Radio outburst from a massive (proto)star. When accretion turns into
  ejection
  Context. Recent observations of the massive young stellar object S255 NIRS 3
have revealed a large increase in both methanol maser flux density and IR
emission, which have been interpreted as the result of an accretion outburst,
possibly due to instabilities in a circumstellar disk. This indicates that this
type of accretion event could be common in young/forming early-type stars and
in their lower mass siblings, and supports the idea that accretion onto the
star may occur in a non-continuous way. Aims. As accretion and ejection are
believed to be tightly associated phenomena, we wanted to confirm the accretion
interpretation of the outburst in S255 NIRS 3 by detecting the corresponding
burst of the associated thermal jet. Methods. We monitored the radio continuum
emission from S255 NIRS 3 at four bands using the Karl G. Jansky Very Large
Array. The millimetre continuum emission was also observed with both the
Northern Extended Millimeter Array of IRAM and the Atacama Large
Millimeter/submillimeter Array. Results. We have detected an exponential
increase in the radio flux density from 6 to 45 GHz starting right after July
10, 2016, namely about 13 months after the estimated onset of the IR outburst.
This is the first ever detection of a radio burst associated with an IR
accretion outburst from a young stellar object. The flux density at all
observed centimetre bands can be reproduced with a simple expanding jet model.
At millimetre wavelengths we infer a marginal flux increase with respect to the
literature values and we show this is due to free-free emission from the radio
jet. Abridged.
