Detectability of Nearby Binary Neutron Stars with Future sub-mHz Gravitational Wave Missions

Abstract

Binary neutron stars (BNSs) are one of the most important gravitational wave (GW) sources, which provide key insights to evolution of massive binary stars and nuclear physics. Beyond Laser Interferometer Space Antenna (LISA), Taiji, and Tianqin missions, proposed concepts for next generation space-based GW observatories, including LISAmax, Folkner, and eASTROD, aim to explore the sub-millihertz (mHz) to microhertz ($μ$ Hz) frequency band. Because the proposed designs substantially suppress low-frequency noise, these detectors are expected to outperform LISA, Taiji, and Tianqin in detecting eccentric Galactic BNS systems. In this paper, we estimate the detectability of nearby inspiraling BNSs using future sub-mHz GW detectors. By utilizing compact binary population synthesis simulations to generate mock BNS samples and estimate their signal-to-noise ratios (SNRs) correspondingly for each GW detector over an observation period of $5-10$\,years, we find that LISAmax may detect $\sim 520-900$ Galactic BNSs, whereas Folkner and eASTROD may detect $\sim 780-1370$ Galactic BNSs. Notably, LISAmax excels in detecting highly eccentric systems $(e>0.90)$ owing to its higher sensitivity at relatively higher sub-mHz frequencies. We further identify seven observed radio BNSs as viable candidates for validation, in particular J0737-3039, which reaches an SNR of $\sim 100$. The expected detection number of LMC inspiraling BNSs is about $\sim 4-18$ for these sub-mHz detectors over an observation period of $5-10$\,years, while detecting inspiraling BNSs in SMC is challenging. This study highlights the significant potential of future sub-mHz GW missions in unraveling BNS formation and evolution physics.

Figures (6)

• Figure 1: The position $(z,r)$ of mock inspiralling BNS in MW cylindrical coordinates at the formation epoch time $t_{\rm f}$ (orange) and present observation time (blue) respectively, considering the orbital adjustment due to the second supernova explosion natal kick $\vec{v}_{\rm k,2}$.
• Figure 2: The joint distribution of orbital periods $P_{\rm orb}$, eccentricity $e$ and distance $d_{\rm L}$ of mock MW inspiralling BNS simulated by CBPS method.
• Figure 3: The characteristic noise strain $h_{\rm noise}$ of future sub-mHz GW detectors with respect to GW frequency $f_{\rm GW}$, i.e., LISAmax (red solid line), Folkner (blue dashed line) and eASTROD (green dashed dotted line). The purple dashed line shows the DWD residual foreground estimated. For comparison, we also plot the $h_{\rm noise}$ of mHz detectors LISA (brown dotted), Taiji (magenta dotted) and Tianqin (grey dotted). The black dots present the effective strain $h_{\rm eff}$ at each harmonics of a BNS inspiral with $e\sim 0.90$ and $P_{\rm orb}\sim 0.5$ days placing at a (typical) distance of $d_{\rm L}\sim 11.8$ kpc.
• Figure 4: The orbital period-eccentricity $P_{\rm orb}-e$ distribution of mock MW inspiralling BNS (filled circles) detected by eASTROD with $T_{\rm obs}=10$ yrs and their color show the logrithemic SNR $\log \rho_{\rm E}$. Other symbols (as labelled at the bottom right) show seven real MW field BNS systems observed by radio telescopes that can be detected by eASTROD, i.e., $\rm B1913+16$ (diamond), $\rm J1757-1854$ (square), $\rm J0509+3801$ (up triangle), $\rm B1534+12$ (left triangle), $\rm J1906+0746$ (right triangle) and $\rm J1947+2052$ (pentagon), and $\rm J0737-3039$. The star symbol represents $\rm J0737-3039$ with the largest SNR of $\sim 100$.
• Figure 5: The effective strain $h_{{\rm eff},n}$ of seven real MW Inspiralling BNS systems with pulsar components detectable by eASTROD assuming $T_{\rm obs}=10$ yr. The markers are the same as in Fig. \ref{['fig:f4']}. The magenta and grey lines show the characteristic noise strain $h_{\rm noise}$ of mHz GW detector Taiji and sub-mHz detector eASTROD respectively. For comparison, dots, plus, and cross symbols show $h_{{\rm eff},n}$ of the mock inspiralling BNS with the largest SNR among those detectable by Taiji in MW, eASTROD in LMC, and MW, respectively.