When Did the Remnant of GW170817 Collapse to a Black Hole?

Abstract

The main hard pulse of prompt gamma-ray emission in GRB 170817A had a duration of ∼ 0.5 s and its onset was delayed with respect to the gravitational-wave chirp signal by t del ≈ 1.74 s. Detailed follow-up of the subsequent broadband kilonova emission revealed a two-component ejecta -a lanthanide-poor ejecta with mass M ej,blue ≈ 0.025 M that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass M ej,red ≈ 0.04 M that powered the longer lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with modest electron fraction, 0.2 Y e 0.3, can be explained if the collapse to a black hole was delayed by the formation of a hypermassive neutron star (HMNS). Here, we determine the survival time of the merger remnant by combining two different constraints, namely, the time needed to produce the requisite blue-ejecta mass and that necessary for the relativistic jet to bore its way out of the expanding ejecta. In this way, we determine that the remnant of GW170817 must have collapsed to a black hole after t coll = 0.98 +0.31 −0.26 s. We also discuss how future detections and the delays between the gravitational and electromagnetic emissions can be used to constrain the properties of the merged object.

Quantum Black Holes in the Sky

“…GW echoes signal found by Abedi and Afshordi following a NS collapse to BH Independent determination of BH collapse, from EM emission byGill et al, 2019 …”

“…22 and 23, the main signal is also accompanied by secondary lower significance resonances at 73 Hz and t − t merger = 32.9 sec. It is worth noting that after this detection, Gill et al [145] used independent Astrophysical considerations, based GW170817 electromagnetic follow-ups, to determine that the remnant of GW170817 must have collapsed into a BH after t coll = 0.98 +0. 31 −0.26 sec, which coincides with the detected GW echo signal at 1.0 second (see Fig.…”

“…31 −0.26 sec. Error-bar (in blue) is the time of collapse considering this independent observation in [145] compared to the detected signal of echoes which is also as a consequence of BH collapse. The shaded region is 0-1 sec prior range after the merger.…”

What Constraints on the Neutron Star Maximum Mass Can One Pose from GW170817 Observations?

“…Within this picture, the remnant is an HMNS, which must have collapsed before the GRB trigger time, which is 1.7 s after the merger (Abbott et al 2017c;Goldstein et al 2017). In particular, a good fraction of the observed 1.7 s delay has to be attributed to the HMNS phase, during which significant mass ejection is warranted to account for the observed bright kilonova emission (Siegel & Metzger 2017;Gill et al 2019). If this is the case, the multi-messenger observations of GW170817 can be used to provide an upper bound on M TOV (Margalit & Metzger 2017;Rezzolla et al 2018;Ruiz et al 2018;Shibata et al 2019), e.g.…”

Jet Propagation in Neutron Star Mergers and GW170817

“…Analytic modeling of the jet propagation in a BNS merger has been presented using ideas from the collapsar case (Murguia-Berthier et al 2017;Ioka & Nakamura 2018;Matsumoto & Kimura 2018;Gill et al 2019;Geng et al 2019;Lazzati & Perna 2019). These studies tried to make constraints on the equation of state of neutron stars (Lazzati & Perna 2019), on the properties of the ejecta (or the wind) (Murguia-Berthier et al 2017;Lazzati & Perna 2019), and on the timescale between the merger and BH formation (Murguia-Berthier et al 2017;Matsumoto & Kimura 2018;Gill et al 2019;Geng et al 2019). However, previous analytic models of the jet propagation did not properly take the expansion of the medium into account; also, there has been no evidence of their consistency with numerical simulations.…”