Pulsar timing residual ($r_n=t_n^{\mathrm{arr}}-t_n^{\mathrm{model}}$)

The difference between an observed arrival time and its model prediction. Positive pulsar timing residuals mean late arrivals in this convention. A periodic residual can expose an omitted orbit; a smooth long-term residual can also come from a wrong spin model or clock correction. Fitting and checking alternative causes are part of the inference, rather than interpreting every residual as gravitational radiation.