Binary neutron star mergers after GW170817

Riccardo Ciolfi1, 2

1INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy2INFN, Sezione di Padova, Via Francesco Marzolo 8, I-35131 Padova, Italy

(Dated: May 7, 2020)

The first combined detection of gravitational waves and electromagnetic signals from a binary neutron star(BNS) merger in August 2017 (event named GW170817) represents a major landmark for the ongoing investi-gation on these extraordinary systems. In this short review, we introduce BNS mergers as events of the utmostimportance for astrophysics and fundamental physics and discuss the main discoveries enabled by this first mul-timessenger observation, which include compelling evidence that such mergers produce a copious amount ofheavy r-process elements and can power short gamma-ray bursts. We further discuss key open questions leftbehind on this event and BNS mergers in general, focussing the attention on the current status and limitationsof theoretical models and numerical simulations.

I. INTRODUCTION

Binary neutron star (BNS) mergers are among the most in-triguing events known in the Universe, characterized by an im-pressive scientific potential spanning many different researchfields in physics and astrophysics. Investigating them offersa unique opportunity to understand hadronic interactions atsupranuclear densities and the equation of state (EOS) of mat-ter in such extreme conditions and, at the same time, to gaincrucial insights on the strong gravity regime, high energy as-trophysical phenomena of primary importance like gamma-ray bursts (GRBs), the origin of heavy elements in the localUniverse, formation channels of compact object binaries, andcosmology (e.g., [1, 2] and refs. therein).

The merger of two neutron stars (NSs) is accompanied bya strong emission of gravitational waves (GWs) and by a richvariety of electromagnetic (EM) signals covering the entirespectrum, from gamma-rays to radio. Such a unique combi-nation of signals makes these systems ideal multimessengersources and allows us to observe them up to cosmologicaldistances. Moreover, among their EM “counterparts”, BNSmergers have long been thought to be responsible for shortGRBs (SGRBs) [3–9] as well as radioactively-powered “kilo-nova” transients associated with r-process nucleosynthesis ofheavy elements [10–12].1

A major step forward in the study of BNS mergers wasmade possible by the first GW detection of this type of eventsby the LIGO and Virgo Collaboration in August 2017 (eventnamed GW170817) [14]. This merger was also observed inthe EM spectrum, via a collection of gamma-ray, X-ray, UV,optical, IR, and radio signals, thus providing as well the firstmultimessenger observation of a GW source [15]. Such abreakthrough led to a number of key discoveries, including astriking confirmation that BNS mergers can launch SGRB jets[16–28] and are ideal sites for r-process nucleosynthesis (e.g.,

1 Merging mixed binaries composed by a NS and a back hole (BH) sharemost of the above features, being also promising GW sources, poten-tial SGRB central engines, and potential sources of radioactively-poweredkilonovae. However, the properties of the emitted signals might be verydifferent. Here, we focus on BNS mergers only and refer the reader toother reviews for the case of NS-BH binary mergers (e.g., [13]).

[29–33]; see also [34] and refs. therein), the first GW-basedconstraints on the NS EOS [35] and the Hubble constant [36],and more. The most important lessons learned from this eventare discussed in the next Section (II).

Together with the remarkable results mentioned above, theGW170817 event also left behind a number of open questions,in part related to details of the merging process that remainedonly poorly constrained. For instance, the remnant object re-sulting from the merger appears to be most likely a metastablemassive NS that eventually collapsed into a BH, but the lackof clear indications on its survival time until collapse leavesdoubts on the nature of the SGRB central engine, which couldhave been either the massive NS or the accreting BH (see,e.g., [37] for a recent review). Theoretical modelling of themerger process via general relativistic magnetohydrodynam-ics (GRMHD) simulations (see Fig. 1) offers the best chanceto tackle the open issues and to establish a reliable connectionbetween the merger and post-merger dynamics and the ob-servable GW and EM emission (e.g., [38] and refs. therein).In Section III, we briefly report on the status of the investi-gation in this direction, with reference to specific challengesposed by the GW170817 event. Finally, concluding remarksare given in Section IV.

II. THE BNS MERGER OF AUGUST 2017

The characteristic “chirp” signal of GW170817, with bothfrequency and amplitude increasing in time up to a maxi-mum, leaves no doubts that the source was a merging com-pact binary with component masses fully consistent with be-ing NSs [14]. In addition, the BNS nature of the source isarguably reinforced by the EM counterparts observed alongwith GWs [15]. Under the BNS assumption, this single detec-tion significantly improved our estimate for the correspond-ing local coalescence rate (the value reported in [14] beingR = 1540+3200

−1220 Gpc−3yr−1).2

2 Under the assumption that GW190425 was also a BNS merger, the updatedrate would be R = 250− 2810Gpc−3yr−1 [40].

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FIG. 1. Example of BNS merger simulation in GRMHD (from the models presented in [39]). The temporal sequence shows the bulk of theNS(s) in white together with color-coded isodensity surfaces.

For this event, most of the information inferred from GWscame from the inspiral phase up to merger, while the lower de-tector sensitivity at frequencies above 1 kHz did not allow fora confident detection of the post-merger signal [14]. Despitesuch limitation, it was possible to start placing the first limitson the NS tidal deformability and thus to constrain the rangeof NS EOS compatible with the event (e.g., [35]; see also[41]), by measuring finite-size effects (i.e. deviations from thepoint-mass waveform) in the last orbits of the inspiral. More-over, combining the luminosity distance derived from GWswith the EM redshift measurement allowed by the identifica-tion of the host galaxy (NGC 4993), it was possible to obtainthe first constraints on the Hubble constant based on a GWstandard siren determination [36].

The observation of a gamma-ray signal emerging about1.74 s after the estimated time of merger enabled us to con-firm that GWs propagate at the speed of light with a precisionbetter than 10−14 [16], which excluded a whole range of grav-itational theories beyond general relativity. At the same time,this high energy signal (named GRB 170817A) was found po-tentially consistent with a SGRB, although orders of magni-tude less energetic than any other known SGRB [16]. Com-bining the prompt gamma-ray emission with the multiwave-length afterglows (in X-ray, optical, and radio) monitored forseveral months, it was possible to eventually converge to thefollowing picture [16–28]: (i) the merger remnant launched ahighly relativistic jet (Lorentz factor >10), in agreement withthe consolidated GRB paradigm (e.g., [42, 43]); (ii) the burstwas observed off-axis by 15◦−30◦ and the low energy gamma-ray signal detected was not produced by the jet core, but ratherby a mildly relativistic outflow moving along the line of sight;(iii) the on-axis observer would have seen a burst energeti-cally consistent with the other known SGRBs. This providedthe long-awaited compelling evidence that BNS mergers cangenerate SGRBs. Furthermore, the off-axis view of a nearby(∼ 40Mpc distance) SGRB jet gave us an unprecedented op-portunity to study its full angular structure.

The other major result related to GW170817 is the firstclear photometric and spectroscopic identification of a kilo-nova, i.e. a UV/optical/IR transient powered by the radioac-tive decay of heavy r-process elements synthesized within thematter ejected by the merger process (e.g., [29–33]; see also[34] and refs. therein). This confirmed that BNS mergers pro-

duce a significant amount of elements heavier than iron, up tovery large atomic mass numbers (A>140).

III. OPEN QUESTIONS AND ONGOING RESEARCH

The discoveries connected with GW170817 certainly repre-sent a breakthrough in the field, but a lot remains to be un-derstood on both this event and BNS mergers in general. Partof our ignorance can be ascribed to current observational lim-its. For instance, much better constraints on the NS EOS willbecome available with the considerably higher sensitivity ofthird generation GW detectors [44, 45], allowing also for aconfident detection of the post-merger GW signal, while themerger rate, the formation scenarios of BNS systems, and theGW-based Hubble constant determination will strongly im-prove with the increasing number of detections. On the otherhand, there are many other aspects for which the informationencoded in the observed signals (in particular in the EM coun-terparts) could not be fully exploited due to the present lim-itations of theoretical models. This urgently calls for furtherdevelopment on the theory side and in particular in the con-text of BNS merger simulations in general relativity, whichrepresents the leading approach to unravel the physical mech-anisms at work when two NSs merge together.

In the following, we discuss recent results of BNS mergersimulations and the associated limitations, focussing the at-tention on the interpretation of the August 2017 event. In par-ticular, we consider the two most important EM counterpartsof this event, (i) the SGRB and its multiwavelength afterglowsand (ii) the kilonova transient.

A. SGRB central engines and GRB 170817A

Understanding the launching mechanism of a SGRB jet froma BNS merger and the nature of the remnant object actingas central engine is among the main driving motivations forthe development of numerical relativity simulations of suchmergers (e.g., [39, 46–50]). The great progress of this type ofsimulations, especially over the last decade, allowed to reachimportant conclusions, even though the final solution of theSGRB puzzle is still ahead of us.

3

FIG. 2. Collimated helical magnetic field structure emerging alongthe spin axis of a long-lived BNS merger remnant (from a simula-tion presented in [53]). Several semi-transperent isodensity surfacesare also shown for the highest rest-mass density region (with densityincreasing from grey to red).

According to the most discussed scenario, a SGRB jetwould be launched by a spinning BH surrounded by a mas-sive (∼ 0.1M�) accretion disk, which is a likely outcome ofa BNS merger. Recent simulations [51, 52] showed that a jetpowered by neutrino-antineutrino annihilation would not bepowerful enough to explain the phenomenology of SGRBs,reinforcing the idea that SGRB jets should be instead mag-netically driven. Various GRMHD simulations explored thelatter possibility (e.g., [46–49]), confirming the formation ofa low density funnel along the BH spin axis and finding in-dications of an emerging helical magnetic field structure thatis favourable for accelerating an outflow. In addition, simula-tions reported in [49] were the first to show the actual produc-tion of a magnetically-dominated mildly relativistic outflowand the authors argued that such an outflow could in principlereach terminal Lorentz factors compatible with a SGRB jet.While the results obtained so far do not provide the ultimateanswer, current simulations suggest that the accreting BH sce-nario is a promising one (see, e.g., [37, 38] for a more detaileddiscussion).

The alternative scenario in which the central engine is amassive NS remnant is also investigated via GRMHD BNSmerger simulations, although a systematic study started onlyrecently [39, 50, 53]. In this case, the higher level of baryonpollution along the spin axis may hamper the formation of anincipient jet. The longest (to date) simulations of this kind, re-cently presented in [53], showed for the first time that the NSdifferential rotation can still build up a helical magnetic fieldstructure able to accelerate a collimated outflow (see Fig. 2),

although such an outcome is not ubiquitous.3 In addition, forthe case under consideration, the properties of the collimatedoutflow (and in particular the very low terminal Lorentz fac-tor) were found largely incompatible with a SGRB jet [53].This result reveals serious difficulties in powering a SGRBthat might apply to massive NS remnants in general, thuspointing in favour of the alternative BH central engine. In or-der to confirm the above conclusion, however, a larger varietyof physical conditions needs to be explored (e.g., by includingneutrino radiation).

For the case of GRB 170817A, neither the observations norcurrent theoretical models can confidently exclude any of thetwo scenarios. Nevertheless, BNS merger simulations alreadyprovided valuable hints in favour of the accreting BH one [49,53] and the continuous improvement of numerical codes andthe degree of realism of their physical description might soonlead to a final solution.

Another important limiting factor for the interpretation ofGRB 170817A is represented by the considerable gap be-tween the relatively small timescales and spatial scales probedby GRMHD merger simulations (up to order ∼ 100ms and∼1000 km) and those relevant for the propagation of an incip-ient jet through the baryon-polluted environment surroundingthe merger site (& 1 s and & 105 km). The ultimate angularstructure and energetics of the escaping jet, which are directlyrelated with the prompt and afterglow SGRB emission, aretherefore very hard to associate with specific properties of themerging system and a specific launching mechanism. Oneof the most important challenges to be addressed in the nearfuture will then be to obtain a self-consistent model able todescribe the full evolution from the pre-merger stage up to thefinal escaping jet.

B. Merger ejecta and the kilonova transient AT 2017gfo

During and after a BNS merger, a relatively large amount ofmaterial (up to ∼0.1M�) can be ejected, either via dynamicalmechanisms associated with the merger process (tidally- andshock-driven ejecta) or via baryon-loaded winds launched bythe (meta)stable massive NS remnant and/or by the accretiondisk around the newly formed BH (if any). Depending on thethermodynamical history and composition (in particular theelectron fraction Ye) of each fluid element within the ejecta,the r-process nucleosynthesis takes place producing a certainamount of heavy elements (i.e. heavier than iron). Later on,the radioactive decay of these elements powers the thermaltransient commonly referred to as a kilonova (e.g., [34] andrefs. therein).

For a given ejecta component, the peak luminosity, peaktime, and peak frequency (or temperature) of the correspond-ing kilonova are mainly determined by the ejecta mass, veloc-ity, and opacity (e.g., [57]). While mass and velocity depend

3 Note that a collimated outflow was also reported in studies where an ad-hoc dipolar field was superimposed by hand on a differentially rotating NSremnant (e.g., [54–56]).

4

on the mass ejection mechanism, the opacity is directly re-lated to the nucleosynthesis yields. In particular, high electronfractions (Ye&0.25) typically produce elements up to atomicmass numbers A. 140, maintaining a relatively low opacityof ∼0.1−1 cm2/g, while more neutron rich ejecta (Ye.0.25)allow for the production of elements up to A>140 (including,e.g., the group of lanthanides), which leads to much higheropacities of ∼10 cm2/g (e.g., [58, 59]).

When applied to the kilonova of August 2017, the abovepicture reaveals that the observed transient (AT 2017gfo) wasgenerated by at least two distinct ejecta components (e.g.,[33]),4 one having mass ≈ 1.5− 2.5× 10−2 M�, velocity≈ 0.2− 0.3 c, and a relatively low opacity of ≈ 0.5 cm2/g,leading to a “blue” kilonova peaking at ∼ 1 day after merger,and the other having mass ≈4−6×10−2 M�, velocity ≈0.1 c,and a much higher opacity of ∼ 10 cm2/g, leading to a “red”kilonova emerging on a timescale of ∼ 1week. One of thecurrent challenges is to identify the mass ejection mechanismsresponsible for these components. In such a quest, numericalrelativity simulations of BNS mergers play a pivotal role.

The “red” part of the 2017 kilonova is perhaps the easiestto accomodate (among the two). The very large mass and lowvelocity would exclude dynamical mass ejection and point toa baryon-loaded wind. In particular, the mass expelled by theaccretion disk around the BH (i.e. after the collapse of theNS remnant) appears to match the requirements, including arelatively high opacity or, equivalently, a low electron fractionfor at least for part of the material (e.g., [61]).

The origin of the “blue” kilonova is more debated. Ejectamass is rather high, but so is also the velocity (v & 0.2 c).The former still poses doubts on a dynamical ejection, whilethe latter represents a potential problem for post-mergerbaryon-loaded winds. The magnetically driven wind from the

(meta)stable NS remnant offers a viable solution [62], thanksto the enhanced mass ejection and the simultaneous accelera-tion due to the magnetic field (as previously suggested, e.g., in[63]). In this case, neutrino irradiation would also be funda-mental to raise the Ye of the material, limit the r-process nucle-osynthesis, and thus maintain a low opacity [63]. We stress,however, that other viable scenarios exist (e.g., [64, 65]).

Current kilonova models are still affected by several un-certainties on the microphysical parameters, on the radiationtransport (which is treated with strong approximations), andon the mass ejection mechanisms. Nonetheless, we are wit-nessing a rapid theoretical and numerical progress that willkeep guiding us towards a more solid interpretation of the ob-servational data.

IV. CONCLUDING REMARKS

The growing interest for BNS mergers over the last decadeswas recently boosted by the multimessenger observation ofthe August 2017 event. Among the numerous breakthroughresults, this BNS merger provided fundamental confirma-tions of theoretical predictions, namely the association withSGRBs, already supported by indirect evidence but still un-proven, and the production of heavy r-process elements andthe related kilonova transients. This success on the theory sidecertainly strengthens the motivation for the development ofmodels and in particular numerical simulations. At the sametime, the case of GW170817 showed that the present and nearfuture observations are likely to contain much more informa-tion than we are currently capable to exploit, making a furtheradvancement in our ability to interpret the data more urgentthan ever.

[1] J. A. Faber and F. A. Rasio, Liv. Rev. Rel. 15, 8 (2012).[2] L. Baiotti and L. Rezzolla, Rep. Prog. Phys. 80, 096901 (2017).[3] B. Paczynski, Astrophys. J. Lett. 308, L43 (1986).[4] D. Eichler, M. Livio, T. Piran, and D. N. Schramm, Nature 340,

126 (1989).[5] R. Narayan, B. Paczynski, and T. Piran, Astrophys. J. Lett. 395,

L83 (1992).[6] S. D. Barthelmy, G. Chincarini, D. N. Burrows, N. Gehrels,

S. Covino, A. Moretti, P. Romano, P. T. O’Brien, C. L. Sarazin,C. Kouveliotou, et al., Nature 438, 994 (2005).

[7] D. B. Fox, D. A. Frail, P. A. Price, S. R. Kulkarni, E. Berger,T. Piran, A. M. Soderberg, S. B. Cenko, P. B. Cameron, A. Gal-Yam, et al., Nature 437, 845 (2005).

[8] N. Gehrels, C. L. Sarazin, P. T. O’Brien, B. Zhang, L. Barbier,S. D. Barthelmy, A. Blustin, D. N. Burrows, J. Cannizzo, J. R.Cummings, et al., Nature 437, 851 (2005).

[9] E. Berger, Ann. Rev. Astron. Astrophys. 52, 43 (2014).[10] L.-X. Li and B. Paczynski, Astrophys. J. 507, L59 (1998).[11] S. Rosswog, Astrophys. J. 634, 1202 (2005).

4 Three-component models were also proposed (e.g., [60]).

[12] B. D. Metzger, G. Martınez-Pinedo, S. Darbha, E. Quataert,A. Arcones, D. Kasen, R. Thomas, P. Nugent, I. V. Panov, andN. T. Zinner, Mon. Not. R. Astron. Soc. 406, 2650 (2010).

[13] M. Shibata and K. Taniguchi, Liv. Rev. Rel. 14, 6 (2011).[14] B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley,

C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya,et al., Phys. Rev. Lett. 119, 161101 (2017).

[15] B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley,C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya,et al., Astrophys. J. Lett. 848, L12 (2017).

[16] B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley,C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya,et al., Astrophys. J. Lett. 848, L13 (2017).

[17] A. Goldstein, P. Veres, E. Burns, M. S. Briggs, R. Hamburg,D. Kocevski, C. A. Wilson-Hodge, R. D. Preece, S. Poolakkil,O. J. Roberts, et al., Astrophys. J. Lett. 848, L14 (2017).

[18] V. Savchenko, C. Ferrigno, E. Kuulkers, A. Bazzano, E. Bozzo,S. Brandt, J. Chenevez, T. J.-L. Courvoisier, R. Diehl,A. Domingo, et al., Astrophys. J. Lett. 848, L15 (2017).

[19] E. Troja, L. Piro, H. van Eerten, R. T. Wollaeger, M. Im, O. D.Fox, N. R. Butler, S. B. Cenko, T. Sakamoto, C. L. Fryer, et al.,Nature 551, 71 (2017).

5

[20] R. Margutti, E. Berger, W. Fong, C. Guidorzi, K. D. Alexan-der, B. D. Metzger, P. K. Blanchard, P. S. Cowperthwaite,R. Chornock, T. Eftekhari, et al., Astrophys. J. Lett. 848, L20(2017).

[21] G. Hallinan, A. Corsi, K. P. Mooley, K. Hotokezaka, E. Nakar,M. M. Kasliwal, D. L. Kaplan, D. A. Frail, S. T. Myers, T. Mur-phy, et al., Science 358, 1579 (2017).

[22] K. D. Alexander, E. Berger, W. Fong, P. K. G. Williams,C. Guidorzi, R. Margutti, B. D. Metzger, J. Annis, P. K. Blan-chard, D. Brout, et al., Astrophys. J. Lett. 848, L21 (2017).

[23] K. P. Mooley, E. Nakar, K. Hotokezaka, G. Hallinan, A. Corsi,D. A. Frail, A. Horesh, T. Murphy, E. Lenc, D. L. Kaplan, et al.,Nature 554, 207 (2018).

[24] D. Lazzati, R. Perna, B. J. Morsony, D. Lopez-Camara,M. Cantiello, R. Ciolfi, B. Giacomazzo, and J. C. Workman,Phys. Rev. Lett. 120, 241103 (2018).

[25] J. D. Lyman, G. P. Lamb, A. J. Levan, I. Mandel, N. R. Tanvir,S. Kobayashi, B. Gompertz, J. Hjorth, A. S. Fruchter, T. Kan-gas, et al., Nature Astr. 2, 751 (2018).

[26] K. D. Alexander, R. Margutti, P. K. Blanchard, W. Fong,E. Berger, A. Hajela, T. Eftekhari, R. Chornock, P. S. Cowperth-waite, D. Giannios, et al., Astrophys. J. Lett. 863, L18 (2018).

[27] K. P. Mooley, A. T. Deller, O. Gottlieb, E. Nakar, G. Halli-nan, S. Bourke, D. A. Frail, A. Horesh, A. Corsi, and K. Ho-tokezaka, Nature 561, 355 (2018).

[28] G. Ghirlanda, O. S. Salafia, Z. Paragi, M. Giroletti, J. Yang,B. Marcote, J. Blanchard, I. Agudo, T. An, M. G. Bernardini,et al., Science 363, 968 (2019).

[29] I. Arcavi, G. Hosseinzadeh, D. A. Howell, C. McCully, D. Poz-nanski, D. Kasen, J. Barnes, M. Zaltzman, S. Vasylyev,D. Maoz, et al., Nature 551, 64 (2017).

[30] D. A. Coulter, R. J. Foley, C. D. Kilpatrick, M. R. Drout, A. L.Piro, B. J. Shappee, M. R. Siebert, J. D. Simon, N. Ulloa,D. Kasen, et al., Science 358, 1556 (2017).

[31] E. Pian, P. D’Avanzo, S. Benetti, M. Branchesi, E. Brocato,S. Campana, E. Cappellaro, S. Covino, V. D’Elia, J. P. U.Fynbo, et al., Nature 551, 67 (2017).

[32] S. J. Smartt, T. W. Chen, A. Jerkstrand, M. Coughlin,E. Kankare, S. A. Sim, M. Fraser, C. Inserra, K. Maguire, K. C.Chambers, et al., Nature 551, 75 (2017).

[33] D. Kasen, B. Metzger, J. Barnes, E. Quataert, and E. Ramirez-Ruiz, Nature 551, 80 (2017).

[34] B. D. Metzger, Liv. Rev. Rel. 23, 1 (2019).[35] B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley,

C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya,et al., Phys. Rev. X 9, 011001 (2019).

[36] B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley,C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya,et al., Nature 551, 85 (2017).

[37] R. Ciolfi, Int. J. Mod. Phys. D 27, 1842004 (2018).[38] R. Ciolfi, arXiv e-prints , arXiv:2003.07572 (2020).

[39] R. Ciolfi, W. Kastaun, B. Giacomazzo, A. Endrizzi, D. M.Siegel, and R. Perna, Phys. Rev. D 95, 063016 (2017).

[40] B. P. Abbott, R. Abbott, T. D. Abbott, S. Abraham, F. Acernese,K. Ackley, C. Adams, R. X. Adhikari, V. B. Adya, C. Affeldt,et al., Astrophys. J. Lett. 892, L3 (2020).

[41] W. Kastaun and F. Ohme, Phys. Rev. D 100, 103023 (2019).[42] T. Piran, Rev. Mod. Phys. 76, 1143 (2004).[43] P. Kumar and B. Zhang, Phys. Rep. 561, 1 (2015).[44] M. Punturo, M. Abernathy, F. Acernese, B. Allen, N. Anders-

son, K. Arun, F. Barone, B. Barr, M. Barsuglia, M. Beker, et al.,Classical and Quantum Gravity 27, 194002 (2010).

[45] B. P. Abbott, R. Abbott, T. D. Abbott, M. R. Abernathy, K. Ack-ley, C. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, C. Af-feldt, et al., Classical and Quantum Gravity 34, 044001 (2017).

[46] L. Rezzolla, B. Giacomazzo, L. Baiotti, J. Granot, C. Kouve-liotou, and M. A. Aloy, Astrophys. J. Lett. 732, L6 (2011).

[47] K. Kiuchi, K. Kyutoku, Y. Sekiguchi, M. Shibata, and T. Wada,Phys. Rev. D 90, 041502 (2014).

[48] T. Kawamura, B. Giacomazzo, W. Kastaun, R. Ciolfi, A. En-drizzi, L. Baiotti, and R. Perna, Phys. Rev. D 94, 064012(2016).

[49] M. Ruiz, R. N. Lang, V. Paschalidis, and S. L. Shapiro, Astro-phys. J. Lett. 824, L6 (2016).

[50] R. Ciolfi, W. Kastaun, J. V. Kalinani, and B. Giacomazzo, Phys.Rev. D 100, 023005 (2019).

[51] O. Just, M. Obergaulinger, H.-T. Janka, A. Bauswein, andN. Schwarz, Astrophys. J. Lett. 816, L30 (2016).

[52] A. Perego, H. Yasin, and A. Arcones, J. Phys. G Nucl. Phys.44, 084007 (2017).

[53] R. Ciolfi, Mon. Not. R. Astron. Soc. Lett. 495, L66 (2020).[54] M. Shibata, Y. Suwa, K. Kiuchi, and K. Ioka, Astrophys. J.

Lett. 734, L36 (2011).[55] D. M. Siegel, R. Ciolfi, and L. Rezzolla, Astrophys. J. Lett.

785, L6 (2014).[56] P. Mosta, D. Radice, R. Haas, E. Schnetter, and S. Bernuzzi,

arXiv e-prints , arXiv:2003.06043 (2020).[57] D. Grossman, O. Korobkin, S. Rosswog, and T. Piran, Mon.

Not. R. Astron. Soc. 439, 757 (2014).[58] D. Kasen, N. R. Badnell, and J. Barnes, Astrophys. J. 774, 25

(2013).[59] M. Tanaka and K. Hotokezaka, Astrophys. J. 775, 113 (2013).[60] A. Perego, D. Radice, and S. Bernuzzi, Astrophys. J. Lett. 850,

L37 (2017).[61] D. M. Siegel and B. D. Metzger, Astrophys. J. 858, 52 (2018).[62] R. Ciolfi and J. V. Kalinani, arXiv e-prints , arXiv:2004.11298

(2020).[63] B. D. Metzger, T. A. Thompson, and E. Quataert, Astrophys.

J. Lett. 856, 101 (2018).[64] K. Kawaguchi, M. Shibata, and M. Tanaka, Astrophys. J. Lett.

865, L21 (2018).[65] V. Nedora, S. Bernuzzi, D. Radice, A. Perego, A. Endrizzi, and

N. Ortiz, Astrophys. J. Lett. 886, L30 (2019).