An asymmetric electron-scattering photosphere around optical tidal disruption events

Giorgos Leloudas*, Mattia Bulla, Aleksandar Cikota, Lixin Dai, Lars L. Thomsen, Justyn R. Maund, Panos Charalampopoulos, Nathaniel Roth, Iair Arcavi, Katie Auchettl, Daniele B. Malesani, Matt Nicholl, Enrico Ramirez-Ruiz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

A star crossing the tidal radius of a supermassive black hole will be spectacularly ripped apart with an accompanying burst of radiation. A few tens of such tidal disruption events have now been identified in optical wavelengths, but the exact origin of the strong optical emission remains inconclusive. Here we report polarimetric observations of three tidal disruption events. The continuum polarization appears independent of wavelength, while emission lines are partially depolarized. These signatures are consistent with photons being scattered and polarized in an envelope of free electrons. An almost axisymmetric photosphere viewed from different angles is in broad agreement with the data, but there is also evidence for deviations from axial symmetry before the peak of the flare and significant time evolution at early times, compatible with the rapid formation of an accretion disk. By combining a super-Eddington accretion model with a radiative transfer code, we simulate the polarization degree as a function of disk mass and viewing angle and we show that the predicted levels are compatible with the observations for extended reprocessing envelopes of ~1,000 gravitational radii. Spectropolarimetry therefore constitutes a new observational test for tidal disruption event models, and opens an important new line of exploration in the study of tidal disruption events.

Original languageEnglish
Pages (from-to)1193-1202
Number of pages10
JournalNature Astronomy
Volume6
Issue number10
Early online date29 Sep 2022
DOIs
Publication statusPublished - Oct 2022

Bibliographical note

Funding Information:
We thank N. Patat and S. González-Gaitán for discussions concerning instrumental polarization corrections. We acknowledge the use of routines from the FUSS code ( https://github.com/HeloiseS/FUSS ) by H. Stevance. G.L., P.C. and D.B.M. were supported by a research grant (19054) from VILLUM FONDEN. M.B. acknowledges support from the Swedish Research Council (Reg. no. 2020-03330). L.D. and L.L.T. acknowledge support from the Hong Kong RGC (GRF grant HKU27305119 and HKU 17304821) and the NSFC Excellent Young Scientists Fund (HKU 12122309). Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the US Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. I.A. is a CIFAR Azrieli Global Scholar in the Gravity and the Extreme Universe Program and acknowledges support from that programme, from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement number 852097), from the Israel Science Foundation (grant number 2752/19), from the United States–Israel Binational Science Foundation (BSF) and from the Israeli Council for Higher Education Alon Fellowship. Parts of this research were supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. D.B.M. acknowledges support from ERC grant number 725246. M.N. acknowledges funding from the ERC under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 948381) and a Fellowship from the Alan Turing Institute. This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 0102.D-0116(A) and 0103.D-0350(A). This work is based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.

Publisher Copyright:
© 2022, The Author(s), under exclusive licence to Springer Nature Limited.

ASJC Scopus subject areas

  • Astronomy and Astrophysics

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