Medium-band observation of the neutrino emitting blazar, TXS 0506+056
  TXS 0506+056 is a blazar that has been recently identified as the counterpart
of the neutrino event IceCube-170922A. Understanding blazar type of TXS
0506+056 is important to constrain the neutrino emission mechanism, but the
blazar nature of TXS 0506+056 is still uncertain. As an attempt to understand
the nature of TXS 0506+056, we report the medium-band observation results of
TXS 0506+056, covering the wavelength range of 0.575 to 1.025 $\mu$m. The use
of the medium-band filters allow us to examine if there were any significant
changes in its spectral shapes over the course of one month and give a better
constraint on the peak frequency of synchrotron radiation with
quasi-simultaneous datasets. The peak frequency is found to be $10^{14.28}$ Hz,
and our analysis shows that TXS 0506+056 is not an outlier from the blazar
sequence. As a way to determine the blazar type, we also analyzed if TXS
0506+056 is bluer-when-brighter (BL Lac type and some flat spectrum radio
quasars, FSRQs) or redder-when-brighter (found only in some FSRQs). Even though
we detect no significant variability in the spectral shape larger than
observational error during our medium-band observation period, the comparison
with a dataset taken at 2012 shows a possible redder-when-brighter behavior of
FSRQs. Our results demonstrate that medium-band observations with small to
moderate-sized telescopes can be an effective way to trace the spectral
evolution of transients such as TXS 0506+056.
