Simultaneous photometric, spectroscopic, and FAST radio observations of AD Leo

Aims. AD Leo is a magnetically active M-dwarf star, well known for its frequent and intense flare activity. These flares manifest across a broad range of wavelengths, from radio to optical: radio observations probe non-thermal processes in the magnetosphere, while optical photometry and spectroscopy trace the responses from the chromosphere and photosphere. However, multi-wavelength coordinated observations of AD Leo remain scarce. In this study, we conducted simultaneous multi-band observations of AD Leo, including radio and optical light curves as well as spectroscopic data, to investigate the relationship between chromospheric activity and radio emission during flare events. Methods. The Five-hundred-metre Aperture Spherical radio Telescope (FAST) data cover the 1-1.5 GHz frequency range, and dynamic spectra were used to identify radio signals from AD Leo. Optical spectroscopic observations span the 3900-9800,Å wavelength range, and chromospheric activity was analysed using key diagnostic lines such as Hα, Hβ, Hγ, and He I D3. For the photometric data, flare events were detected and characterized by applying a repeated fitting method to the light curves obtained from Transiting Exoplanet Survey Satellite (TESS) and the 85 cm telescope. Results. We detected four radio burst events with FAST during two observing epochs, characterized by high circular polarization, narrow bandwidths, and millisecond to sub-millisecond fine structures. The polarization handedness is stable within individual events but differs between epochs. Optical spectroscopy reveals 16 flare events with significant enhancements in multiple chromospheric lines, with Hα flare energies of ∼ 9.97 -6.27 29 Optical photometry identifies 32 flares from the 85 cm telescope, with flare energies ranging from to 10^33.55,erg and a flare frequency distribution slope of α ≈ -1.02, and 144 flares from TESS, spanning bolometric energies of with a slope of α ≈ -0.94. For the 2025 January 12 event, no reliable optical counterpart could be established because simultaneous 85 cm photometry was unavailable and the spectroscopic observations did not cover the radio-burst interval. On 2025 February 06, one radio burst temporally overlapped the decay phase of a small optical flare and enhanced chromospheric emission, whereas the other radio bursts occurred later. These observations therefore suggest a possible, but not definitive, temporal association between the radio and optical activity.