{
  "id": "2208.12452",
  "version": "v1",
  "published": "2022-08-26T06:19:40.000Z",
  "updated": "2022-08-26T06:19:40.000Z",
  "title": "Deciphering the extreme X-ray variability of the nuclear transient eRASSt J045650.3-203750: A likely repeating partial tidal disruption event",
  "authors": [
    "Zhu Liu",
    "A. Malyali",
    "M. Krumpe",
    "D. Homan",
    "A. J. Goodwin",
    "I. Grotova",
    "A. Kawka",
    "A. Rau",
    "A. Merloni",
    "G. E. Anderson",
    "J. C. A. Miller-Jones",
    "A. G. Markowitz",
    "S. Ciroi",
    "F. Di Mille",
    "M. Schramm",
    "Shenli Tang",
    "D. A. H. Buckley",
    "M. Gromadzki",
    "Chichuan Jin",
    "J. Buchner"
  ],
  "comment": "26 pages, 12 figures, submitted to A&A, comments are welcome",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "(Abridged) In this paper, we present the results of an exceptional repeating X-ray nuclear transient, eRASSt J045650.3-203750 (hereafter J0456-20), uncovered by SRG/eROSITA in a quiescent galaxy at redshift of z~0.077. The main results are: 1) J0456-20 cycles through four distinctive phases: an X-ray rising phase leading into an X-ray plateau phase which lasts for ~2 months. This is terminated by a rapid X-ray flux drop phase during which the X-ray flux can drastically drop by more than a factor of 100 within 1 week followed by an X-ray faint state for about two months before it starts the X-ray rising phase again; 2) the X-ray spectra are generally soft in the rising phase with a photon index >3.0, and become harder as the X-ray flux increases. There is evidence of a multi-colour disk with inner region temperature of $T_\\text{in}=70$ eV at the beginning of the X-ray rising phase. The high quality XMM-Newton data suggest that a warm and hot corona could be responsible for the X-ray emission, through inverse Comptonisation of soft disk seed photons, during the plateau phase and at the bright end of the rising phase; 3) J0456-20 shows only moderate UV variability and no significant optical variability; 4) radio emission is only detected (as yet) in the X-ray plateau phase, and shows a rapid decline on a time-scale of 2 weeks. We conclude that J0456-20 is likely a repeating nuclear transient with a tentative recurrence time of ~223 days. We discuss several possibilities to explain J0456-20's observational properties, and currently favour a repeating partial tidal disruption event (TDE) as the most likely scenario. The long-term X-ray evolution is explained as a transition between a thermal disk-dominated soft state and a steep power-law state, implying that the corona can be formed within a few months and destroyed within a few weeks.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2022-08-26T06:19:40.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "repeating partial tidal disruption event",
      "nuclear transient erasst j045650",
      "extreme x-ray variability",
      "rising phase",
      "x-ray flux drop phase"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 26,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}