Gamma-Ray Emission Produced by r-process Elements from Neutron Star Mergers
Meng-Hua Chen, Li-Xin Li, Da-Bin Lin, En-Wei Liang
Abstract
Abstract The observation of a radioactively powered kilonova AT 2017gfo associated with the gravitational wave event GW170817 from a binary neutron star merger proves that these events are ideal sites for the production of heavy r -process elements. The gamma-ray photons produced by the radioactive decay of heavy elements are unique probes for the detailed nuclide compositions. Based on the detailed r -process nucleosynthesis calculations and considering radiative transport calculations for the gamma rays in different shells, we study the gamma-ray emission in a merger ejecta on a timescale of a few days. It is found that the total gamma-ray energy generation rate evolution is roughly depicted as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.3</mml:mn> </mml:mrow> </mml:msup> </mml:math> . For the dynamical ejecta with a low electron fraction ( Y e ≲ 0.20), the dominant contributors of gamma-ray energy are the nuclides around the second r -process peak ( A ∼ 130) and the decay chain of 132 Te ( t 1/2 = 3.21 days) → 132 I ( t 1/2 = 0.10 days) → 132 Xe produces gamma-ray lines at 228, 668, and 773 keV. For the case of a wind ejecta with Y e ≳ 0.30, the dominant contributors of gamma-ray energy are the nuclides around the first r -process peak ( A ∼ 80) and the decay chain of 72 Zn ( t 1/2 = 1.93 days) → 72 Ga ( t 1/2 = 0.59 days) → 72 Ge produces gamma-ray lines at 145, 834, 2202, and 2508 keV. The peak fluxes of these lines are 10 −9 ∼ 10 −7 ph cm −2 s −1 , which are marginally detectable with the next-generation MeV gamma-ray detector ETCC if the source is at a distance of 40 Mpc.