Various quantum gravity approaches that extend beyond the Standard Model predict Lorentz invariance and charge-parity-time violation at energies approaching the Planck scale. These models frequently predict a wavelength-dependent speed of light, which would result in time delays between promptly emitted photons at different energies, as well as a wavelength-dependent rotation of the plane of linear polarization for photons resulting from vacuum birefringence. Here, we describe a pilot program with an automated system of small telescopes that can simultaneously conduct high cadence optical photometry and polarimetry of active galactic nuclei (AGN) in multiple passbands. We use these observations as a proof of principle to demonstrate how such data can be used to test various Lorentz violation models, including special cases of the Standard Model extension (SME). In our initial campaign with this system, the Array Photo Polarimeter, we observed two AGN sources, including BL Lacertae at redshift $z=0.069$, and S5 $\mathrm{B}0716+714$ at $z=0.31$. We demonstrate that optical polarimetry with a broadband Luminance filter combined with simultaneous $I_c$-band observations yields SME parameter constraints that are up to $\sim 10$ and $\sim 30$ times more sensitive than with a standard $I_c$-band filter, for SME models with mass dimension $d=5$ and $d=6$, respectively. Using only a small system of telescopes with an effective 0.45-m aperture, we further demonstrate $d=5$ constraints for individual lines of sight that are within a factor of $\sim 1–10$ in sensitivity to comparable constraints from optical polarimetry with a 3.6-m telescope. Such an approach could significantly improve existing SME constraints via a polarimetric all-sky survey of AGN with multiple 1-meter class telescopes.

• Received 24 September 2018

DOI:https://doi.org/10.1103/PhysRevD.99.035045