Long-term scintillation observations of five pulsars at 1540 MHz

From 2001 January to 2002 June, we monitored PSRs B0329+54, B0823+26, B1929+10, B2020+28 and B2021+51 using the Nanshan 25-m radio telescope of the Urumqi Observatory to study their diffractive interstellar scintillation (DISS). The average interval between observations was about 9 d and the observation duration ranged between 2 and 6 h depending on the pulsar. Wide variations in the DISS parameters were observed over the 18-month data span. Despite this, the average scintillation velocities are in excellent agreement with the proper motion velocities. The average two-dimensional autocorrelation function for PSR B0329+54 is well described by a thin-screen Kolmogorov model, at least along the time and frequency axes. Observed modulation indices for the DISS time-scale and bandwidth and the pulsar flux density are greater than values predicted for a Kolmogorov spectrum of electron density fluctuations. Correlated variations over times that are long compared to the nominal refractive scintillation time are observed, suggesting that larger scale density fluctuations are important. For these pulsars, the scintillation bandwidth as a function of frequency has a power-law index  (～3.6)  much less than that expected for Kolmogorov turbulence (～4.4). Sloping fringes are commonly observed in the dynamic spectra, especially for PSR B0329+54. The detected range of fringe slopes are limited by our observing resolution. Our observations are sensitive to larger-scale fringes and hence smaller refractive angles, corresponding to the central part of the scattering disc.