Spectropolarimetry of the tidal disruption event AT 2019qiz: a quasi-spherical reprocessing layer

Abstract

We present optical spectropolarimetry of the tidal disruption event (TDE) AT 2019qiz on days +0 and +29 relative to maximum brightness. Continuum polarization, which informs the shape of the electron-scattering surface, was found to be consistent with 0 per cent at peak brightness. On day +29, the continuum polarization rose to ∼1 per cent, making this the first reported spectropolarimetric evolution of a TDE. These findings are incompatible with a naked eccentric disc that lacks significant mass outflow. Instead, the spectropolarimetry paints a picture wherein, at maximum brightness, high-frequency emission from the accretion disc is reprocessed into the optical band by a nearly spherical, optically thick, electron-scattering photosphere located far away from the black hole. We estimate the radius of the scattering photosphere to be $\sim 100\rm \, au$ at maximum brightness — significantly larger than the tidal radius ($\sim 1\rm \, au$) and the thermalisation radius ($\sim 30\rm \, au$) where the optical continuum is formed. A month later, as the fallback rate drops and the scattering photosphere recedes, the continuum polarization increases, revealing a moderately aspherical interior. We also see evidence for smaller-scale density variations in the scattering photosphere, inferred from the scatter of the data in the Stokes q − u plane. On day +29, the Hα emission-line peak is depolarized to ∼0.3 per cent (compared to ∼1 per cent continuum polarization), and displays a gradual rise toward the line’s redder wavelengths. This observation indicates the Hα line formed near the electron-scattering radius.

Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event

“…While it likely does not inhibit the formation of a disk, nodal precession -assuming the SMBH has a modest spin -is probably important for its subsequent evolution: over many orbits of the material in the innermost regions of the disk, nodal and apsidal precession, coupled to the (likely) large misalignment angle between the spin axis of the SMBH and the angular momentum of the gas, will cause fluid annuli to precess independently instead of conforming to a smooth, warped disk (Nixon et al 2012;Liska et al 2021). The orbit of the returning star also precesses and leads to a time-dependent feeding angle of the flow, and thus the gas is likely morphologically complex and, we suggest, closer to spherically symmetric than in the form of a traditional disc (see also Patra et al 2022). If we assume that the returning debris stream is cylindrical with cross-sectional radius ∼ R and length a a 2 (M/M ) 2/3 (Cufari et al 2022b), then taking a = 0.01 AU, M = 10 7.7 M , and with 0.05 M contained in the stream (see Section 4),…”

Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event

“…While it likely does not inhibit the formation of a disk, nodal precession-assuming the SMBH has a modest spin-is probably important for its subsequent evolution; over many orbits of the material in the innermost regions of the disk, nodal and apsidal precession, coupled to the (likely) large misalignment angle between the spin axis of the SMBH and the angular momentum of the gas, will cause fluid annuli to precess independently instead of conforming to a smooth, warped disk (Nixon et al 2012;Liska et al 2021). The orbit of the returning star also precesses and leads to a time-dependent feeding angle of the flow; thus, the gas is likely morphologically complex and, we suggest, closer to spherically symmetric than in the form of a traditional disk (see also Patra et al 2022). If we assume that the returning debris stream is cylindrical with a cross-sectional radius of ∼R e and length ( )…”

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

“…They further model the polarization with the super-Eddington accretion model of Dai et al (2018) and the radiative transfer code POSSIS (Bulla 2019), yielding polarization predictions between 0% and 6% for different observing angles, values that are broadly consistent with the observations. A simultaneous study (Patra et al 2022) presented spectral polarimetry of AT 2019qiz, showing zero polarization at peak but increasing with time, one month later. Finally, the study by Liodakis et al (2022a) presented time-varying polarization for AT 2020mot, reaching an extraordinary 25% ± 4% in one epoch.…”

“…Figure 4 shows a comparison of the linear polarization of AT 2022cmc with other transients, including ordinary optical TDEs (Leloudas et al 2022;Liodakis et al 2022b;Patra et al 2022), relativistic (Wiersema et al 2012(Wiersema et al , 2020 and extreme TDEs (ASASSN-15lh; Maund et al 2020), and a sample of GRBs selected from Covino & Gotz (2016) with N > 4 epochs.…”