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AAssociation of GW170817/GRB170817A/AT2017gfo provides the first directevidence for neutron star mergers as significant sources of $r$-processnucleosynthesis. A gamma-ray transient (GRT) would be powered by theradioactive decay of the freshly-synthesized $r$-process elements. By analyzingthe composition and gamma-ray opacity of the kilonova ejecta in details, wecalculate the lightcurve and spectrum of the GRT for a range of sphericallysymmetric merger ejecta models with mass $M_{\rm ej}=0.001 \sim 0.05M_{\odot}$and expansion velocity $v_{\rm ej}= 0.1\sim 0.4c$. It is found that the peak ofthe GRT lightcurve depends on $M_{\rm ej}$ and $v_{\rm ej}$ as $t_{\rm pk}\approx 0.5~{\rm days} ~ (M_{\rm ej}/0.01M_{\odot})^{1/2}(v_{\rmej}/0.1c)^{-1}$ and $L_{\rm pk} \approx 2.0\times10^{41} ~{\rm erg~s} ^{-1}(M_{\rm ej}/0.01M_{\odot})^{1/2}(v_{\rm ej}/0.1c)$. Most radiating photons arein the $100-3000$ keV band and the spectrum peaks at about 800~keV fordifferent nuclear physics inputs. The line features are blurred out by theDoppler broadening effect. Adopting the ejecta parameters reported inliterature, we examine the detection probability of the possible GRT associatedwith AT2017gfo. We show that the GRT cannot be convincingly detected neitherwith current nor with the proposed missions in the MeV band, such as ETCC andAMEGO. The low gamma-ray flux, together with the extremely low event rate atlocal universe, makes a discovery of GRTs a great challenge.

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