External authors:

• H. M. Tedila ( National Astronomical Observatory, CAS , Arba Minch University )
• D. Li ( Tsinghua University , National Astronomical Observatory, CAS , State Key Lab Radio Astron & Technol )
• P. Wang ( Beijing Normal University , National Astronomical Observatory, CAS , State Key Lab Radio Astron & Technol )
• R. Yuen ( Xinjiang Astronomy Observatory, CAS , Xinjiang Key Lab Radio Astrophys )
• Z. W. Wu ( National Astronomical Observatory, CAS )
• S. J. Dang ( Guizhou Normal University )
• J. P. Yuan ( Xinjiang Astronomy Observatory, CAS , Xinjiang Key Lab Radio Astrophys )
• N. Wang ( Xinjiang Astronomy Observatory, CAS , Xinjiang Key Lab Radio Astrophys )
• J. S. Zhang ( University of Chinese Academy of Sciences, CAS , National Astronomical Observatory, CAS )
• J. T. Bai ( Henan Academy of Sciences )
• D. Zhao ( Xinjiang Astronomy Observatory, CAS )

Abstract:

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.