Complete Second-Order Relativistic Derivation of the Observed Pulsar Timing Modulations

Author(s)

Magi, Matteo, Yoo, Jaiyul

Abstract

We present a fully nonlinear relativistic description of the observed pulsar timing modulations, defined through the ratio of the proper-time intervals between two successive emissions of radio pulses and their observations by a pulsar timing array. Working in the limit of a vanishing emission interval, we derive the observed pulsar timing modulation to second order in relativistic perturbation theory around a Minkowski background, without fixing a gauge and retaining the full scalar, vector, and tensor content of the metric perturbations. Although the derivation naturally introduces several gauge-dependent intermediate quantities, we explicitly demonstrate that they combine into a coordinate-independent expression. Moreover, we explicitly show that the second-order timing modulation from two neighboring radio pulses coincides with the observed redshift, defined as the fractional change in wavelength along a single geodesic. The agreement represents a nontrivial second-order realization of the exact nonlinear equivalence proven in Magi & Yoo 2026.

Figures

Caption

Illustration of the geometric setup: ${O}(\tau)$ is the observer worldline parametrized by its proper time~$\tau$, while ${S}(\sigma)$ is the pulsar worldline parametrized by~$\sigma$. The propagation of two radio pulses is described by two null geodesics~$\gamma(\lam)$ and $\gamma'(\lam')$ with the corresponding affine parameters~$\lam$ and~$\lam'$.