ABSTRACT:
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has completed its commissioning and begun the Commensal Radio Astronomy FasT Survey (CRAFTS) in 2020. We present predictions for the number of radio-flaring ultracool dwarfs (UCDs) that are likely to be detected by CRAFTS, which will cover $\sim$23,000 deg$^2$ at its completion. Based on the observed flaring UCDs from a number of targeted radio surveys in the literature, we derived a lower limit of the detection rate to be $\sim$3\%. Assuming a flat radio spectrum $\nu L _{\nu}\propto \nu^{\beta+1} $ with $\beta$ = -1.0 for UCD flares, we constructed a flare luminosity function $d N/d L \propto L^{-1.96 \pm 0.45}$ (here $L=\nu L_\nu$).
CRAFTS was found to be sensitive enough for flares from UCDs up to $\sim$260 pc, comparable to their scale height in the Galactic thin disk. Considering a thin disk, we carried out a 3D Monte Carlo simulation of the UCD population, which was then fed to mock CRAFTS observations. We estimated that $\sim$450 flaring UCDs would be detected through transient searches, and $\sim$330 can also be seen in Stokes $V$, assuming 100\% circularly polarized emission. 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 -2.0, the number of expected detections increases dramatically from $\sim$50 to $\sim$4630. We also contemplated the strategies for following up candidates of flaring UCDs.
