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Title: Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs

Abstract

Abstract We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class, AT2018cow, result from an electron-capture collapse to a neutron star following the merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less-massive WD during the merger and the formation of a shell-burning non-degenerate star incorporating the ONeMg core. During the shell-burning stage, a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after ∼102–104 yr. During the formation of a neutron star, as little as $${\sim } 10^{-2} \, \mathrm{M}_\odot$$ of the material is ejected at the bounce-off with mildly relativistic velocities and total energy of about a few 1050 erg. This ejecta becomes optically thin on a time-scale of days – this is the FBOT. During the collapse, the neutron star is spun up and the magnetic field is amplified. The ensuing fast magnetically dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high-energy emission in a manner similar to pulsar wind nebulae. If the secondary WD was of the DA type, the wind will likely have $${\sim } 10^{-4} \, \mathrm{M}_\odot$$ of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high-energy emission of similar luminosity to the optical, and late infrared features.

Authors:
 [1];  [2]
  1. Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907-2036, USA
  2. Anton Pannekoek Institute for Astronomy, University of Amsterdam, PO Box 94249, 1090 GE, Amsterdam, the Netherlands
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1530651
Grant/Contract Number:  
SC0016369
Resource Type:
Published Article
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Name: Monthly Notices of the Royal Astronomical Society Journal Volume: 487 Journal Issue: 4; Journal ID: ISSN 0035-8711
Publisher:
Oxford University Press
Country of Publication:
United Kingdom
Language:
English

Citation Formats

Lyutikov, Maxim, and Toonen, Silvia. Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs. United Kingdom: N. p., 2019. Web. doi:10.1093/mnras/stz1640.
Lyutikov, Maxim, & Toonen, Silvia. Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs. United Kingdom. doi:10.1093/mnras/stz1640.
Lyutikov, Maxim, and Toonen, Silvia. Tue . "Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs". United Kingdom. doi:10.1093/mnras/stz1640.
@article{osti_1530651,
title = {Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs},
author = {Lyutikov, Maxim and Toonen, Silvia},
abstractNote = {Abstract We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class, AT2018cow, result from an electron-capture collapse to a neutron star following the merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less-massive WD during the merger and the formation of a shell-burning non-degenerate star incorporating the ONeMg core. During the shell-burning stage, a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after ∼102–104 yr. During the formation of a neutron star, as little as ${\sim } 10^{-2} \, \mathrm{M}_\odot$ of the material is ejected at the bounce-off with mildly relativistic velocities and total energy of about a few 1050 erg. This ejecta becomes optically thin on a time-scale of days – this is the FBOT. During the collapse, the neutron star is spun up and the magnetic field is amplified. The ensuing fast magnetically dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high-energy emission in a manner similar to pulsar wind nebulae. If the secondary WD was of the DA type, the wind will likely have ${\sim } 10^{-4} \, \mathrm{M}_\odot$ of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high-energy emission of similar luminosity to the optical, and late infrared features.},
doi = {10.1093/mnras/stz1640},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 4,
volume = 487,
place = {United Kingdom},
year = {2019},
month = {7}
}

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