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Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?

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journal contribution
posted on 30.04.2019, 13:27 by NPM Kuin, K Wu, S Oates, A Lien, S Emery, J Kennea, MD Pasquale, Q Han, PJ Brown, A Tohuvavohu, A Breeveld, DN Burrows, SB Cenko, S Campana, A Levan, C Markwardt, JP Osborne, MJ Page, KL Page, B Sbarufatti, M Siegel, E Troja
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 gamma-ray emission continuing for at least 8 days, 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 - 0.4 Msun Helium White Dwarf (WD) by a 100,000 to one million solar mass Black Hole (BH) located in the outskirts of galaxy Z~137-068 could provide an explanation for most of the characteristics shown in the multi-wavelength 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.

Funding

We acknowledge the efforts of the Swift planners. Swift and NuSTAR Data were retrieved from the Swift and NuSTAR archive at HEASARC/GSFC, and from the UK Swift Science Data Centre. We also used the CFHT archive hosted at the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency, and the WFCAM UKIRT data from the UKIDSSDR10PLUS data release from the WFCAM archive at the Royal Observatory Edinburgh. This work has been supported by the UK Space Agency under grant ST/P002323/1 and the UK Science and Technology Facilities Council under grant ST/N00811/1. QH is supported by a UCL MSSL Summer Research Studentship. SRO gratefully acknowledges the support of the Leverhulme Trust Early Career Fellowship. SC acknowledges the support of under ASI-INAF contract I/004/11/1.

History

Citation

Monthly Notices of the Royal Astronomical Society, 2019, stz053

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

AM (Accepted Manuscript)

Published in

Monthly Notices of the Royal Astronomical Society

Publisher

Oxford University Press (OUP), Royal Astronomical Society

eissn

1365-2966

Copyright date

2019

Available date

30/04/2019

Publisher version

https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stz053/5287985

Language

en