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An asymmetric electron-scattering photosphere around optical tidal disruption events

Nature Astronomy volume 6, pages 1193–1202 (2022)Cite this article

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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.

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Fig. 1: Spectral polarimetry of optical TDEs.
Fig. 2: Emission line and polarization profiles.
Fig. 3: The Stokes plane.
Fig. 4: The evolution of polarization with time for two TDEs.
Fig. 5: Polarization modelling for a TDE super-Eddington disk.

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Data availability

All raw data are publicly available through the ESO (https://archive.eso.org/) and NOT (http://www.not.iac.es/observing/forms/fitsarchive/) archives. Reduced data are available from the first author on reasonable request.

Code availability

The radiative transfer code POSSIS used in this work is not publicly available. Results presented in this work are available from Mattia Bulla (mattia.bulla@unife.it) upon reasonable request.

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Acknowledgements

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.

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Authors and Affiliations

  1. DTU Space, National Space Institute, Technical University of Denmark, Kgs. Lyngby, Denmark

    Giorgos Leloudas, Panos Charalampopoulos & Daniele B. Malesani

  2. The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, Stockholm, Sweden

    Mattia Bulla

  3. European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Santiago de Chile, Chile

    Aleksandar Cikota

  4. Department of Physics, The University of Hong Kong, Hong Kong, Hong Kong SAR

    Lixin Dai & Lars L. Thomsen

  5. Department of Physics and Astronomy, The University of Sheffield, Sheffield, UK

    Justyn R. Maund

  6. Lawrence Livermore National Laboratory, Livermore, CA, USA

    Nathaniel Roth

  7. The School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel

    Iair Arcavi

  8. CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Ontario, Canada

    Iair Arcavi

  9. School of Physics, The University of Melbourne, Melbourne, Victoria, Australia

    Katie Auchettl

  10. ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Melbourne, Victoria, Australia

    Katie Auchettl

  11. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA

    Katie Auchettl & Enrico Ramirez-Ruiz

  12. Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands

    Daniele B. Malesani

  13. Birmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Birmingham, UK

    Matt Nicholl

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  1. Giorgos Leloudas
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Contributions

G.L. initiated the project, was PI of the observing proposals, reduced all broad-band polarimetry, analysed the data and wrote most of the manuscript. M.B. provided the radiative transfer models and contributed with text and figures. A.C. reduced all spectral polarimetry, helped with the data analysis and contributed text. L.D. initiated the project with G.L., provided the disk models and contributed with text. L.L.T. contributed to the disk modelling and Markov chain Monte Carlo fitting. J.R.M. helped with data analysis and interpretation. P.C. helped with the host correction and figure production. N.R. contributed with theoretical predictions and interpretation. D.B.M. helped with observation coordination and triggered the telescope. All authors contributed to discussions at different stages of the project and provided comments on the manuscript.

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Correspondence to Giorgos Leloudas or Lixin Dai.

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Extended data

Extended Data Fig. 1 Polarization properties as a function of wavelength.

This figure is similar to Fig. 1. The different panels below the flux spectra show the polarization angle θ, the normalized Stokes parameters q, u, and the rotated Stokes parameters (from Extended Data Figure 7). Dashed lines mark the wavelengths of major emission lines: red for Balmer lines, blue for He II and green for N III. All error bars are 1σ uncertainties and Fλ is in units of erg s−1cm−2 Å−1. All data is shown here after correcting for the ISP and for the host dilution.

Extended Data Fig. 2 Host galaxy contamination at the time of the VLT spectropolarimetry.

The host contribution is defined here as the flux ratio α(λ) = Ihost(λ)/Itot(λ) and is computed by dividing a spectrum of the host with the VLT spectrum including the TDE (after proper absolute flux calibration). Solid lines represent smoothed versions for each dataset, using a Savitzky-Golay filter. The location of prominent emission features is marked with vertical dashed lines and regions of significant telluric absorption are shown as shaded regions (their location differs for each TDE due to their different redshifts).

Extended Data Fig. 3 Spectral polarimetry of optical TDEs without the ISP and the host corrections.

This figure is similar to Fig. 1 but the data is shown before applying the ISP and the host galaxy corrections. The polarization appears to increase towards the blue but this is primarily an effect of the host contribution being a strong function of wavelength (Extended Data Figure 2) and it is not an intrinsic property of the TDEs. All error bars are 1σ uncertainties.

Extended Data Fig. 4 Polarization properties as a function of wavelength without the ISP and the host corrections.

This figure is similar to Extended Data Figure 1 but the data is shown before applying the ISP and the host galaxy corrections. All error bars are 1σ uncertainties and Fλ is in units of erg s−1 cm−2 Å−1.

Extended Data Fig. 5 Emission line and polarization profiles without the ISP and the host corrections.

This figure is similar to Fig. 2 but the data is shown before applying the ISP and the host galaxy corrections. All error bars are 1σ uncertainties.

Extended Data Fig. 6 Stokes plane without the ISP and the host corrections.

This figure is similar to Fig. 3 but the data is shown before applying the ISP and the host galaxy corrections. The location of the adopted ISP (see Methods) is highlighted with a black circle. All error bars are 1σ uncertainties.

Extended Data Fig. 7 Rotated Stokes plane.

The data from Fig. 3 is shown after rotating anti-clockwise so that qrot becomes parallel to the dominant axis. As no reliable fit was obtained for AT 2019dsg, this TDE is not shown on the rotated Stokes plane. We note that the pre-maximum data of AT 2018dyb shows a large systematic offset from urot = 0, while the data at + 50 days only shows some scatter around this axis. All error bars are 1σ uncertainties. All data is shown here after correcting for the ISP and for the host dilution.

Extended Data Fig. 8 The impact of a depolarising absorption opacity on the wavelength dependence of polarization.

Panel a: Ratio between absorption and electron scattering opacity, κabs/κes, as a function of wavelength for Model 1 (ES+Abs 1, orange) and Model 2 (ES+Abs 2, green). Panel b: Models fit to the polarization spectrum of AT2018dyb at − 17 days for the pure electron-scattering Model 0 (ES, cyan) and Model 1 (ES+Abs 1, orange) and Model 2 (ES+Abs 2, green) with both electron scattering and absorption opacity. See Methods for a detailed discussion. The fits are restricted to the wavelength ranges 5000 − 6000 and 7050 − 7250 Å (highlighted in black) that are free from strong line features in the flux spectrum. Deviations from the expected constant level in the ES model are due to Monte Carlo noise in the simulations. All error bars are 1σ uncertainties.

Extended Data Fig. 9 Polarization predictions for the TDE disk model.

The q signal is shown as a function of viewing angle θ. Different lines show predictions for different disk masses going from mdisk = 0.01 to mdisk = 0.1 M⊙. We observe that if mdisk decreases with time (as might be expected), then the polarization is also generally expected to decrease for a given viewing angle.

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Supplementary discussion and Fig. 1.

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Leloudas, G., Bulla, M., Cikota, A. et al. An asymmetric electron-scattering photosphere around optical tidal disruption events. Nat Astron 6, 1193–1202 (2022). https://doi.org/10.1038/s41550-022-01767-z

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