On the possibility of GW190425 being a black hole–neutron star binary merger. (arXiv:2001.04474v1 [astro-ph.HE])

On the possibility of GW190425 being a black hole–neutron star binary merger. (arXiv:2001.04474v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kyutoku_K/0/1/0/all/0/1">Koutarou Kyutoku</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fujibayashi_S/0/1/0/all/0/1">Sho Fujibayashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hayashi_K/0/1/0/all/0/1">Kota Hayashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kawaguchi_K/0/1/0/all/0/1">Kyohei Kawaguchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kiuchi_K/0/1/0/all/0/1">Kenta Kiuchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shibata_M/0/1/0/all/0/1">Masaru Shibata</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tanaka_M/0/1/0/all/0/1">Masaomi Tanaka</a>

We argue that the kilonova/macronova associated with a gravitational-wave
event GW190425 could have been bright enough to be detected if it was caused by
the merger of a low-mass black hole and a neutron star. Although tidal
disruption occurs for such a low-mass black hole as is generally expected, the
masses of the dynamical ejecta are limited to <~10^{-3}M_sun, consistently with previous work in the literature. The remnant disk could be as massive as 0.05--0.1M_sun, and the disk outflow of ~0.01--0.03M_sun is likely to be driven by the viscous or magnetohydrodynamic effects. The disk outflow may not be neutron rich enough to synthesize abundance of lanthanide elements even in the absence of strong neutrino emitter if the ejection is driven on the viscous time scale of >~0.3s. If this is the case, the opacity of the disk outflow is
kept moderate and the kilonova/macronova at the distance of GW190425 reaches
detectable 20–21mag at 1day after merger for most viewing angles. If some disk
activity ejects the mass within ~0.1s, instead, lanthanide-rich outflows would
be launched and the detection of emission becomes challenging. Future possible
detections of kilonovae/macronovae from GW190425-like systems will disfavor
prompt collapse of binary neutron stars and a non-disruptive low-mass black
hole–neutron star binary associated with a neutron-star small radius, whose
mass ejection is negligible. The host-galaxy distance will constrain the
viewing angle and deliver further information about the mass ejection.

We argue that the kilonova/macronova associated with a gravitational-wave
event GW190425 could have been bright enough to be detected if it was caused by
the merger of a low-mass black hole and a neutron star. Although tidal
disruption occurs for such a low-mass black hole as is generally expected, the
masses of the dynamical ejecta are limited to <~10^{-3}M_sun, consistently with
previous work in the literature. The remnant disk could be as massive as
0.05–0.1M_sun, and the disk outflow of ~0.01–0.03M_sun is likely to be driven
by the viscous or magnetohydrodynamic effects. The disk outflow may not be
neutron rich enough to synthesize abundance of lanthanide elements even in the
absence of strong neutrino emitter if the ejection is driven on the viscous
time scale of >~0.3s. If this is the case, the opacity of the disk outflow is
kept moderate and the kilonova/macronova at the distance of GW190425 reaches
detectable 20–21mag at 1day after merger for most viewing angles. If some disk
activity ejects the mass within ~0.1s, instead, lanthanide-rich outflows would
be launched and the detection of emission becomes challenging. Future possible
detections of kilonovae/macronovae from GW190425-like systems will disfavor
prompt collapse of binary neutron stars and a non-disruptive low-mass black
hole–neutron star binary associated with a neutron-star small radius, whose
mass ejection is negligible. The host-galaxy distance will constrain the
viewing angle and deliver further information about the mass ejection.

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