Modeling continuum polarization levels of tidal disruption events based on the collision-induced outflow model

¹ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
e-mail: pngchr@space.dtu.dk
² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
³ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁴ INFN − Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁵ Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark

Received: 19 September 2022
Accepted: 9 December 2022

Abstract

Tidal disruption events (TDEs) have been observed in the optical and ultraviolet (UV) for more than a decade, but the underlying emission mechanism still remains a puzzle. It has been suggested that viewing angle effects could potentially explain their large photometric and spectroscopic diversity. Polarization is indeed sensitive to the viewing angle and the first polarimetry studies of TDEs are now available, calling for a theoretical interpretation. In this study, we model the continuum polarization levels of TDEs using the three-dimensional (3D) Monte Carlo radiative transfer code POSSIS and the collision-induced outflow (CIO) TDE emission scenario, where unbound shocked gas originating from a debris stream intersection point offset from the black hole (BH), reprocesses the hard emission from the accretion flow into UV and optical bands. We explore two different cases of peak mass fallback rates Ṁ_p (∼ 3 M_⊙ yr⁻¹ and ∼ 0.3 M_⊙ yr⁻¹) while varying the following geometrical parameters: the distance R_int from the BH to the intersection point where the stellar debris stream self intersects; the radius of the photosphere around the BH R_ph, on the surface of which the optical and UV photons are generated; and the opening angle Δθ that defines the fraction of the surface of the photosphere on which the photons are generated (anisotropic emission). For the high mass fallback rate case, we find for every viewing angle polarization levels below one (P < 1%) and P < 0.5% for ten out of 12 simulations. The absolute value of polarization reaches its maximum (P_max) for equatorial viewing angles. For the low mass fallback rate case, the model can produce a wide range of polarization levels for different viewing angles and configurations. The maximum value predicted is P ≈ 8.8% and P_max is reached for intermediate viewing angles. We find that the polarization strongly depends on (i) the optical depths at the central regions (between the emitting photosphere and the intersection point) set by the different Ṁ_p values and (ii) the viewing angle. With time, there is a drop in densities and optical depths leading to a general increase in polarization values and P_max, although the opposite trend can be observed for specific viewing angles. Increasing the distance R_int between the intersection point and the BH seems to generally favor higher polarization levels. Finally, by comparing our model predictions to polarization observations of a few TDEs, we attempt to constrain their observed viewing angles and we show that multi-epoch polarimetric observations can become a key factor in constraining the viewing angle of TDEs.