Swift spectra of AT2018cow: A white dwarf tidal disruption event?

N. Paul M. Kuin, Kinwah Wu, Samantha Oates, Amy Lien, Sam Emery, Jamie A. Kennea, Massimiliano De Pasquale, Qin Han, Peter J. Brown, Aaron Tohuvavohu, Alice Breeveld, David N. Burrows, S. Bradley Cenko, Sergio Campana, Andrew Levan, Craig Markwardt, Julian P. Osborne, Mat J. Page, Kim L. Page, Boris SbarufattiMichael Siegel, Eleonora Troja

Research output: Contribution to journalArticlepeer-review

23 Scopus citations

Abstract

The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay, and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint γ-ray emission continuing for at least 8 d, and featureless spectra in the ultraviolet bands - both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1-0.4M helium white dwarf (WD) by a 105-106M black hole located in the outskirts of galaxy Z 137-068 could provide an explanation for most of the characteristics shown in the multiwavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10-150 keV band.

Original languageEnglish (US)
Pages (from-to)2505-2521
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Volume487
Issue number2
DOIs
StatePublished - Aug 1 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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