Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?. (arXiv:1808.08492v2 [astro-ph.HE] UPDATED)

Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?. (arXiv:1808.08492v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Kuin_N/0/1/0/all/0/1">N. Paul M. Kuin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wu_K/0/1/0/all/0/1">Kinwah Wu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Oates_S/0/1/0/all/0/1">Samantha Oates</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lien_A/0/1/0/all/0/1">Amy Lien</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Emery_S/0/1/0/all/0/1">Sam Emery</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kennea_J/0/1/0/all/0/1">Jamie Kennea</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pasquale_M/0/1/0/all/0/1">Massimiliano de Pasquale</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Han_Q/0/1/0/all/0/1">Qin Han</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brown_P/0/1/0/all/0/1">Peter J. Brown</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tohuvavohu_A/0/1/0/all/0/1">Aaron Tohuvavohu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Breeveld_A/0/1/0/all/0/1">Alice Breeveld</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Burrows_D/0/1/0/all/0/1">David N. Burrows</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cenko_S/0/1/0/all/0/1">S. Bradley Cenko</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Campana_S/0/1/0/all/0/1">Sergio Campana</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Levan_A/0/1/0/all/0/1">Andrew Levan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Markwardt_C/0/1/0/all/0/1">Craig Markwardt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Osborne_J/0/1/0/all/0/1">Julian P. Osborne</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Page_M/0/1/0/all/0/1">Mat J. Page</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Page_K/0/1/0/all/0/1">Kim L. Page</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sbarufatti_B/0/1/0/all/0/1">Boris Sbarufatti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siegel_M/0/1/0/all/0/1">Michael Siegel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Troja_E/0/1/0/all/0/1">Eleonora Troja</a>

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.

http://arxiv.org/icons/sfx.gif