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

Abstract

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 ∼0.05M ⊙ and expansion velocity v ej = 0.1c to ∼0.4c. 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.

The production of actinides in neutron star mergers

“…Besides the impact of actinide production on kilonova lightcurves, their presence might also contribute to the γray emission from individual BNSM event [101,[104][105][106][107][108] or from old BNSM remnants (∼ 10 4 -10 6 years old) residing inside the Milky Way [105,109,110]. In particular, [109] suggested that if future sub-MeV γ -ray missions can reach a line sensitivity of ∼ 10 −8 γ cm −2 s −1 , then there is a non-negligible chance (∼ 10 − 30%) to detect lines from the decay of 230 Th with a number fraction of ∼ 10 −5 − 10 −6 per merger in Milky Way's old BNSM remnants.…”