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

Mergers of binary neutron stars 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 discs orbiting an HMNS of variable lifetime, and characterize the effect of this delay to BH formation on the disc wind ejecta. When BH formation is relatively prompt (≲100 ms), outflows from the disc are sufficiently neutron rich to form heavy r-process elements, 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 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. 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.