  Including millimeter-wave (mm-wave) data in multi-wavelength studies of the
variability of active galactic nuclei (AGN) can provide insights into AGN
physics that are not easily accessible at other wavelengths. We demonstrate in
this work the potential of cosmic microwave background (CMB) telescopes to
provide long-term, high-cadence mm-wave AGN monitoring over large fractions of
sky. We report on a pilot study using data from the SPTpol instrument on the
South Pole Telescope (SPT), which was designed to observe the CMB at arcminute
and larger angular scales. Between 2013 and 2016, SPTpol was used primarily to
observe a single 500 deg^2 field, covering the entire field several times per
day with detectors sensitive to radiation in bands centered at 95 and 150 GHz.
We use SPT 150 GHz observations to create AGN light curves, and we compare
these mm-wave light curves to those at other wavelengths, in particular
gamma-ray and optical. In this Letter, we focus on a single source, PKS
2326-502, which has extensive, day-timescale monitoring data in gamma-ray,
optical, and now mm-wave between 2013 and 2016. We find PKS 2326-502 to be in a
flaring state in the first two years of this monitoring, and we present a
search for evidence of correlated variability between mm-wave, optical R band,
and gamma-ray observations. This pilot study is paving the way for AGN
monitoring with current and upcoming CMB experiments such as SPT-3G, Simons
Observatory, and CMB-S4, including multi-wavelength studies with facilities
such as VRO-LSST.
