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Context. The two nuclei of the starburst galaxy Arp220 contain multiplecompact radio sources previously identified as radio supernovae or supernovaremnants. Aims. In order to search for an embedded radio AGN, or other possibleexotic objects, we have carried out a program of VLBI monitoring at 6 cm overthree epochs each separated by four months. Methods. Combining the new datawith existing data at 6 cm and 18 cm (spanning 4 and 12 years respectively) weare able to characterise source flux density variability on a range oftime-scales. Additionally we analyse the variability of sources in shape andposition. Results. We detect rapid (< 4 months) variability in three sources(W7, W26 and W29). These sources show possible superluminal motion (> 4c) ofjet-like features near rapidly varying almost stationary components. Theseenigmatic sources might be associated with an AGN or a highly beamedmicroquasar (i.e. microblazar). Other hypotheses include that the apparentvariability is intrinsic and is produced by neutron star powered centralcomponents within a supernova remnant, by a sequence of several supernovaewithin super star clusters, or is extrinsic and is produced by Galacticinterstellar scintillation of very compact non-varying objects. Conclusions. Amicroquasar/microblazar origin seems to be the best explanation for the natureof the variable sources in Arp220.

Rapid variability of the compact radio sources in Arp220. Evidence for a population of microblazars?