Effects of spin on magnetized binary neutron star mergers and jet launching.

Events GW170817 and GRB 170817A provide the best confirmation so far that binary neutron star (BNS) mergers can be the progenitors of short gamma-ray bursts (sGRBs). An open question for GW170817 remains the spin of the remnant black hole (BH), which may have a strong impact on the formation and lifetime of a jet and the outgoing Poynting luminosity. We report GRMHD simulations of spinning, magnetized, BNSs undergoing merger and delayed collapse. The BNSs consist of two identical NS, modeled as $\Gamma=2$ polytropes, in quasicircular orbit endowed with a pulsar-like B-fields. Following merger, magnetic instabilities and the loss of angular momentum due to gravitational radiation in the hypermassive NS (HMNS) induce the formation of a nearly uniformly rotating inner core and a Keplerian disk envelope. The HMNS collapses to a BH, with spin $a/M_{\rm BH} \sim 0.75$ independent of the initial spin of the NS. After $3000M-4000 M \sim 45-60(M_{\rm NS}/1.625M_\odot)\rm ms$ following BH formation, an incipient jet is launched. The lifetime of the jets [$\Delta t\sim 100-140(M_{\rm NS}/1.625M_\odot)\rm ms$] and their Poynting luminosities [$L_{jet}\sim 10^{51.5\pm 1}\rm erg/s$] are consistent with typical sGRBs, as well as with the Blandford--Znajek mechanism for launching jets.