Physical origin of a negative chromatic index process in PSR J1012+5307

Identify the physical mechanism that produces an apparent chromatic timing process with a negative radio-frequency scaling index (χ < 0) in PSR J1012+5307, as inferred from anti-correlations between achromatic red noise and chromatic noise when analyzing the NANOGrav 15 yr dataset with both DMX and CustomGP models, and determine how such a mechanism manifests in pulsar timing residuals and impacts noise modeling.

Background

The paper analyzes six pulsars from the NANOGrav 15 yr dataset using multiple chromatic noise models and finds that PSR J1012+5307 exhibits anti-correlated behavior between achromatic red noise and a chromatic component. This behavior is consistent with a chromatic process that scales with radio frequency with a negative exponent (χ < 0), which is unusual relative to standard interstellar medium effects such as dispersion (χ = 2) or scattering (χ ≈ 4).

The authors note that, while the behavior is consistent with χ < 0, they do not know which physical process could cause such a scaling. They refer to independent evidence from EPTA DR2+InPTA DR1 analyses that also favor a negative chromatic index for this pulsar, underscoring the significance of understanding the physical origin to improve pulsar noise modeling and gravitational-wave inference.

References

The behavior is consistent with the presence of a χ < 0 chromatic process as explored at the end of Appendix~\ref{appendix:chromatic_modeling}. Its unknown to us what type of physical process this may correspond to, however this idea is supported by \citetalias{EPTA_noise}, who found χ = -0.65{+0.46}_{-0.41} after a free-chromatic analysis of PSR J1012+5307 from the joint EPTA DR2 and InPTA DR1 dataset \citep{Tarafdar+2022, EPTA_data}.

— The NANOGrav 15 yr Data Set: Chromatic Gaussian Process Noise Models for Six Pulsars (2405.14941 - Larsen et al., 23 May 2024) in Section 6 (Summary and Discussion)