The Earth term in pulsar timing residuals is out of phase among the pulsars

Abstract

We aim to resolve a misunderstanding about whether the so-called "Earth term" in the pulsar timing response to a gravitational wave is in phase among a set of pulsars. We note that the misunderstanding has potentially arisen from the statements that the Earth term is "coherent" or "builds up coherently" among the pulsars. We clarify what authors mean by "coherent" in these statements, pointing out that "coherent" does not indicate that the Earth terms are in phase among the pulsars. Using the pulsar timing residuals induced by a continuous gravitational wave, we show that the Earth term does not align across different pulsars except for the special case when the gravitational-wave source is edge-on, i.e. when the orbital inclination angle of the source is either $\iota=\pi/2$ or $3\pi/2$. We demonstrate the same concept using the pulsar timing software libstempo by plotting the Earth terms from a set of pulsars.