  We present our discovery of dramatic variability in SDSS J1100+4421 by the
high-cadence transient survey Kiso Supernova Survey (KISS). The source
brightened in the optical by at least a factor of three within about half a
day. Spectroscopic observations suggest that this object is likely a
narrow-line Seyfert 1 galaxy (NLS1) at z=0.840, however with unusually strong
narrow emission lines. The estimated black hole mass of ~ 10^7 Msun implies
bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is
also extremely radio-loud, with a radio loudness parameter of R ~ 4 x 10^2 - 3
x 10^3, which implies the presence of relativistic jets. Rapid and
large-amplitude optical variability of the target, reminiscent of that found in
a few radio- and gamma-ray loud NLS1s, is therefore produced most likely in a
blazar-like core. The 1.4 GHz radio image of the source shows an extended
structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine
NLS1, as suggested here, this radio structure would then be the largest ever
discovered in this type of active galaxies.
