Long-Term Scintillation Studies of Pulsars: III. Testing Theoretical Models of Refractive Scintillation

Abstract

Refractive interstellar scintillation (RISS) is thought to be the cause behind a variety of phenomena seen at radio wavelengths in pulsars and compact radio sources. Though there is substantial observational data to support several consequences of it, the quantitative predictions from theories have not been thoroughly tested. In this paper, data from our long-term scintillation study of 18 pulsars are used to test the predictions. The fluctuations of decorrelation bandwidth ($ν_d$), scintillation time scale ($τ_d$) and flux density (F) are examined for their cross-correlations and compared with the predictions. The theory predicts a strong correlation between $ν_d$ and $τ_d$, and strong anti-correlations between $ν_d$ and F, and $τ_d$ and F. For 5 pulsars, we see a reasonable agreement. There is considerable difficulty in reconciling the results for the rest of the pulsars. Our analysis shows the underlying noise sources can sometimes reduce the correlation, but cannot cause an absence of correlation. It is also unlikely that the poor flux correlations arise from a hitherto unrecognized intrinsic flux variations. For PSR B0834+06, which shows anomalous behaviour of persistent drift slopes, positive correlation is found between $τ_d$ and the drift-corrected $ν_d$. Many pulsars show an anti-correlation between $ν_d$ and the drift slope, and this is in accordance with the simple models of RISS. The detections of correlated variations of observables and a reasonable agreement between the predicted and measured correlations for some pulsars confirm RISS as the primary cause of the observed fluctuations. However, the complexity seen with the detailed results suggests the necessity of more comprehensive theoretical treatments for describing refractive fluctuations and their correlations.