[1402.4803] Red or blue? A potential kilonova imprint of the delay until black hole formation following a neutron star merger

[1402.4803] Red or blue? A potential kilonova imprint of the delay until black hole formation following a neutron star merger:



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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