Long-term scintillation studies of EPTA pulsars I. Observations and basic results

Context. Interstellar scintillation analysis of pulsars allows us to probe the small-scale distribution and inhomogeneities of the ionized interstellar medium. From the frequency scale of scintillation, one can estimate the geometric time delays from multipath propagation, a source of (typically) unmodeled, correlated noise in pulsar timing. Interstellar scintillation analysis of well-timed pulsars is useful to quantify the effects of time delays and may lead to improved timing precision, enhancing the probability of detecting gravitational waves. Aims. Our priority is to present the data set and the basic measurements of scintillation parameters of pulsars, employing long-term scintillation observations carried out from 2011 January to 2020 August by the European Pulsar Timing Array radio telescopes in the 21-cm and 11-cm bands. Additionally, we aim to identify future possible lines of study using this long-term scintillation data set. Methods. The autocorrelation function of dynamic spectra has been used to estimate the scintillation bandwidth v(d) and scintillation timescale tau(d). Results. We present the long-term time series of v(d) and tau(d) for 13 pulsars. Sanity checks and comparisons indicate that the scintillation parameters of our work and previously published works are mostly consistent. For two pulsars, PSRs J1857+0943 and J1939+2134, we were able to obtain measurements of the v(d) at both bands, which allowed us to derive the time series of frequency scaling indices with a mean and a standard deviation of 2.82 +/- 1.95 and 3.18 +/- 0.60, respectively. We found some interesting features which will be studied in more detail in subsequent papers in this series: (i) in the time series of PSR J1939+2134, where v(d) and tau(d) sharply decrease associated with a sharp increase in the dispersion measure; (ii) PSR J0613-0200 and PSR J0636+5126 show a strong annual variation in the time series of the tau(d); and (iii) PSR J1939+2134 shows a weak anticorrelation between the scintillation timescale and the dispersion in Westerbork Synthesis Radio Telescope data.