The Potential of Detecting Radio-flaring Ultracool Dwarfs at L band in the FAST Drift-scan Survey

TL;DR

This paper predicts the detection of approximately 170 radio-flaring ultracool dwarfs by the FAST CRAFTS survey, highlighting the survey's sensitivity, the importance of spectral index assumptions, and implications for understanding UCD magnetic activity.

Contribution

The study provides the first detailed prediction of UCD flare detections in the L band with FAST CRAFTS, incorporating a flare luminosity function and Monte Carlo simulations.

Findings

CRAFTS can detect UCD flares up to 180 pc.

Detection numbers vary from 20 to 3460 depending on spectral index.

Approximately 170 flaring UCDs are expected to be detected.

Abstract

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) completed its commissioning and began the Commensal Radio Astronomy FasT Survey (CRAFTS), a multi-year survey to cover 60% of the sky, in 2020. We present predictions for the number of radio-flaring ultracool dwarfs (UCDs) that are likely to be detected by CRAFTS. Based on the observed flaring UCDs from a number of unbiased, targeted radio surveys in the literature, we derive a detection rate of $\ge$3%. Assuming a flat radio spectrum $\nu L _{\nu}\propto \nu^{\beta+1} $ with $\beta$ = -1.0 for UCD flares, we construct a flare luminosity function $d N/d L \propto L^{-1.96 \pm 0.45}$ (here $L=\nu L_\nu$). CRAFTS is found to be sensitive enough for flares from UCDs up to $\sim$180 pc. Considering the Galactic thin disk, we carry out a 3D Monte Carlo simulation of the UCD population, which is then fed to mock CRAFTS observations. We estimate that $\sim$170 flaring UCDs would be detected through transient searches in circular polarization. Though only marginally sensitive to the scale height of UCDs, the results are very sensitive to the assumed spectral index $\beta$. For $\beta$ from 0 to -2.5, the number of expected detections increases dramatically from $\sim$20 to $\sim$3460. We also contemplate the strategies for following up candidates of flaring UCDs, and discuss the implications of survey results for improving our knowledge of UCD behavior at $L$ band and dynamos.