Archival Multiband Gravitational-Wave Signals from Massive Black Hole Binary Mergers

TL;DR

This paper explores how low-frequency signals from massive black hole binary mergers, undetectable directly, can be reconstructed from pulsar timing array data by analyzing the pulsar term, enabling multiband gravitational-wave astronomy.

Contribution

It introduces a method to extract and stack orphaned pulsar terms from PTA data for multiband gravitational-wave analysis of MBHB mergers.

Findings

Pulsar terms contain low-frequency signals from MBHBs above PTA sensitivity.

Stacking pulsar terms enhances detection prospects for multiband signals.

Potential for archival searches combining PTA data with LISA and astrometric observations.

Abstract

While massive black hole binaries (MBHBs) merge at gravitational-wave frequencies above the pulsar timing array (PTA) sensitivity band, we show that they leave orphaned low-frequency contributions in the PTA pulsar term. Due to the light-propagation time between each pulsar in the array and Earth, the pulsar term acts as a time-delayed probe of a chirping merger with a specific frequency response determined by the direction of origin and intrinsic properties of the MBHB. We provide a detailed consideration of how such a multiband signal would manifest in a full PTA, demonstrate an approach to stack these orphaned pulsar terms across the array, and discuss prospects for an archival, multiband search in conjunction with MBHB mergers observed in astrometric data or spaceborne interferometers like LISA.