Rapid variability of the compact radio sources in Arp220. Evidence for a
  population of microblazars?
  Context. The two nuclei of the starburst galaxy Arp220 contain multiple
compact radio sources previously identified as radio supernovae or supernova
remnants. Aims. In order to search for an embedded radio AGN, or other possible
exotic objects, we have carried out a program of VLBI monitoring at 6 cm over
three epochs each separated by four months. Methods. Combining the new data
with existing data at 6 cm and 18 cm (spanning 4 and 12 years respectively) we
are able to characterise source flux density variability on a range of
time-scales. Additionally we analyse the variability of sources in shape and
position. Results. We detect rapid (< 4 months) variability in three sources
(W7, W26 and W29). These sources show possible superluminal motion (> 4c) of
jet-like features near rapidly varying almost stationary components. These
enigmatic sources might be associated with an AGN or a highly beamed
microquasar (i.e. microblazar). Other hypotheses include that the apparent
variability is intrinsic and is produced by neutron star powered central
components within a supernova remnant, by a sequence of several supernovae
within super star clusters, or is extrinsic and is produced by Galactic
interstellar scintillation of very compact non-varying objects. Conclusions. A
microquasar/microblazar origin seems to be the best explanation for the nature
of the variable sources in Arp220.
