Quasars That Have Transitioned from Radio-quiet to Radio-loud on Decadal
  Timescales Revealed by VLASS and FIRST
  We have performed a search over 3440 deg$^2$ of Epoch 1 (2017-2019) of the
Very Large Array Sky Survey to identify unobscured quasars in the optical ($0.2
< z < 3.2$) and obscured active galactic nuclei (AGN) in the infrared that have
brightened dramatically in the radio over the past one to two decades. These
sources would have been previously classified as "radio-quiet" quasars based on
upper limits from the Faint Images of the Radio Sky at Twenty Centimeters
survey (1993-2011), but they are now consistent with "radio-loud" quasars
($L_{\rm 3\,GHz} = 10^{40 - 42} \,\, {\rm erg} \,{\rm s}^{-1}$). A
quasi-simultaneous, multiband ($\sim1-18$ GHz) follow-up study of 14 sources
with the VLA has revealed compact sources ($<0.1^{\prime \prime}$ or $<1$ kpc)
with peaked radio spectral shapes. The high-amplitude variability over decadal
timescales at 1.5 GHz (100% to $>$2500%), but roughly steady fluxes over a few
months at 3 GHz, are inconsistent with extrinsic variability due to propagation
effects, thus favoring an intrinsic origin. We conclude that our sources are
powerful quasars hosting compact/young jets. This challenges the generally
accepted idea that "radio-loudness" is a property of the quasar/AGN population
that remains fixed on human timescales. Our study suggests that frequent
episodes of short-lived AGN jets that do not necessarily grow to large scales
may be common at high redshift. We speculate that intermittent but powerful
jets on subgalactic scales could interact with the interstellar medium,
possibly driving feedback capable of influencing galaxy evolution.
