Summary of the First Year of the Space Weather Around Young Suns Program: 900 Hours of Low-frequency Radio and Optical Data Dedicated to Young, Solar-type Stars

Technical Summary of the SWAYS Program First Year Results

Problem and Motivation
Understanding the early evolution of the Sun requires studying young, solar-type stars, which are inferred to have been significantly more magnetically active than the present-day Sun. This heightened activity likely drove stronger stellar winds and more frequent, energetic Coronal Mass Ejections (CMEs), impacting the angular momentum evolution of the star and the environment of early Earth. While methods exist to detect mass motion in stellar coronae (e.g., Doppler spectroscopy, X-ray dimming), they are limited to material relatively close to the stellar surface. Detecting genuine mass escape into the interplanetary medium remains difficult. Low-frequency radio emission (< 1 GHz) offers a unique diagnostic: Type II bursts (associated with CME-driven shocks) and Type III bursts (associated with electron beams) are generated by plasma emission at frequencies scaling with the ambient electron density ($\nu_p \propto \sqrt{n_e}$). Because stellar coronae are often denser and hotter than the Sun's, the plasma frequencies for these events may occur at much lower frequencies or require material to travel significantly further from the star before becoming observable. The Space Weather Around Young Suns (SWAYS) program was established to address these challenges by coordinating low-frequency radio and optical observations to identify plasma emission and the reconnection events responsible for accelerating material.

Methodology
The SWAYS program utilizes a coordinated observing strategy between two instruments:

1. Flarescope: A 0.5 m optical photometer on Palomar Mountain operating in the Sloan $g'$ band (400–550 nm). It employs engineered diffusers and frame-transfer imaging to achieve sub-milli-magnitude precision, capable of detecting flares with bolometric energies $\gtrsim 10^{33}$ erg.
2. OVRO-LWA: The Owens Valley Radio Observatory Long Wavelength Array, a 352-element cross-dipole interferometer operating between 13–87 MHz. It collects both cross-correlated data (for all-sky imaging and burst candidate identification) and beamformed data (high-time-resolution power sums).

The first season (November 2023–June 2024) monitored six young, solar-type stars ($\epsilon$ Eridani, $\chi^1$ Orionis, $\kappa^1$ Ceti, $\pi^1$ Ursa Majoris, EK Draconis, and V889 Herculis), accumulating approximately 900 hours of data with ~70% temporal overlap between the optical and radio instruments.

The analysis focused on a specific superflare event detected on EK Draconis (EK Dra), a highly active G-dwarf often used as a proxy for the young Sun. Following the optical detection, the authors performed a rigorous search for associated radio bursts in the OVRO-LWA data:

• Data Reduction: Cross-correlated data were calibrated (delay, bandpass, imaging) to produce dynamic spectra and light curves.
• Type III Search: Given the expectation that EK Dra's dense corona would slow the drift rate of Type III bursts, the team performed a de-dispersion search across a wide parameter space ($a$ and $\alpha$ values for drift rates) to account for potentially slower, longer-duration bursts compared to solar analogs.
• Type II Search: Due to the unconstrained nature of Type II drift rates (dependent on CME speed and geometry), the team binned data into coarse time resolutions (2–10 min) and frequency bands to search for slowly drifting signatures without de-dispersion.
• Modeling: The authors employed radial, isothermal wind models (Parker and Constant Wind Speed) to estimate the distance a CME or electron beam must travel to reach densities conducive to emission in the 41–87 MHz band. They also modeled magnetic confinement to assess if the CME could escape the stellar magnetic field.

Key Results

1. Optical Detection: Flarescope detected a single, unambiguous superflare on EK Dra on May 24, 2024. The event exhibited a steep rise and exponential decay over ~20 minutes, with a peak amplitude of 0.031 relative to the median flux. The estimated bolometric energy was $E_{flare} \approx 3.95 \times 10^{34}$ erg, consistent with previous superflare observations of this star.
2. Radio Non-Detection: Despite the high energy of the flare and the presence of complementary radio data, no Type II or Type III radio bursts were detected from EK Dra in the 5.5 hours following the flare onset.
   • The 10-second integration sensitivity limit was $\approx 3.2$ Jy ($5\sigma$), corresponding to a solar-equivalent burst of $\sim 1.6 \times 10^{10}$ SFU.
   • De-dispersion searches for Type III bursts and coarse-binning searches for Type II bursts yielded no signals distinguishable from noise.
3. Physical Constraints: The authors investigated several physical explanations for the non-detection:
   • Propagation Delay: Modeling suggests that due to EK Dra's high coronal density ($n_0 \approx 4 \times 10^{10}$ cm$^{-3}$), a CME or electron beam must travel $\gtrsim 10 R_*$ to excite plasma frequencies in the OVRO-LWA band. This implies a delay of several hours between the flare and any potential radio signal.
   • Instability Conditions: The high coronal temperatures ($T_{cor} \approx 10^7$ K) may require electron beams to travel at exceptionally high velocities ($v_b \gg 0.4c$) to develop the bump-on-tail instability required for Type III bursts, or the instability growth rate may be insufficient.
   • Confinement: Magnetic pressure calculations suggest that for certain CME energy partitions, the overlying magnetic field of EK Dra could confine the eruption, preventing mass from escaping to the distances required for low-frequency emission.
   • Brightness Limits: Even if a burst occurred, plasma emission saturation limits and source size broadening could render the flux density below the detection threshold of OVRO-LWA.

Significance and Claims
The paper claims that this first season of SWAYS provides the longest dedicated monitoring program for solar-type stars below 100 MHz and the longest targeted radio monitoring with complementary optical data. The primary significance of the results lies in the non-detection of radio bursts associated with a massive superflare.

The authors argue that this result challenges the simple extrapolation of solar flare-CME scaling laws to young, active stars. Specifically:

• Inhibition of Radio Signatures: The exceptionally hot and dense coronae of active stars may inhibit the development of the plasma instabilities required for Type II and III bursts, or delay their occurrence by hours relative to the flare.
• Mass Loss Implications: The lack of a radio signature for a $\sim 10^{34}$ erg flare suggests that mass loss via CMEs may not contribute as substantially to the overall mass and angular momentum loss of young stars as previously hypothesized, or that CMEs are frequently confined by strong magnetic fields.
• Observational Strategy: The findings highlight a "two-fold problem" for space weather studies: active stars flare frequently but may produce radio signals that are inhibited or delayed, while less active stars may produce detectable bursts but lack the high-energy flares necessary to drive them.

The paper concludes that effective characterization of stellar space weather requires rethinking coordination strategies, potentially moving toward flare-agnostic radio searches and expanding de-dispersion parameter spaces to account for the extreme conditions of young stellar environments.