Radiative Transfer Simulations for Neutron Star Merger Ejecta

The merger of binary neutron stars (NSs) is among the most promising gravitational wave (GW) sources. Next-generation GW detectors are expected to detect signals from the NS merger within 200 Mpc. Detection of electromagnetic wave (EM) counterpart is crucial to understand the nature of GW sources. Among possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, prediction so far does not take into account detailed r-process element abundances in the ejecta. We perform radiative transfer simulations for the NS merger ejecta including all the r-process elements from Ga to U for the first time. We show that the opacity in the NS merger ejecta is about kappa = 10 cm^2 g^{-1}, which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ~ 100. As a result, the emission is fainter and longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of two NSs and a softer equation of states adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, expected brightness of the emission is i = 22 - 25 AB mag, z = 21 - 23 AB mag, and 21 - 24 AB mag in NIR JHK bands. Thus, observations with wide-field 4m- and 8m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.