We present a systematic analysis of the X-ray emission of a sample of 17 optically selected, X-ray-detected tidal disruption events (TDEs) discovered between 2014 and 2021. The X-ray light curves show a diverse range of temporal behaviors, with most sources not following the expected power-law decline. The X-ray spectra are mostly extremely soft and consistent with thermal emission from the innermost region of an accretion disk, which cools as the accretion rate decreases. Three sources show formation of a hard X-ray corona at late times. The spectral energy distribution shape, probed by the ratio (LBB/LX) between the UV/optical and X-ray, shows a wide range of LBB/LX ∈ (0.5, 3000) at early times and converges to disklike values of LBB/LX ∈ (0.5, 10) at late times. We estimate the fraction of optically discovered TDEs with LX ≥ 1042 erg s−1 to be at least 40% and show that X-ray loudness is independent of black hole mass. We argue that distinct disk formation timescales are unlikely to be able to explain the diverse range of X-ray evolution. We combine our sample with X-ray-discovered ones to construct an X-ray luminosity function, best fit by a broken power law, with a break at LX ≈ 1044 erg s−1. We show that there is no dichotomy between optically and X-ray-selected TDEs; instead, there is a continuum of early-time LBB/LX, at least as wide as LBB/LX ∈ (0.1, 3000), with optical/X-ray surveys selecting preferentially, but not exclusively, from the higher/lower end of the distribution. Our findings are consistent with unification models for the overall TDE population.
A Systematic Analysis of the X-Ray Emission in Optically Selected Tidal Disruption Events: Observational Evidence for the Unification of the Optically and X-Ray-selected Populations
Abstract
Acknowledgement
© 2024. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
M.G. thanks T. Wevers, J. Krolik, P. Jonker, and E. Coughlin for comments, suggestions, and discussions. M.G. and S.G. are supported in part by NASA XMM-Newton grants 80NSS23K0621 and 80NSSC22K0571. Y.Y. acknowledges support by NASA XMM-Newton grant 80NSSC23K0482. E.H. acknowledges support by NASA under award No. 80GSFC21M0002. This work is based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, the University of Warwick, Ruhr University Bochum, and Northwestern University. Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry service was funded under Heising-Simons Foundation grant No. 12540303 (PI: Graham). This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The ATLAS project is primarily funded to search for near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.
Facilities
XMM - Newton X-Ray Multimirror Mission satellite, Swift - Swift Gamma-Ray Burst Mission, eROSITA, PO:1.2 m, OGLE
Software References
astropy (Astropy Collaboration et al. 2013), emcee (Foreman-Mackey et al. 2013), heasoft (Heasarc 2014), matplotlib (Hunter 2007), Prospector (Johnson et al. 2021), scipy (Virtanen et al. 2020), xspec (Arnaud 1996)
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Additional details
- ISSN
- 1538-4357
- National Aeronautics and Space Administration
- 80NSS23K0621
- National Aeronautics and Space Administration
- 80NSSC22K0571
- National Aeronautics and Space Administration
- 80NSSC23K0482
- National Aeronautics and Space Administration
- 80GSFC21M0002
- National Science Foundation
- AST-2034437
- Heising-Simons Foundation
- 12540303
- National Aeronautics and Space Administration
- NN12AR55G
- National Aeronautics and Space Administration
- 80NSSC18K0284
- National Aeronautics and Space Administration
- 80NSSC18K1575
- Caltech groups
- Zwicky Transient Facility