Finding the Remnants of the Milky Way's Last Neutron Star Mergers

Abstract

The discovery of a binary neutron star merger (NSM) through both its gravitational wave and electromagnetic emission has revealed these events to be key sites of r-process nucleosynthesis. Here, we evaluate the prospects of finding the remnants of Galactic NSMs by detecting the gamma-ray decay lines from their radioactive r-process ejecta. We find that 126 Sn, which has several lines in the energy range 415-695 keV and resides close to the second r-process peak, is the most promising isotope, because of its half-life t 1/2 = 2.30(14) × 10 5 yr being comparable to the ages of recent NSMs. Using a Monte Carlo procedure, we predict that multiple remnants are detectable as individual sources by next-generation γ-ray telescopes which achieve sub-MeV line sensitivities of ∼ 10 −8 -10 −6 γ cm −2 s −1 . However, given the unknown locations of the remnants, the most promising search strategy is a systematic survey of the Galactic plane and bulge extending to high Galactic latitudes. Individual known supernova remnants which may be mis-classified NSM remnants could also be targeted, especially those located outside the Galactic plane. Detection of a moderate sample of Galactic NSM remnants would provide important clues to unresolved issues such as the production of actinides in NSMs, properties of merging NS binaries, and even help distinguish them from rare supernovae as current Galactic rprocess sources. We also investigate the diffuse flux from longer-lived nuclei (e.g. 182 Hf) that could in principle trace the Galactic spatial distribution of NSMs over longer timescales, but find that the detection of the diffuse flux appears challenging even with next-generation telescopes.

Kilonovae

“…A related, but potentially more promising near-term strategy is a gamma-ray search for remnants of past NS mergers in our Galaxy (Wu et al. 2019a; Korobkin et al. 2019).…”

“…Wu et al. (2019a) estimate that multiple remnants are detectable as individual sources by next-generation gamma-ray satellites with line sensitivities .…”

Neutron star mergers and how to study them

“…They may be similar to supernova remnants but have lower total kinetic energies and will tend to occur in regions with lower surrounding material (due to occurring outside of their host galaxies). Even long after merger they will be radioactive, with emission dominated by isotopes with half-lives of similar order to the age of the remnant (Wu et al 2019;Korobkin et al 2020). Longer still, the kinetic energy will eventually be used up and the shock-front will dissipate.…”

“…Longer still, the kinetic energy will eventually be used up and the shock-front will dissipate. Ejecta that is bound to the host galaxy will eventually return and become part of the diffuse galactic material where long-term mixing distributes the heaviest elements throughout the galaxy (Wu et al 2019). Some will eventually join new planets and stars, and a bit may eventually be dug out of the ground by advanced life.…”

Dual MeV gamma-ray and dark matter observatory - GRAMS Project

“…Table 1 shows the sensitivity for the balloon (satellite) experiment (3σ, T e f f = 10 6 s) to gamma-ray lines from positron annihilation (511 keV), 56 Co (847 keV) from ∼2 ×10 −5 ∼15 (∼75) 44 Ti (1157 keV) 1.0 × 10 −6 (1.9 × 10 −7 ) ∼2 ×10 −5 ∼20 (∼110) 60 Fe (1173 keV) 1.0 × 10 −6 (1.9 × 10 −7 ) ∼2 ×10 −5 ∼20 (∼110) 60 Fe (1333 keV) 9.1 × 10 −7 (1.7 × 10 −7 ) ∼2 ×10 −5 ∼20 (∼120) 26 Al (1809 keV) 7.2 × 10 −7 (1.3 × 10 −7 ) 2.5 ×10 −5 ∼35 (∼190) 2 H (2223 keV) 6.4 × 10 −7 (1.1 × 10 −7 ) ∼2 ×10 −5 ∼30 (∼180) 12 C* (4438 keV) 4.9 × 10 −7 (7.3 × 10 −8 ) ∼1 ×10 −5 ∼20 (∼140) thermonuclear supernovae, 44 Ti (1157 keV) from corecollapse supernovae, 60 Fe (1173 keV and 1333 keV) from core-collapse supernovae, 26 Al (1809 keV) from collapse supernovae or massive stars, 2 H (2223 keV) from neutron capture by protons, and 12 C* (4438 keV) from cosmic ray interactions [35,36]. GRAMS could also detect gamma-ray lines from Galactic neutron star merger remnants (666 keV and 695 keV from 126 Sn) [37]. With this larger effective area, GRAMS would have more than an order of magnitude improved sensi-tivity compared with SPI/INTEGRAL [28,29,30,31].…”