OISTER Optical and Near-Infrared Monitoring Observations of a Peculiar
  Radio-Loud Active Galactic Nucleus SDSS J110006.07+442144.3
  We present monitoring campaign observations at optical and near-infrared
(NIR) wavelengths for a radio-loud active galactic nucleus (AGN) at z=0.840,
SDSS~J110006.07+442144.3 (hereafter, J1100+4421), which was identified during a
flare phase in late February, 2014. The campaigns consist of three intensive
observing runs from the discovery to March, 2015, mostly within the scheme of
the OISTER collaboration. Optical-NIR light curves and simultaneous spectral
energy distributions (SEDs) are obtained. Our measurements show the strongest
brightening in March, 2015. We found that the optical-NIR SEDs of J1100+4421
show an almost steady shape despite the large and rapid intranight variability.
This constant SED shape is confirmed to extend to $\sim5~\mu$m in the observed
frame using the archival WISE data. Given the lack of absorption lines and the
steep power-law spectrum of $\alpha_{\nu}\sim-1.4$, where
$f_{\nu}\propto\nu^{\alpha_{\nu}}$, synchrotron radiation by a relativistic jet
with no or small contributions from the host galaxy and the accretion disk
seems most plausible as an optical-NIR emission mechanism. The steep
optical-NIR spectral shape and the large amplitude of variability are
consistent with this object being a low $\nu_{\rm{peak}}$ jet-dominated AGN. In
addition, sub-arcsec resolution optical imaging data taken with Subaru Hyper
Suprime-Cam does not show a clear extended component and the spatial scales are
significantly smaller than the large extensions detected at radio wavelengths.
The optical spectrum of a possible faint companion galaxy does not show any
emission lines at the same redshift and hence a merging hypothesis for this
AGN-related activity is not supported by our observations.
