Title: 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 ¹²⁶Sn, 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 $$\gamma$$-ray telescopes which achieve sub-MeV line sensitivities of ~18 ^-8-10 ^-6$$\gamma$$ 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-classi ed 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 $r-$process sources. We also investigate the diffuse flux from longer-lived nuclei (e.g. ¹⁸²Hf) 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.

Authors:

Wu, Meng-Ru  ^[1];

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Banerjee, Projjwal  ^[2];

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Metzger, Brian D.  ^[3];

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Martínez-Pinedo, Gabriel  ^[4];

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Aramaki, Tsuguo  ^[5];

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Burns, Eric ^[6]; Hailey, Charles J. ^[3]; Barnes, Jennifer  ^[3];

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Karagiorgi, Georgia ^[7]

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+ Show Author Affiliations

1. Academia Sinica, Taipei (Taiwan). Inst. of Physics, and Inst. of Astronomy and Astrophysics
2. Shanghai Jiao Tong Univ. (China). Dept. of Astronomy, School of Physics and Astronomy; Indian Inst. of Technology Palakkad, Palakkad, Kerala (India). Dept. of Physics
3. Columbia Univ., New York, NY (United States). Columbia Astrophysics Lab., and Dept. of Physics
4. GSI Helmholtz Centre for Heavy Ion Research, Darmstadt (Germany); Technische Universität Darmstadt, Darmstadt (Germany). Inst. fur Kernphysik (Theoriezentrum)
5. SLAC National Accelerator Lab., Menlo Park, CA (United States). Kavli Inst. for Particle Astrophysics and Cosmology
6. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
7. Columbia Univ., New York, NY (United States). Dept. of Physics

Publication Date:

2019-07-19

Research Org.:

SLAC National Accelerator Lab., Menlo Park, CA (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1560650

Grant/Contract Number:  

107-2119-M-001-038; 11533006; NNX16AB30G; 279384907; SFB 1245; AC02-76SF00515; NNG08FD60C; PF7-180162; PHY-1404209

Resource Type:

Accepted Manuscript

Journal Name:

The Astrophysical Journal (Online)

Additional Journal Information:

Journal Name: The Astrophysical Journal (Online); Journal Volume: 880; Journal Issue: 1; Journal ID: ISSN 1538-4357

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS