The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

The millisecond-duration radio flashes known as fast radio bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (similar to 100 mas precision) of FRB. 121102 using the Very Large Array has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here, we report very-long-baseline radio interferometric observations using the European VLBI Network and the 305 m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of less than or similar to 40 pc (less than or similar to 12 mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (similar to 2-4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be less than or similar to 0.7 pc (less than or similar to 0.2 mas angular extent at 5.0 GHz) and a lower limit for the brightness temperature of T-b greater than or similar to 5 x 10(7) K. Together, these observations provide strong evidence for a direct physical link between FRB. 121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB. 121102 that best match the observed data.