Testing the nature of compact objects in the lower mass gap using gravitational wave observations

As the compact binary catalog continues to grow rapidly, developing and refining tests to probe the nature of compact objects is essential for a comprehensive understanding of both the observed data and the underlying astrophysics of the binary population. We investigate the effectiveness of spin-induced multipole moments (SIQM) and tidal deformability measurements in distinguishing lower mass-gap black hole (BH) binaries from non-BH binaries with different mass and spin configurations. We perform model-agnostic tests on binary BH (BBH) simulations using full Bayesian inference, evaluating the independent and joint measurability of SIQM and tidal parameters across the parameter space. We extend the analysis to simulations of self-interacting spinning boson stars, using synthetic signals that exhibit (a) both SIQM and tidal effects and (b) each effect individually. For case (a), recovery is performed using (i) a BBH model, (ii) a model incorporating both SIQM and tidal effects, and (iii) models including either SIQM or tidal effects. For case (b), we employ (i) a BBH model and (ii) models incorporating either SIQM or tidal effects, consistent with the injection. Simulations assume binaries of total mass of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>8</a:mn> <a:msub> <a:mi>M</a:mi> <a:mo stretchy="false">⊙</a:mo> </a:msub> </a:math> , varying mass ratios and spin magnitudes, using the inspiral-only aylor2 waveform model. A three-detector network consists of Advanced LIGO and Virgo at design sensitivity is assumed, with results scalable to future detector sensitivities. We find that employing an incorrect model to analyze the signal can lead to severe biases in parameter inference. Notably, when analyzing a simulated binary boson starlike signal with component masses <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mo stretchy="false">(</d:mo> <d:mn>4</d:mn> <d:mo>,</d:mo> <d:mn>4</d:mn> <d:mo stretchy="false">)</d:mo> <d:msub> <d:mi>M</d:mi> <d:mo stretchy="false">⊙</d:mo> </d:msub> </d:math> using a BBH model, the system is incorrectly identified as having masses <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mo stretchy="false">(</i:mo> <i:mn>8</i:mn> <i:mo>,</i:mo> <i:mn>2</i:mn> <i:mo stretchy="false">)</i:mo> <i:msub> <i:mi>M</i:mi> <i:mo stretchy="false">⊙</i:mo> </i:msub> </i:math> . In contrast, using the correct recovery model that includes both SIQM and tidal deformability effects, successfully recovers the true masses, highlighting the significance of waveform model accuracy in performing reliable distinguishability tests for compact objects in the low-mass gap.