Testing the nature of GW200105 by probing the frequency evolution of eccentricity

TL;DR

This work addresses whether GW200105 truly exhibits orbital eccentricity or whether mimickers could fake such signatures. It applies the model-independent EECT, which compares the GR-predicted eccentricity evolution $e_{ m GR}(f)$ with the observed $e_{ m obs}(f)$ via the deviation $\\delta_e(f)=2\frac{e_{ m GR}(f)-e_{ m obs}(f)}{e_{ m GR}(f)+e_{ m obs}(f)}$, across multiple reference frequencies. The results show $\\delta_e(f)$ is consistent with zero within $90\%$ (and $68\%$) confidence for GW200105 and a GW200105-like injection, lending independent support to the eccentricity hypothesis and validating GR-consistency in the eccentric evolution. This approach offers a computationally efficient alternative to Bayesian model selection for assessing eccentricity, highlighting the importance of GR-based checks and setting the stage for future, more sensitive tests with enhanced detector sensitivity.

Abstract

GW200105 is a compact binary coalescence (CBC) event, consisting of a neutron star and a black hole, observed in LIGO-Virgo-KAGRA's (LVK's) third observing run (O3). Recent reanalyses of the event using state-of-the-art waveform models have claimed observation of signatures of an eccentric orbit. It has nevertheless been pointed out in the literature that certain physical or modified gravity effects could mimic eccentricity by producing a spurious non-zero eccentricity value, at a given reference frequency, when recovered with an eccentric waveform model. We recently developed a model-independent Eccentricity Evolution Consistency Test (EECT, S. A. Bhat et al. 2025) to identify such mimickers, by comparing the measured frequency $\textit{evolution}$ of eccentricity, $e(f)$, with that expected from General Relativity (GR). In this $\textit{Letter}$, we apply EECT to GW200105 and find that it satisfies EECT within 68% confidence. Our analysis therefore lends complementary support in favour of the eccentricity hypothesis, while also providing a novel test of the consistency of $e(f)$ with GR.

Figures (5)

• Figure 1: Left: Violins of the eccentricity deviation $\delta_e$ at different reference frequencies. The horizontal line at $\delta_e = 0$ represents zero deviation from GR. Right: Individual $e_{\rm obs}\xspace$ (right side) and $e_{\rm GR}\xspace$ (left side) posteriors at different reference frequencies for the same. In both panels, the dashed and the dotted lines represent the 90% and 68% credible intervals (CI), respectively.
• Figure 2: Same as Figure \ref{['fig: violin_plot_ps']}, but for a GW200105-like injection. Left: Violins of eccentricity deviation $\delta_e$ at different reference frequencies. The horizontal line at $\delta_e = 0$ represents zero deviation from GR. Right: Individual $e_{\rm obs}\xspace$ (right side) and $e_{\rm GR}\xspace$ (left side) posteriors at different reference frequencies for the same. In both panels, the dashed and the dotted lines represent the 90% and 68% credible intervals (CI), respectively. The vertical dash-dotted gray lines between 30 and 35 Hz indicate the onset of $e_{\rm obs}\xspace$ posteriors (35 and 40 Hz ones) that fail the zero-exclusion criterion described in Section \ref{['sec:method']}.
• Figure 3: Priors used for PE of GW200105. $\mathcal{U}(a, b)$ refers to uniform distribution in the range $(a, b)$ while PL$(a,b)$ refers to a power law distribution in the range $(a,b)$.
• Figure 4: Detector-frame chirp mass posteriors recovered at different reference frequencies.
• Figure 5: Corner plot of posteriors recovered at 18 Hz (blue) and 25 Hz (maroon) for GW200105.