Exploring the remarkable subpulse drift and polarization properties of PSR B0818−41

PSR B0818−41 is one of the few pulsars which show multiple drift regions having well defined phase relationship. In this paper we report new results from the multifrequency observations of this pulsar with the Giant Metrewave Radio Telescope (GMRT). We determine the mean flux of the pulsar at multiple frequencies and find evidence for a low frequency turnover in its spectrum. Significant linear polarization is observed with depolarization at the edge of the profile, which is likely to be due to orthogonal polarization mode jumps. Circular polarization changes sign near the middle of the pulse profile at 1060 MHz which is not observed at 325 and 610 MHz. Remarkable frequency evolution of polarization angle (PA) is observed for this pulsar, which is commonly not seen in other pulsars. Based on the frequency evolution of average profile, observed PA swing and results from subpulse drifting, we propose two possible emission geometries, α ∼11, β ∼ −5.4and α ∼175.4, β ∼ −6.9. Simulation of the pulsar radiation pattern with both these geometries reproduces the observed features in the drift pattern quite well. In addition to the remarkable subpulse drifting observed at 325 MHz, we report subpulse drifting at 244 and 610 MHz. At 244 MHz we observe drifting in the outer regions but not in the inner region whereas at 610 MHz subpulse drift is observed in both inner and outer regions. Sometimes, we see changes of drift rates, transitions from negative to stationary and stationary to negative drift rates, many of which appear to have some connection with nulls. We determine Pm 3 (the measured time interval between the recurrence of successive drift bands at a given pulse longitude), Pm 2 (the measured longitude separation between two adjacent drift bands) and ∆Φ (subpulse width) values at different frequencies for both inner and outer drift regions. Though Pm 3 is the same for all the drift regions, P m 2 and ∆Φ values are different. The peak emission from the leading and the trailing outer regions are offset by ∼ 9 P1. Utilising this information, we solve the aliasing problem and argue that drifting is first order aliased with corresponding carousel rotation period P4 ∼ 10 s, which makes it the fastest known carousel. The drift pattern from the two rings are phase locked for PSR B0818−41. The same is found to be true for, all pulsars showing drifting in multiple rings of emission. This finding puts constraints on the theoretical models of pulsar emission mechanism and, favors a pan magnetospeheric radiation mechanism.