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

Interstellar scintillation analysis of pulsars allows us to probe the small-scale distribution and inhomogeneities of the ionized interstellar medium. 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 dataset. We present the long-term time series of $\nu_{\rm d}$ and $\tau_{\rm d}$ 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 $\nu_{\rm d}$ at both bands, which allows us to derive the time series of frequency scaling indices with a mean and a standard deviation of 2.82$\pm$1.95 and 3.18$\pm$0.60, respectively. We found some interesting features which will be studied in more detail in subsequent papers in this series: (i) in the time series of PSR~J1939+2134, where the scintillation bandwidth sharply increases or decreases associated with a sharp change of dispersion measure; (ii) PSR~J0613$-$0200 and PSR~J0636+5126 show a strong annual variation in the time series of the $\tau_{\rm d}$; (iii) PSR~J1939+2134 shows a weak anti-correlation between scintillation timescale and dispersion in WSRT data.