly assumed dipole-like global field but highly possible in localized strong-field structures, i.e. starspots. Our findings suggest that a concentrated magnetic field above starspots could be responsible for some of the most intense radio bursts from M dwarfs, supporting a solar-like electron acceleration mechanism.

Figures (12)

• Figure 1: Stokes V dynamic spectra of the radio bursts from AD Leo. (A) An overview of the event, presented with a resolution of 6.3 ms and 0.49 MHz. Negative values indicate right-hand circularly polarized emission, following the PSR/IEEE convention where the left-hand emission is positive and the right-hand is negative2010PASA...27..104V. Three dashed rectangular frames denote the time ranges of the detailed fine structures shown below. Horizontal blank spaces result from radio-frequency interference (RFI) flagging. (B to D) Examples of the fine structures in the radio emission, presented with a resolution of 0.39 ms and 0.49 MHz. The frequency drift rates of the fine bursts are indicated by the dashed lines with their respective values.
• Figure 2: Magnetic field properties of AD Leo from a flattened polar view based on two input models (A to E: ZDI map, F to J: ZDI map with a pair of starspots). (A) Surface radial magnetic field ($B_r$). Blue (red) color denotes inward (outward) magnetic field. A nonlinear colorbar is used for better illustration: values between -0.5 kG -- 0.5 kG are displayed linearly in colors, while values beyond this range are shown on a logarithmic scale. The latitudes are shown by concentric circles, from outer to inner marking -30$^{\circ}$, 0$^{\circ}$, 30$^{\circ}$ (dashed), and 60$^{\circ}$ (dashed) and the longitudes (45$^{\circ}$, 135$^{\circ}$, 225$^{\circ}$, 315$^{\circ}$) are also indicated. (B, C) Magnetic scale height ($L_B$) at the radial distance (to the center) of $r=1.05\; r_\star$ and $r=1.1\; r_\star$. The possible source regions of the fundamental (second-harmonic) emission are enclosed by the black (green) contour lines, represented as either the gap between the two lines (i.e. the donut shapes in panel B) or an area encircled by the line (i.e. the round shape in panel C). (D, E) Distributions of $L_B$ from the $L_B$ maps. Light-colored histograms represent counts from all pixels in the maps and dark-colored histograms represent counts from pixels within the fundamental and second-harmonic regions. The light grey (dark grey) area marks the $L_B<0.15\; r_\star$ ($L_B\lesssim0.08\;r_\star$) threshold. (F to J) Cases of adding a pair of starspots, shown in the same format. The right-bottom sub-panels in F to H zoom in the starspots within the annulus sectors. The grey dashed lines denote the central axis of the starspots.
• Figure 3: The magnetic scale height distributions for adding starspots of different sizes and at different latitudes.(A) Radial magnetic field incorporating the ZDI map and the starspots. From left to right are the maps without starspots and with starspots of different sizes ($\rho=30^{\circ}, 15^{\circ}, 8^{\circ}$, $\rho$: the longitudinal distance between the centroids of the pair of starspots). The two dashed lines represent $10^{\circ}$ and $70^{\circ}$ latitudes. Starspots are initially placed at $10^{\circ}$ latitude. (B) The magnetic scale height ($L_B$) in a 2D slice along the central axis of the starspots ($10^{\circ}$ latitude). X-axis denotes the longitude in degree and Y-axis denotes the radial distance ($r$) in stellar radius ($r_\star$). The black curves represent $L_B=0.15\;r_\star$ contour lines. (C) Cases of starspots placed at $70^{\circ}$ latitude, shown in the same format as in (B). (D) The $L_B$ variation with radial distance. At each height, the lowest $L_B$ is selected and plotted. The blue curves represent the cases of $10^{\circ}$ latitude starspots and the orange curves represent those at $70^{\circ}$ latitude.
• Figure 4: Drift rate analysis using the 2D FFT diagrams of the dynamic spectra.(A to C) The 2D FFT diagrams of the dynamic spectra shown in Fig. \ref{['fig:figure1']} (B to D). Darker color indicates stronger power. The red dashed lines denote the best-fit slopes of the central-symmetric patterns. (D to F) The corresponding integrated power along the lines that pass through the origin with different slopes. The black dashed curves show the normalized integrated value as a function of the slope and the blue curves show the Gaussian fit. The best-fit peak slopes and the uncertainties are shown as the red dashed lines and the shaded regions.
• Figure 5: The variation of the frequency drift rate of the emission fine structures at a time step of $\sim 1$ s. The shaded regions mark the time periods of Fig. \ref{['fig:figure1']} (B, C, D).