Radioactively Powered Gamma-Ray Transient Associated with a Kilonova from Neutron Star Merger

The association of GW170817/GRB 170817A/AT2017gfo provides the first direct evidence for neutron star mergers as significant sources of r -process nucleosynthesis. A gamma-ray transient (GRT) would be powered by the radioactive decay of the freshly synthesized r -process elements. By analyzing the composition and gamma-ray opacity of the kilonova ejecta in detail, we calculate the lightcurve and spectrum of the GRT for a range of spherically symmetric merger ejecta models with mass M ej = 0.001 to ∼0.05 M ⊙ and expansion velocity v ej = 0.1 c to ∼0.4 c . It is found that the peak of the GRT lightcurve depends on M ej and v ej as t pk ≈ 0.5 days ( M ej / 0.01 M ⊙ ) 1 / 2 ( v ej / 0.1 c ) − 1 and L pk ≈ 2.0 × 10 41 erg s − 1 ( M ej / 0.01 M ⊙ ) 1 / 2 ( v ej / 0.1 c ) . Most radiating photons are in the 100–3000 keV band and the spectrum peaks at about 800 keV for different nuclear physics inputs. The line features are blurred out by the Doppler broadening effect. Adopting the ejecta parameters reported in the literature, we examine the detection probability of the possible GRT associated with AT2017gfo. We show that the GRT cannot be convincingly detected with either current or proposed missions in the MeV band, such as ETCC and AMEGO. The low gamma-ray flux, together with the extremely low event rate at local universe, makes a discovery of GRTs a great challenge.