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

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 e ﬀ ects 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 3-D 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 o ﬀ set from the black hole, reprocesses the hard emission from the accretion ﬂow into UV and optical bands. We explore two di ﬀ erent cases of peak mass fallback rates ˙ M p ( ∼ 3 M (cid:12) yr − 1 and ∼ 0 . 3 M (cid:12) yr − 1 ) while varying the following geometrical parameters: the distance R int from the black hole (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 / UV photons are generated, and the opening angle ∆ θ that deﬁnes the fraction of the surface of the photosphere that photons are generated on (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 10 / 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 di ﬀ erent 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 depends strongly on i) the optical depths at the central regions (between the emitting photosphere and the intersection point) set by the di ﬀ erent ˙ M p values and ii) the viewing angle. With time, there is a drop in densities and optical depths leading to 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 black hole seems to generally favour 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.