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Very long baseline interferometric (VLBI) localisations of repeating fastradio bursts (FRBs) have demonstrated a diversity of local environments: fromnearby star-forming regions to globular clusters. Here we report the VLBIlocalisation of FRB 20201124A using an ad-hoc array of dishes that alsoparticipate in the European VLBI Network (EVN). In our campaign, we detected 18total bursts from FRB 20201124A at two separate epochs. By combining thevisibilities from both observing epochs, we were able to localise FRB 20201124Awith a 1-$\sigma$ error of 4.5 milliarcseconds (mas). We use the relativelylarge burst sample to investigate astrometric accuracy, and find that for$\gtrsim20$ baselines ($\gtrsim7$ dishes) that we can robustly reachmilliarcsecond precision even using single-burst data sets. Sub-arcsecondprecision is still possible for single bursts, even when only $\sim$ sixbaselines (four dishes) are available. We explore two methods for determiningthe individual burst positions: the peaks of the dirty maps and a Gaussian fitto the cross fringe pattern on the dirty maps. We found the latter to be morereliable due to the lower mean and standard deviation in the offsets from theFRB position. Our VLBI work places FRB 20201124A 705$\pm$26 mas (1-$\sigma$errors) from the optical centre of the host galaxy, and consistent withoriginating from within the recently-discovered extended radio structureassociated with star-formation in the host galaxy. Future high-resolutionoptical observations, e.g. with Hubble Space Telescope, can determine theproximity of our FRB 20201124A VLBI position to nearby knots of star formation.

Milliarcsecond localisation of the repeating FRB 20201124A