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 are substantial observational data to support several observable 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 in the DM range 3-35 pc cm^-3 are used to test the relevant theoretical predictions. The variabilities of decorrelation bandwidth (v_d)scintillation timescale (τ_d), and flux density (F) are examined for their cross-correlation properties and compared with the existing predictions. The theory predicts a strong correlation between the fluctuations of v_d and τ_d and strong anticorrelations between those of v_d and F, and τ_dand F. For five pulsars, we see a reasonable agreement with the predictions. There is considerable difficulty in reconciling the results for the rest of the pulsars, most of which show the positive correlation between v_d and τ_d but are characterized by poor flux correlations. In general, the measured correlations are lower than the predicted values. Our analysis shows that while the underlying noise sources can sometimes reduce the degree of correlation, they cannot give rise to an absence of correlation. It is also unlikely that the observed poor flux correlations arise from a hitherto unrecognized form of intrinsic flux variations of pulsars. For PSR B0834+06, which shows anomalous behavior in the form of persistent drift slopes, positive correlation is found between τ_d and the drift-corrected v_d. Many pulsars show an anticorrelation between the fluctuations of v_d and the drift rate of intensity patterns, and this is in accordance with simple minded expectations from theory. The detections of correlations between the fluctuations of different observables and a reasonable agreement seen 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 cross-correlations.