All good things come in threes: the third image of the lensed quasar
  PKS1830-211
  Strong gravitational lensing distorts our view of sources at cosmological
distances but brings invaluable constraints on the mass content of foreground
objects and on the geometry and properties of the Universe. We report the
detection of a third continuum source toward the strongly lensed quasar
PKS1830-211 in ALMA multi-frequency observations of high dynamic range and high
angular resolution. This third source is point-like and located slightly to the
north of the diagonal joining the two main lensed images, A and B, 0.3 arcsec
away from image B. It has a flux density that is ~140 times weaker than images
A and B and a similar spectral index, compatible with synchrotron emission. We
conclude that this source is most likely the expected highly de-magnified third
lensed image of the quasar. In addition, we detect, for the first time at
millimeter wavelengths, weak and asymmetrical extensions departing from images
A and B that correspond to the brightest regions of the Einstein ring seen at
centimeter wavelengths. Their spectral index is steeper than that of compact
images A, B, and C, which suggests that they arise from a different component
of the quasar. Using the GravLens code, we explore the implications of our
findings on the lensing model and propose a simple model that accurately
reproduces our ALMA data and previous VLA observations. With a more precise and
accurate measurement of the time delay between images A and B, the system
PKS1830-211 could help to constrain the Hubble constant to a precision of a few
percent.
