Detection of GW200105 with a targeted eccentric search

TL;DR

This work tests a targeted eccentric search to evaluate gains from including orbital eccentricity in GW template banks for NSBH mergers. By employing a five-dimensional aligned-spin eccentric bank with $1.553{,}811$ templates and a reweighted-SNR statistic, GW200105 is recovered with exclusive IFARs exceeding 1000 years and a significance around $4.08\sigma$, modestly higher than a parallel quasi-circular search. The eccentric search delivers a substantial increase in sensitive volume for high-eccentricity signals, reducing selection bias against $e_{20}\ge 0.1$ and suggesting an appreciable improvement (approximately $4$–$6$×) over quasi-circular searches in the relevant regime. These results underscore the value of integrating eccentric templates into GW analyses to better characterize dynamical formation channels and to enhance detection prospects for eccentric NSBH mergers, at the cost of increased computational resources. The study also highlights the importance of accurate population-level selection functions for eccentric sources in future hierarchical analyses.

Abstract

The neutron star -- black hole (NSBH) binary GW200105 was recently found to have significant residual orbital eccentricity at a gravitational-wave frequency of 20 Hz~\cite{Morras:2025xfu}. The event was originally identified with moderate significance by matched-filter searches that employ non-eccentric templates. The neglect of relevant physical effects, such as orbital eccentricity, can severely reduce the sensitivity of the search and, consequently, also the significance of an event candidate. Here, we present a targeted eccentric search for GW200105. The eccentric search identifies GW200105 as the most significant event with a signal-to-noise ratio of $13.4$ and a false alarm rate of less than 1 in 1000 years. The best-matching template parameters are consistent with the Bayesian inference result, supporting the interpretation of GW200105 as an NSBH that formed through dynamical mechanisms and not isolated binary evolution.

Figures (6)

• Figure 1: Density distribution of the templates in the eccentric template bank for the targeted search, shown in the $\mathcal{M}^{ecc}_c - \chi_{\rm eff}^{}$ and $\mathcal{M}_c^{}- e^{}_{20}$ spaces in the left and right panels, respectively. The filled diamond denotes the median posterior value for GW200105 from Morras:2025xfu (labeled as MPS 2025), while the star marks the triggering template for GW200105 in each panel. The quasi-circular template bank spans the same parameter space as the eccentric bank in the zero-eccentricity limit.
• Figure 2: The SNR, $\rho$ and $\chi_r^2$ values of noise triggers from each detector during periods of coincident operation of at least two detectors (black), eccentric injections (colored) and the triggers for GW200105 from the quasi-circular and eccentric searches in the left and right panel, respectively. The injections are colored by to the eccentricity of the injected signals at 20 Hz. The blue filled diamond markers indicate the $(\rho, \chi_r^{2})$-values of the GW200105 trigger in the eccentric and quasi-circular search, respectively.
• Figure 4: The ratio of the sensitive spacetime volume between the eccentric bank $\langle VT \rangle^{}_{\rm EC}$ and the quasi-circular bank $\langle VT \rangle^{}_{\rm QC}$, estimated using eccentric injections at various IFAR thresholds. The error bar of each point denotes the $1\sigma$-uncertainty. The eccentric search is significantly more sensitive to eccentric signals than the quasi-circular search beyond an IFAR of $\sim$ 500 years.
• Figure 5: Left: Points with black cross and red plus marks denote the number of triggers with exclusive IFAR greater than or equal to a given value in foregrounds of the quasi-circular and eccentric searches, respectively. The solid black line is the number of expected background events, which is the same for both searches as the live-times are the same for both searches. The gray bands indicate the $1\sigma$ to $4\sigma$ uncertainties. The $x$-axis is a mix of log and linear scales to showcase differences in the significance of GW200105 between the two searches. Right: The rate of events ranked louder or equal to the ranking statistics $\Lambda_s^{}$ in the LV networks' exclusive background for the two searches given by graphs labeled as LV:EC and LV:QC, respectively. The curves with labels L:EC and L:QC denote the same quantity for the LIGO Livingston only background for the eccentric and the quasi-circular searches. The black cross and red plus symbol give the rate of events at the ranking statistics of GW200105 in the foreground of respective searches.
• Figure 6: Differences $(\Delta)$ between parameters of eccentric injections and their best-matching templates in the eccentric search. Contours denote the 50% and 90% confidence intervals, while dashed vertical lines in the one-dimensional marginalized distributions indicate $1\sigma$ regions. Solid lines denote the offset for GW200105 in the eccentric search relative to parameter estimation results of Morras:2025xfu.