TDI-infinity: time-delay interferometry without delays

TL;DR

This paper introduces TDI-infinity, a novel approach to time-delay interferometry for LISA that simplifies modeling, handles data gaps, and generalizes to arbitrary armlength variations by directly formulating the likelihood in terms of basic phase measurements.

Contribution

It presents an implicit, numerical formulation of TDI that marginalizes over laser noise and accommodates complex orbital dynamics and data gaps.

Findings

Simplifies TDI modeling for LISA.

Handles data gaps and missing measurements.

Generalizes to arbitrary armlength variations.

Abstract

The space-based gravitational-wave observatory LISA relies on a form of synthetic interferometry (time-delay interferometry, or TDI) where the otherwise overwhelming laser phase noise is canceled by linear combinations of appropriately delayed phase measurements. These observables grow in length and complexity as the realistic features of the LISA orbits are taken into account. In this paper we outline an implicit formulation of TDI where we write the LISA likelihood directly in terms of the basic phase measurements, and we marginalize over the laser phase noises in the limit of infinite laser-noise variance. Equivalently, we rely on TDI observables that are defined numerically (rather than algebraically) from a discrete-filter representation of the laser propagation delays. Our method generalizes to any time dependence of the armlengths; it simplifies the modeling of gravitational-wave signals; and it allows a straightforward treatment of data gaps and missing measurements.