Electromagnetic counterparts of compact object mergers powered by the radioactive decay of r-process nuclei

Abstract

The most promising astrophysical sources of kHz gravitational waves (GWs) are the inspiral and merger of binary neutron star(NS)/black hole systems. Maximizing the scientific return of a GW detection will require identifying a coincident electromagnetic (EM) counterpart. One of the most likely sources of isotropic EM emission from compact object mergers is a supernova‐like transient powered by the radioactive decay of heavy elements synthesized in ejecta from the merger. We present the first calculations of the optical transients from compact object mergers that self‐consistently determine the radioactive heating by means of a nuclear reaction network; using this heating rate, we model the light curve with a one‐dimensional Monte Carlo radiation transfer calculation. For an ejecta mass ∼10−2 M⊙ (10−3 M⊙) the resulting light‐curve peaks on a time‐scale ∼1 d at a V‐band luminosity νLν∼ 3 × 1041 (1041) erg s−1[MV=−15(−14)]; this corresponds to an effective ‘f’ parameter ∼3 × 10−6 in the Li–Paczynski toy model. We argue that these results are relatively insensitive to uncertainties in the relevant nuclear physics and to the precise early‐time dynamics and ejecta composition. Since NS merger transients peak at a luminosity that is a factor of ∼103 higher than a typical nova, we propose naming these events ‘kilo‐novae’. Because of the rapid evolution and low luminosity of NS merger transients, EM counterpart searches triggered by GW detections will require close collaboration between the GW and astronomical communities. NS merger transients may also be detectable following a short‐duration gamma‐ray burst or ‘blindly’ with present or upcoming optical transient surveys. Because the emission produced by NS merger ejecta is powered by the formation of rare r‐process elements, current optical transient surveys can directly constrain the unknown origin of the heaviest elements in the Universe.

The dynamic ejecta of compact object mergers and eccentric collisions

“…15 in Rosswog et al [20] for the collision cases. Other groups [6][7][8] find similar results for NSNS merger cases.…”

“…They rely on estimates for the ejected mass and its velocity distribution (or alternatively dm/dv), which can be straightforwardly extracted from hydrodynamic simulations. Via nuclear network calculations along hydrodynamic trajectories, one can extract the nuclear energy injection rate,˙ , which shows little sensitivity to the exact details [6,31]. The latter authors find that˙ provides a good fit to the results of the network calculations.…”

Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes

“…Electromagnetically, such mergers are also postulated to generate a relatively rapidly evolving optical/infrared transient-referred to as kilonova or macronova (e.g., Li & Paczyński 1998;Metzger et al 2010;Roberts et al 2011;Barnes & Kasen 2013;Kasen, Fernández, & Metzger 2015;Metzger et al 2015;Barnes et al 2016). The combination of an sGRB and kilonova is considered the 'smoking gun' signature of such mergers.…”

Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

“…1999; Argast et al 2004;Hirai et al 2015), possibly followed by a so-called "kilonova" (e.g., Metzger et al 2010;Barnes & Kasen 2013;Tanaka & Hotokezaka 2013). However, to produce a double NS system a massive binary must survive two SN explosions.…”