VVV-WIT-04: an extragalactic variable source caught by the VVV Survey
  We report the discovery of VVV-WIT-04, a near-infrared variable source
towards the Galactic disk located ~0.2 arcsec apart from the position of the
radio source PMN J1515-5559. The object was found serendipitously in the
near-IR data of the ESO public survey VISTA Variables in the V\'ia L\'actea
(VVV). Our analysis is based on variability, multicolor, and proper motion data
from VVV and VVV eXtended surveys, complemented with archive data at longer
wavelengths. We suggest that VVV-WIT-04 has an extragalactic origin as the
near-IR counterpart of PMN J1515-5559. The Ks-band light-curve of VVV-WIT-04 is
highly variable and consistent with that of an Optically Violent Variable (OVV)
quasar. The variability in the near-IR can be interpreted as the redshifted
optical variability. Residuals to the proper motion varies with the magnitude
suggesting contamination by a blended source. Alternative scenarios, including
a transient event such as a nova or supernova, or even a binary microlensing
event are not in agreement with the available data.
