Periodic Q-mode modulation in PSR J1825$-$0935 (PSR B1822$-$09)

PSR J1825$-$0935 (PSR B1822$-$09) switches between radio-quiet (Q-mode) and radio-bright (B-mode) modes. The Q-mode is known to have a periodicfluctuation that modulates both the interpulse and the main pulse with the same period. Earlier investigators argued that the periodicQ-mode modulation is associated with drifting subpulses. We report on single-pulse observations of PSR J1825$-$0935 that were made using the Parkes 64-m radio telescopewith a central frequency of 1369 MHz. The high-sensitivity observations revealed that the periodicQ-mode modulation is in fact a periodiclongitude-stationary intensity modulationoccurring in the interpulse and the main pulse. The fluctuation spectral analysis showed that the modulation periodis about $43 P_1$, where $P_1$ is the rotation periodof the pulsar. Furthermore, we confirm that the modulation patternsin the interpulse and the main pulse are phase-locked. Specifically, the intensities of the interpulse and the immediately following main pulse are more highly correlated than for the main pulse and interpulse at any other lag. Polarization properties of the strong and weak Q-mode states are different, even for the trailing part of the main pulse which does not show the periodic intensity modulation.