The nuclear starburst in Arp 299-A: From the 5.0 GHz VLBI radio
  light-curves to its core-collapse supernova rate
  The nuclear region of the Luminous Infra-red Galaxy Arp 299-A hosts a recent
($\simeq 10$ Myr), intense burst of massive star formation which is expected to
lead to numerous core-collapse supernovae (CCSNe). Previous VLBI observations,
carried out with the EVN at 5.0 GHz and with the VLBA at 2.3 and 8.4 GHz,
resulted in the detection of a large number of compact, bright, non-thermal
sources in a region $\lsim$150 pc in size. We aim at establishing the nature of
all non-thermal, compact components in Arp 299-A, as well as estimating its
core-collapse supernova rate. We use multi-epoch European VLBI Network (EVN)
observations taken at 5.0 GHz to image with milliarcsecond resolution the
compact radio sources in the nuclear region of Arp 299-A. We also use one
single-epoch 5.0 GHz Multi-Element Radio Linked Interferometer Network (MERLIN)
observation to image the extended emission in which the compact radio sources
--traced by our EVN observations-- are embedded. Twenty-six compact sources are
detected, 8 of them are new objects not previously detected. The properties of
all detected objects are consistent with them being a mixed population of CCSNe
and SNRs. We find clear evidence for at least two new CCSNe, implying a lower
limit to the CCSN rate of $\nu_{\rm SN}\gsim$0.80 SN/yr indicating that the
bulk of the current star formation in Arp 299-A is taking place in the
innermost $\sim 150$ pc. Our MERLIN observations trace a region of diffuse,
extended emission which is co-spatial to the region where all compact sources
are found. From this diffuse, non-thermal radio emission we obtain an
independent estimate for the core-collapse supernova rate, which is in the
range $\nu_{\rm SN}=0.40$ - 0.65 SN/yr, roughly in agreement with previous
estimates and our direct estimate of the CCSN rate from the compact radio
emission.
