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\nthe inspiral and merger of binary neutron star(NS)/black hole systems.\nMaximizing the scientific return of a GW detection will require identifying a\ncoincident electro-magnetic (EM) counterpart. One of the most likely sources of\nisotropic EM emission from compact object mergers is a supernova-like transient\npowered by the radioactive decay of heavy elements synthesized in ejecta from\nthe merger. We present the first calculations of the optical transients from\ncompact object mergers that self-consistently determine the radioactive heating\nby means of a nuclear reaction network; using this heating rate, we model the\nlight curve with a one dimensional Monte Carlo radiation transfer calculation.\nFor an ejecta mass ~1e-2 M_sun[1e-3 M_sun] the resulting light curve peaks on a\ntimescale ~ 1 day at a V-band luminosity nu L_nu ~ 3e41[1e41] ergs/s (M_V =\n-15[-14]); this corresponds to an effective "f" parameter ~3e-6 in the\nLi-Paczynski toy model. We argue that these results are relatively insensitive\nto uncertainties in the relevant nuclear physics and to the precise early-time\ndynamics and ejecta composition. Due to the rapid evolution and low luminosity\nof NS merger transients, EM counterpart searches triggered by GW detections\nwill require close collaboration between the GW and astronomical communities.\nNS merger transients may also be detectable following a short-duration\nGamma-Ray Burst or "blindly" with present or upcoming optical transient\nsurveys. Because the emission produced by NS merger ejecta is powered by the\nformation of rare r-process elements, current optical transient surveys can\ndirectly constrain the unknown origin of the heaviest elements in the Universe.\n