Contemporaneous optical-radio observations of a fast radio burst in a close galaxy pair

We present the MeerKAT discovery and MeerLICHT contemporaneous optical observations of the fast radio burst (FRB) 20230808F, which was found to have a dispersion measure of DM = 653.2 ± 0.4 pc cm⁻³. FRB 20230808F has a scattering time-scale τ_s = 3.1 ± 0.1 ms at 1563.6 MHz, a rotation measure RM = 169.4 ± 0.2 rad m⁻², and a radio fluence F_radio = 1.72 ± 0.01 Jy ms. We find no optical counterpart in the time immediately after the FRB, nor in the 3 months after the FRB during which we continued to monitor the field of the FRB. We set an optical upper flux limit in MeerLICHT’s q-band of 11.7 μJy for a 60 s exposure which started ∼3.4 s after the burst, which corresponds to an optical fluence, F_opt, of 0.039 Jy ms on a time-scale of ∼3.4 s. We obtain an estimate for the q−band luminosity limit of vL_v ∼ 1.3 × 10⁴³ erg s⁻¹. We localize the burst to a close galaxy pair at a redshift of z_spec = 0.3472 ± 0.0002. Our time delay of ∼3.4 s between the FRB arrival time and the start of our optical exposure is the shortest ever for an as yet non-repeating FRB, and hence the closest to simultaneous optical follow-up that exists for such an FRB.