MeerKAT discovery of a hyperactive repeating fast radio burst source

We present the discovery and localisation of a repeating fast radio burst (FRB) source from the MeerTRAP project, a commensal fast radio transient search programme using the MeerKAT telescope. FRB 20240619D was first discovered on 2024 June 19 with three bursts being detected within two minutes in the MeerKAT L-band (856 - 1712MHz). We conducted follow-up observations of FRB 20240619D with MeerKAT using the Ultra-High Frequency (UHF; 544 - 1088MHz), L-band and S-band (1968 - 2843MHz) receivers one week after its discovery, and recorded a total of 249 bursts. The MeerKAT-detected bursts exhibit band-limited emission with an average fractional bandwidth of 0.31, 0.34 and 0.48 in the UHF, L-band and S-band, respectively. We find our observations are complete down to a fluence limit of ~1Jy ms, above which the cumulative burst rate follows a power law $R (>F)\propto (F/1\,\text{Jy}\,\text{ms})^γ$ with $γ=-1.6\pm0.1$ and $-1.7\pm0.1$ in the UHF and L-band, respectively. The near-simultaneous L-band, UHF and S-band observations reveal a frequency dependent burst rate with $3\times$ more bursts being detected in the L-band than in the UHF and S-band, suggesting a spectral turnover in the burst energy distribution of FRB 20240619D. Our polarimetric analysis demonstrates that most of the bursts have $\sim100\%$ linear polarisation fractions and $\sim10\%\text{--}20\%$ circular polarisation fractions. We find no optical counterpart of FRB 20240619D in the MeerLICHT optical observations simultaneous to the radio observations and set a fluence upper limit in MeerLICHT's q-band of 0.76Jy ms and an optical-to-radio fluence ratio limit of 0.034 for a 15s exposure.