Linear and Circular Polarimetry of the Optically Bright Relativistic Tidal Disruption Event AT 2022cmc

Abstract

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, p lin = 0.14% ± 0.73%, and p cir = −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.

AT2022cmc: A Tidal Disruption Event with a Two-component Jet in a Bondi-profile Circumnuclear Medium

“…Eight free parameters are considered in the fitting, i.e., the isotropic kinetic energy E K,iso , the initial lorentz factor Γ 0 of the jet, the jet opening angle θ j , the CNM profile parameter k, the number density of the CNM medium n 18 at R = 10 18 cm, the electron distribution power-law index p, and the energy fraction in electrons ò e and in magnetic field ò B . The viewing angle θ obs is fixed to 0°in the fit due to the on-axis jet as suggested by Andreoni et al (2022) and Pasham et al (2023), and also by the nearly zero observed polarization degrees (Cikota et al 2023).…”

“…The low polarization (consistent with zero) favors the jet with low magnetic field energy density (Cikota et al 2023). We therefore use a relatively smaller value of the lower limit for ò B .…”