Electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers

Abstract

The first hours following a neutron star merger are considered to provide several UV/optical/NIR signals: β-decay emission from free neutrons, radioactive decay of shocked heavy elements in the cocoon and cocoon's cooling emission. Here we consider two additional emission sources: β-decay of free neutrons in the cocoon and synchrotron by the β-decay electrons. We present 3D RHD simulations of jets that propagate in a multi-layer ejecta from the merger and calculate semi-analytically the resulting light curves. We find that the free neutrons emission at high latitudes is enhanced by the cocoon by a factor of a few to power a wide ( 60 • ) and brief (∼ 1 hour) UV signal that can reach an absolute magnitude of -15, comparable with the cooling emission. If the ejected neutron matter mass is M n 10 −4 M , the synchrotron emission may yield a long (∼ 8 hours) quasi-isotropic UV/optical signal with an absolute magnitude between -12 and -15, depending on the magnetic field. Such a high mass of a mildly-relativistic component may partly obscure the cocoon's shocked r-process elements, thereby attenuating its radioactive decay emission. Future observations on these timescales, including null detections, may place constraints on the ejected neutron matter mass and shed light on the ejecta and jet-cocoon characteristics.

Simulations of early kilonova emission from neutron star mergers

“…It is to be noted that most of the previous works have considered a fixed opacity value of 1 cm 2 g −1 or less (Kasliwal et al 2017;Villar et al 2017;& Kollmeier 2018;Gottlieb & Loeb 2020) to calculate blue kilonovae at t < 1 day. This assumption is not valid precisely, as the change in the opacity with time is quite large for even high Y e ejecta.…”

“…Hence, the purely radioactive kilonova models may not be able to explain the observed early light curve, depending on the structure and composition of the outer ejecta. In this case, a heating source other than radioactive decays of r-process nuclei may be necessary, for example, heating by shock or cocoon (Kasliwal et al 2017;& Kollmeier 2018), β decay luminosity from free neutrons (Metzger et al 2015;Gottlieb & Loeb 2020), or some other central power source (Metzger et al 2008;Yu et al 2013;Metzger & Fernández 2014;Matsumoto et al 2018;Metzger et al 2018;Li et al 2018;Wollaeger et al 2019). Although it is difficult to draw firm conclusions due to a lack of atomic data on highly-ionized lanthanides, our new atomic opacities provide the foundation for a discussion on the detailed properties of blue kilonova models at an early phase.…”

“…Other possibilities include emission powered by β-decays of free neutrons (Metzger et al 2015;Gottlieb & Loeb 2020) or by a long-lived central engine (Metzger et al 2008;Yu et al 2013;Metzger & Fernández 2014;Matsumoto et al 2018;Metzger et al 2018;Li et al 2018;Wollaeger et al 2019).…”

“…However, Villar et al (2017) used a fixed value of opacity for different segments of the ejecta in their multicomponent ejecta model. Similarly, in the models of cocoon emission and free neutron decay (Kasliwal et al 2017;Piro & Kollmeier 2018;Gottlieb & Loeb 2020), the opacity was fixed at a certain value.…”

Jet propagation in expanding media

“…The propagation of relativistic jets in expanding medium started to draw special attention in anticipation of the first GW signal from binary neutron star (BNS) mergers (Murguia-Berthier et al 2014Lazzati et al 2017;Gottlieb et al 2018a), and it became the focus of many studies following GW170817 (Kasliwal et al 2017;Gottlieb et al 2018bGottlieb et al , 2021aDuffell et al 2018;Kathirgamaraju et al 2018Kathirgamaraju et al , 2019Lazzati & Perna 2019;Geng et al 2019;Gottlieb & Loeb 2020;Klion et al 2021;Murguia-Berthier et al 2021;Urrutia et al 2021). Most of these studies used numerical simulations showing that the jet-ejecta interplay shapes the emerging jet-cocoon structure, and thus has important observational implications on the prompt emission and the afterglow light curve, and possibly also on the kilonova emission.…”