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

Abstract

PSR B0818-41 is one of the few pulsars that show multiple drift regions having well-defined phase relationship. In this paper, we report new results from 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. A 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, the observed PA swing and the results from subpulse drifting, we propose two possible emission geometries, α~11 °, β~5 °.4 and α~ 175 °.4, β~-6° 9 and compare those in detail with observed properties. 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 apparent drift rates, transitions from negative to stationary (phase stationary intensity modulation) and stationary to negative drift rates, many of which appear to have some connection with nulls. We determine Pm3 (the measured time interval between the recurrence of successive drift bands at a given pulse longitude), P^m₂ (the measured longitude separation between two adjacent drift bands) and Δ Φ (subpulse width) values at different frequencies for both inner and outer drift regions. Although P^m₃ is the same for all the drift regions, P^m₂ and ΔΦ values are different. The peak emission from the leading and the trailing outer regions is offset by ~9 P₁. Utilizing 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 is 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 favours a pan magnetospheric radiation mechanism.