Linear and circular polarimetry of the optically bright relativistic tidal disruption event AT 2022cmc

Tidal disruption events (TDEs) occur when a star orbiting a massive black hole is sufficiently close to being tidally ripped apart by the black hole. AT 2022cmc is the first relativistic TDE that was observed (and discovered) as an optically bright and fast transient, showing signatures of nonthermal radiation induced by a jet that is oriented toward the Earth. In this work, we present optical linear and circular polarization measurements, observed with the Very Large Telescope/FORS2 in the R band (which corresponds to the blue/UV part of the spectrum in the rest frame), ∼7.2 and ∼12.2 rest-frame days after the first detection, respectively, when the light curve of the transient had settled in a bright blue plateau. Both linear and circular polarizations are consistent with zero, plin = 0.14% ± 0.73%, and pcir = −0.30% ± 0.53%. This is the highest signal-to-noise ratio linear polarization measurement obtained for a relativistic TDE and the first circular polarimetry for such a transient. The nondetection of the linear and circular polarizations is consistent with the scenario of AT 2022cmc being a TDE where the thermal component (disk+outflows) is viewed pole-on, assuming an axially symmetric geometry. The presence and effect of a jet and/or external shocks are, however, difficult to disentangle.