Systematic biases in parameter estimation on LISA binaries. II. The effect of excluding higher harmonics for spin-aligned, high-mass binaries

TL;DR

This paper investigates how neglecting higher-order waveform modes affects parameter estimation of massive black hole binaries with LISA, revealing significant biases especially for high-mass, short signals, and proposes an improved analysis method.

Contribution

It extends previous work to include aligned spins and high-mass binaries, showing how higher modes impact parameter biases and introducing a more efficient likelihood optimization scheme.

Findings

Neglecting higher modes causes significant parameter biases in high-mass binaries.

Systematic errors can severely mislead sky localization for the heaviest systems.

Proposed method improves computational efficiency in predicting waveform-induced biases.

Abstract

The Laser Interferometer Space Antenna (LISA) will observe massive black hole binaries (MBHBs) with astoundingly high signal-to-noise ratio, leaving parameter estimation with these signals susceptible to seemingly small waveform errors. Of particular concern for MBHBs are errors due to neglected higher-order modes. We extend Yi et al. [arXiv:2502.12237] to examine errors due to neglected higher-order modes for MBHBs with nonzero (aligned) progenitor spins and total mass up to $10^8\,M_\odot$. For these very massive systems, there can be regions of parameter space in which the $(\ell, |m|)=(2,\,2)$ modes are no longer dominant with respect to higher-order ones. We find that the extent of systematic bias can change significantly when varying the progenitor spins of the binary. We also find that for the heaviest, and therefore shortest, MBHB signals, slight systematic errors can cause severe mis-inference of the sky localization parameters. We propose an improved likelihood optimization scheme with respect to previous work as a way to predict these effects in a computationally efficient manner.