Pulsar scintillation studies with LOFAR. III. Annual variations in PSR J0814+7429

The interstellar scintillation observed in radio pulsars arises from interference between electromagnetic waves scattered by electron density fluctuations in the turbulent interstellar plasma, providing a critical tool for probing the small-scale structure of the ionized interstellar medium and the pulsar system itself. The primary aim of this work is to study long-term scintillation variations for a bright and nearby pulsar, PSR J0814$+$7429, carried out from September 2013 to September 2023 with the LOw-Frequency ARray (LOFAR) High Band Antennae in the frequency range 120 - 170 MHz. We derived the basic scintillation parameters, scintillation bandwidth (Δν_̊m d), and scintillation timescale (Δτ_̊m d) from the two-dimensional (2D) auto-covariance function of the dynamic spectra that are a 2D matrix of pulse intensity as a function of time and frequency. We present a long-term monitoring study of Δν_̊m d and Δτ_̊m d for PSR J0814$+7429$, which shows a strong annual variation in the time series of the Δτ_̊m d. From our modeling of the annual variations of scintillation velocities, the scattering screen is anisotropic and located at 0.23 kpc from the Earth, likely corresponding to the boundary of the Local Bubble.