Multifaceted Emission Properties of PSR J2129+4119 Observed with FAST

We present a detailed single-pulse study of the long-period pulsar PSR J2129+4119 using high-sensitivity Five-hundred-meter Aperture Spherical Radio Telescope (FAST) observations. Despite locating well below the traditional death line, the pulsar exhibits sustained and multimodal emission behavior, including nulls, weak pulses, regular emission, and occasional bright pulses. The nulling fraction is measured to be 8.13% ± 0.51%, with null durations typically under four pulse periods. Fluctuation spectral analysis reveals both phase-modulated subpulse drifting and intermittent beatlike modulation. At the same time, polarization profiles show high linear polarization and stable polarization position angle swings consistent with a near-tangential sight line geometry. Quasiperiodic microstructures are detected in 11.54% of regular pulses, with a mean periodicity and width of 4.57 and 4.30 ms, respectively. A well-defined scintillation arc in the secondary spectrum confirms the presence of a localized scattering screen. These results indicate that PSR J2129+4119 remains magnetospherically active and coherently emitting despite its low energy loss rate, offering key insights into pulsar emission physics near the death line.