Abstract
Mergers of binary neutron stars (NSs) usually result in the formation of a
hypermassive neutron star (HMNS). Whether- and when this remnant collapses to a
black hole (BH) depends primarily on the equation of state and on angular
momentum transport processes, both of which are uncertain. Here we show that
the lifetime of the merger remnant may be directly imprinted in the
radioactively powered kilonova emission following the merger. We employ
axisymmetric, time-dependent hydrodynamic simulations of remnant accretion
disks orbiting a HMNS of variable lifetime, and characterize the effect of this
delay to BH formation on the disk wind ejecta. Our models follow the system
evolution over several seconds, and include the effect of nuclear
recombination, viscous heating, and neutrino irradiation by both the HMNS and
the disk. When BH formation is relatively prompt (< 100 ms), outflows from the
disk are sufficiently neutron rich to form heavy r-process elements with mass
number A > 140, resulting in \~week-long emission with a spectral peak in the
near-infrared (NIR), similar to that produced by the dynamical ejecta. In
contrast, delayed BH formation allows neutrinos from the HMNS to raise the
electron fraction in the polar direction to values such that potentially
Lanthanide-free outflows (A < 140) are generated. The lower opacity would
produce a brighter, bluer, and shorter-lived \~day-long emission (a `blue bump')
prior to the late NIR peak from the dynamical ejecta and equatorial wind. A
long-lived HMNS also increases the ejecta mass significantly compared to the
prompt BH case. Our work motivates efforts to obtain early (\~day) optical
follow-up of mergers detected by Advanced LIGO/Virgo. This new diagnostic of BH
formation should be useful for events with a signal to noise lower than that
required for direct detection of gravitational waveform signatures.
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