Orateur
Description
By monitoring the times of arrival of radio pulses from millisecond
pulsars, Pulsar Timing Arrays (PTAs) serve as unique gravitational wave
(GW) laboratories in the nanohertz band. To date, the primary
astrophysical sources of GWs targeted in this frequency range have been
inspiraling supermassive black hole binaries (SMBHBs) on circular or
eccentric orbits. In this work, we demonstrate that, thanks to the
so-called pulsar term in the timing residual waveform of GW signals,
PTAs can probe individual SMBHBs that merged before timing observations
began. We refer to the latter as zombie binaries. Using SMBHB population
models consistent with current PTA constraints, we find that while the
probability of detecting such systems in existing PTA datasets remains
low, the Square Kilometer Array observatory is expected to achieve
sufficient sensitivity to have a few zombie binaries with a
signal-to-noise ratio exceeding 3 in its data. Although their confident
identification might be challenging, this new class of PTA sources opens
a novel window for studying the most massive SMBHBs in our local universe.