The millisecond-duration radio flashes known as Fast Radio Bursts ( FRBs ) represent an enigmatic astrophysical phenomenon .
Recently , the sub-arcsecond localization ( \sim 100 mas precision ) of FRB 121102 using the VLA 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 \lesssim 40 pc ( \lesssim 12 mas angular separation , at 95 % confidence ) .
We detect consistent angular broadening of the bursts and persistent radio source ( \sim 2 – 4 ~ { } \mathrm { 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 \lesssim 0.7 ~ { } \mathrm { pc } ( \lesssim 0.2 mas angular extent at 5.0 GHz ) and a lower limit for the brightness temperature of T _ { b } \gtrsim 5 \times 10 ^ { 7 } \mathrm { 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 .