Atomic Physics and the Spectral Modeling of Kilonovae*

Neutron star mergers are promising candidates for the observation of an electromagnetic (EM) signal coincident with gravitational waves. The 2017 observation of GW170817 [1] appears to be such an event, with gravitational waves confirmed by subsequent EM signals ranging from the infrared to x-ray portions of the spectrum. The atomic properties of the elements produced during these events are predicted to play an important role in the electromagnetic transients called kilonovae. Specfically, the radiative opacity plays an important role in determining the characteristics of the EM radiation that can penetrate the expanding media and ultimately be observed from Earth. The broad range of EM radiation observed from this recent event suggests that a significant fraction of the periodic table, from the fourth to seventh rows, is relevant. The presence of heavy r-process elements poses a particularly significant challenge to the accurate calculation of opacities and radiation transport, due to the appearance of dense forests of absorption lines arising from near-neutral lanthanide and actinide elements. We use the Los Alamos suite of atomic physics and plasma modeling codes [2] to investigate the use of detailed, fine-structure opacities [3-4] to model the EM emission from kilonovae. Our simulations [5-7] predict emission in a range of EM bands, depending on issues such as the presence of winds, elemental composition, and viewing angle. This talk emphasizes various atomic-physics aspects of the spectral modeling of neutron star mergers.

[1] B.P. Abbott et al, Astrophys. J. Lett. 848 , L12 (2017).

[2] C.J. Fontes et al, J. Phys. B 48, 144014 (2015).

[3] C.J. Fontes et al, Mon. Not. R. Astron. Soc. 493, 4143 (2020).

[4] The NIST-LANL Lanthanide Opacity Database: https://nlte.nist.gov/OPAC/.

[5] R.T. Wollaeger et al, Astrophys. J. 880}, 22 (2019)

[6] W. Even et al, Astrophys. J. 899, 24 (2020).

[7] O. Korobkin et al, preprint, arXiv:2004.00102 (2021).