Single-pulse Emission Variation of Two Pulsars Discovered by FAST

We investigate the single-pulse emission variations of two pulsars, PSRs J0211+4235 and J0553+4111, observed with the Five-hundred-meter Aperture Spherical radio Telescope at the 1.25 GHz central frequency. The observation sessions span from 2020 December to 2021 July, with 21 and 22 observations for them respectively. The integrated pulse profile of PSR J0211+4235 shows that there is a weak pulse component following the main component, and PSR J0553+4111 displays a bimodal profile with a bridge component in the middle. PSR J0211+4235 presents significant nulling phenomenon with nulling duration lasting from 2 to 115 pulses and burst duration lasting from 2 to 113 pulses. The NF of each observation is determined to be 45%-55%. No emission greater than three s is found in the mean integrated profile of all nulling pulses. In most cases, the pulse energy changes abruptly during the transition from null to burst, while in the transition from burst to null there are two trends: abrupt and gradual. We find that the nulling phenomenon of PSR J0211+4235 is periodic by the Fourier transform of the null and burst state. In addition, the single-pulse modulation characteristics of these two pulsars are investigated, and the distributions of modulation index, LRFS and 2DFS are analyzed with PSRSALSA. The left peak of PSR J0553+4111 has intensity modulation. Finally, the polarization properties of these two pulsars are obtained through polarization calibration, and their characteristics are analyzed. The possible physical mechanisms of these phenomena are discussed.