Are fast radio bursts the most likely electromagnetic counterpart of neutron star mergers resulting in prompt collapse?

Abstract

Inspiraling and merging binary neutron stars (BNSs) are important sources of both gravitational waves and coincident electromagnetic counterparts. If the BNS total mass is larger than a threshold value, a black hole ensues promptly after merger. Through a statistical study in conjunction with recent LIGO/Virgo constraints on the nuclear equation of state, we estimate that up to ∼ 25% of BNS mergers may result in prompt collapse. Moreover, we find that most models of the BNS mass function we study here predict that the majority of prompt-collapse BNS mergers have q 0.8. Prompt-collapse BNS mergers with mass ratio q 0.8 may not be accompanied by detectable kilonovae or short gamma-ray bursts, because they unbind a negligible amount of mass and form negligibly small accretion disks onto the remnant black hole. We call such BNS mergers "orphan". However, recent studies have found that 10 41−43 (Bp/10 12 G) 2 erg s −1 electromagnetic signals can be powered by magnetospheric interactions several milliseconds prior to merger. Moreover, the energy stored in the magnetosphere of an orphan BNS merger remnant will be radiated away in O(1 ms). Through simulations in full general relativity of BNSs endowed with an initial dipole magnetosphere, we find that the energy in the magnetosphere following black hole formation is EB ∼ 10 39−41 (Bp/10 12 G) 2 erg. Radiating ∼ 1% of EB in 1 ms, as has been found in previous studies, matches the premerger magnetospheric luminosity. These magnetospheric signals are not beamed, and their duration and power agrees with those of non-repeating fast radio bursts (FRBs). These results combined with our statistical study suggest that a non-repeating FRB may be the most likely electromagnetic counterpart of prompt-collapse BNSs. Detection of a non-repeating FRB coincident with gravitational waves from a BNS merger could settle the extragalactic origin of a fraction FRBs and could be used to place constraints on the nuclear equation of state. FRBs can also initiate triggered searches for weak signals in the LIGO/Virgo data.

Observations and Theory of Short GRBs at the Dawn of the Gravitational Wave Era

“…Overall, this could be similar to the model proposed for FRBs where a supramassive neutron star collapses to a BH [150,161]. Thus, a prompt collapse is lacking many interesting features arising from the delayed collapse, but could provide answers to other mysterious EM signals (see also [162]). We must add that in the event that the magnetic field energy is amplified to above 10 47 erg in the first millisecond after merger and the remnant subsequently collapses to a BH, the interaction of the emergent magnetic pulse with the ejected matter could give rise to a different variety of low luminosity short GRBs.…”

Magnetohydrodynamic simulations of binary neutron star mergers in general relativity: Effects of magnetic field orientation on jet launching

“…In this case, in contrast to the prompt collapse case (see e.g. [29,30] for possible EM counterparts in this case), the formation of a hypermassive neutron star (HMNS) allows magnetic instabilities to amplify the magnetic energy to reach equipartition with the plasma kinetic energy before BH formation [31]. Following the HMNS collapse, a magnetically-supported jet is then launched once the regions above the BH poles approach force-free values (B 2 /8 πρ 0 1).…”

Optical follow-up of gravitational wave triggers with DECam during the first two LIGO/VIRGO observing runs

“…Mergers for which both gravitational and electromagnetic emission are detectable, bright standard sirens, can be used to determine cosmological parameters such as H 0 via the distance-redshift relation: the distance measurement comes from the GW signal and the redshift from identifying the host galaxy of the EM counterpart. Mergers of neutron stars, or of a neutron star and a black hole, have multiple predicted signatures (Metzger et al, 2010;Nissanke et al, 2010;Gaertig et al, 2011;Berger, 2014;Metzger, 2017;Rosswog et al, 2017;Tanaka et al, 2018;Gottlieb et al, 2018;Paschalidis and Ruiz, 2019;Duque et al, 2019): a gravitational wave chirp, a neutrino burst, a gamma-ray burst followed by an afterglow in various wavelengths, and an optical transient referred in the literature as a kilonova, which we aim to detect with DE-Cam. Fainter, redder, and shorter-lived than supernovae, kilonovae are challenging from the observational point of view.…”