Luminosity Functions and Detectability of Binary Neutron Star Merger-nova Signals with Various Merger Remnants

TL;DR

The paper investigates how post-merger remnants (BH vs magnetar) and the neutron-star EOS (e.g., SLy vs DD2) shape the luminosity function of binary neutron star merger-novae and how detectable these signals are with future surveys. The authors synthesize a large population of mergers, classify remnants using EOS-dependent thresholds, and model merger-nova emission with an anisotropic ejecta framework plus magnetar spin-down energy injection, then compute the intrinsic luminosity function and ToO detection prospects for CSST combined with next-generation GW detectors. They find a robust triple-peak luminosity function at $t_{ m d}=1$ day, with the brightest first peak arising from magnetar winds and the other two from BH ejecta anisotropy; the relative peak heights and positions depend on the EOS, with DD2 yielding more magnetar-powered events than SLy. Importantly, magnetar-powered merger-novae can be detected to $z ightarrow 1$–$1.5$, significantly extending the ToO reach compared to BH remnants, which is promising for constraining the NS EOS and merger-nova physics through population-level analyses with CSST and future GW networks.

Abstract

With the rapid advancements in next-generation ground-based gravitational wave (GW) detectors, it is anticipated that $10^3$-$10^5$ binary neutron star (BNS) mergers per year will be detected, with a significant fraction accompanied by observable merger-nova signals through future sky surveys. Merger-novae are typically powered by the radioactive decay of heavy elements synthesized via the r-process. If the post-merger remnant is a long-lived rapid-rotating neutron star, the merger-nova can be significantly enhanced due to strong magnetized winds. In this paper, we generate mock BNS merger samples using binary population synthesis model and classify their post-merger remnants--black hole (BH) and magnetar, (i.e., long-lived supramassive NS and stable NS), based on results from numerical simulations. We then construct merger-nova radiation models to estimate their luminosity function. We find that the luminosity function may exhibit a distinctive triple-peak structure, with the relative positions and heights of these peaks depending on the equation of state (EOS) of the BNS. Furthermore, we estimate the average Target-of-Opportunity (ToO) detection efficiency $\langle f_{\rm eff} \rangle$ with the Chinese Space Station Telescope (CSST) and find that due to possible enhanced luminosity, the largest source redshift with $\langle f_{\rm eff} \rangle>0.1$ can be enlarged from $z_{\rm s}\sim 0.5$ to $z_{\rm s}\sim 1-1.5$. Besides, we also generate the detectable mass spectrum for merger-novae by $\langle f_{\rm eff}\rangle$, which may provide insights to the ToO searching strategy.

Figures (13)

• Figure 1: The normalized redshift evolution of the merger rate density, ${R}(z)$ (solid line), and the number density, $d\dot{N}/{dz}$ (dashed line), of the mock BNS merger samples (marginalized over $m_1$ and $q$) generated by the $\boldsymbol{\alpha10.\rm kb\beta0.9}$ BSE model in 2022MNRAS.509.1557C with observational extinction-corrected SFR in annurev:/content/journals/10.1146/annurev-astro-081811-125615 and the mean metallicity redshift evolution obtained in 2016Natur.534..512B.
• Figure 2: The distributions of the primary ($m_1$) and secondary ($m_2$) masses of BNS mergers are shown for different EOS, i.e., SLy (left panel) and DD2 (right panel). The colored region labeled with "NS", "SMNS" and "BH" represent the mock BNS merger samples that result in a stable NS, a SMNS and a BH remnant, respectively. In both panels, brighter color regions indicate areas with a higher density of samples. The markers with color bars represent the median value and posterior distribution of GW170817 and GW190425, respectively.
• Figure 3: The luminosity function ${d\dot{\Phi}(M_{\rm AB})}/{dM_{\rm AB}dV_{\rm c}}$ of the merger-nova signals at $z_{\rm s}=0.5$ produced by BNS mergers with different EOS, specifically DD2 (green lines) and SLy (pink lines), defined by the intrinsic absolute magnitude at $t_{\rm d}=1$ day in different bands, i.e., $u$, $g$, $r$, $i$ and $z$ bands.
• Figure 4: The normalized probability distribution of BNS mergers with BH remnant associated with merger-nova signals, assuming that the EOS is DD2. The left panel shows the results for the time span above the limiting magnitudes of CSST (if always below the detection threshold, $\Delta T=0$). The right panel shows the results for the localization precision of GW signals with Cosmic Explorer (CE) associated with merger-nova signals with $\Delta T>0$. The red, black, and blue histograms represent the results for the $u$, $r$, and $z$ filters of CSST.
• Figure 5: The legend is the same as in Figure \ref{['fig:dd2_BH']}, except that the merger remnant is a magnetar.