MeerKAT discovery of a hyperactive repeating fast radio burst source

We present the discovery and localization 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 2 min in the MeerKAT L band (856–1712 MHz). We conducted follow-up observations of FRB 20240619D with MeerKAT using the Ultra-High Frequency (UHF; $544\!-\!1088$ MHz), L-band and S-band (1968–2843 MHz) 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 ~ 1 Jy ms, above which the cumulative burst rate follows a power law R (>F)∝ (F/1\, Jy\, ms)^γ with γ =-1.6± 0.1 and -1.7± 0.1 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× 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 ~ 100~\ per\ cent linear polarization fractions and ~ 10\!-\!20~\ per\ cent circular polarization 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 0.76 Jy ms and an optical-to-radio fluence ratio limit of 0.034 for a 15 s exposure.