The nascent milliquasar VT J154843.06+220812.6: tidal disruption event
  or extreme accretion-state change?
  We present detailed multiwavelength follow up of a nuclear radio flare, VT
J154843.06+220812.6, hereafter VT J1548. VT J1548 was selected as a ${\sim}1$
mJy radio flare in 3 GHz observations from the VLA Sky Survey (VLASS). It is
located in the nucleus of a low mass ($\log M_{\rm BH}/M_\odot \sim6$) host
galaxy with weak or no past AGN activity. VT J1548 is associated with a slow
rising (multiple year), bright mid IR flare in the WISE survey, peaking at
${\sim}10\%L_{\rm edd.}$. No associated optical transient is detected, although
we cannot rule out a short, early optical flare given the limited data
available. Constant late time (${\sim}3$ years post-flare) X-ray emission is
detected at ${\sim}10^{42}$ erg s$^{-1}$. The radio SED is consistent with
synchrotron emission from an outflow incident on an asymmetric medium. A
follow-up, optical spectrum shows transient, bright, high-ionization coronal
line emission ($[{\rm Fe\,X}]\,\lambda 6375,[{\rm Fe\,XI}]\,\lambda 7894,[{\rm
S\,XII}]\,\lambda 7612$). Transient broad H$\alpha$ is also detected but
without corresponding broad H$\beta$ emission, suggesting high nuclear
extinction. We interpret this event as either a tidal disruption event or an
extreme flare of an active galactic nucleus, in both cases obscured by a dusty
torus. Although these individual properties have been observed in previous
transients, the combination is unprecedented. This event highlights the
importance of searches across all wave bands for assembling a sample of nuclear
flares that spans the range of observable properties and possible triggers.
