Unraveling the drift behaviour of the remarkable pulsar PSR B0826–34

Abstract

Abstract. Pulsars with drifting subpulses are thought to be an important key to unlocking the mystery of how radio pulsars work. We present new results from high sensitivity GMRT observations of PSR B0826−34 -a wide profile pulsar that exhibits an interesting but complicated drifting pattern. We provide a model to explain the observed subpulse drift properties of this pulsar, including the apparent reversals of the drift direction. In this model, PSR B0826−34 is close to being an aligned rotator. Using information about the polarization and frequency evolution of the pulse profile, we solve for the emission geometry of this pulsar and show that the angle between the rotation axis and the dipole magnetic axis is less than 5• . As a result, our line of sight samples a circular path that is entirely within the emission beam. We see evidence for as many as 6 to 7 drifting bands in the main pulse at 318 MHz, which are all part of a circulating system of about 15 spark-associated subpulse emission beams that form, upon averaging, one conal ring of the mean emission. We also see evidence for a second ring of emission, which becomes dominant at higher frequencies (above 1 GHz) due to the nature of the emission geometry. We model the subpulse drift behaviour of this pulsar in detail, providing quantitative treatments of the aliasing problem and various effects of geometry which play an important role. The observed drift rate is an aliased version of the true drift rate which is such that a subpulse drifts to the location of the adjacent subpulse (or a multiple thereof) in about one pulsar period. We show that small variations, of the order of 3-8%, in the mean drift rate are then enough to explain the apparent reversals of drift direction seen in the data. We find the mean circulation time of the drift pattern to be significantly longer than the predictions of the original Ruderman & Sutherland (1975) model and propose an explanation for this, which relates to modified models with temperature regulated partial ion flow in the polar vacuum gap. The small variations in drift rate are then explained by very small heating and cooling effects -less than 3500 K change in the ∼2.5 × 10 6 K surface temperature of the neutron star polar cap. From a detailed consideration of the variation of the mean subpulse separation across the main pulse window, we show that the circulating spark pattern is not centred around the dipole axis, but around a point much closer (within a degree or so) to the rotation axis. This is an indicator of the presence of a "local pole" corresponding to the non-dipolar magnetic fields that are expected to be present close to the neutron star surface. PSR B0826−34 thus provides a very rich and powerful system in which to explore important aspects of the physics of pulsar radio emission and neutron star magnetospheres.

The topology and polarisation of subbeams associated with the “drifting” subpulse emission of pulsar B0943+10

“…The model implies that the cooling process of the star's surface below the mean temperature leads to a faster than mean drift rate, while the reverse process of heating results in slower than mean drift rates. The drift-rate and direction changes of B0826-34 at 325 MHz were discussed by Gupta et al (2004). A typical time scale for one cycle of drift-rate reversal is about 100 stellar rotation periods, or nearly 3 min for this pulsar.…”

“…In most other such pulsars driftrate variations are associated only with smaller and less orderly changes in the amplitude, shape and/or phase of the pulse profile. Systematic variations of the drift rate in B0826-34 (and probably B2303+30) apparently cross alias boundaries and thus produce changes in the drift direction (Esamdin et al 2004;Gupta et al 2004). An even stronger similarity with B0943+10 is the discovery by Esamdin et al that B0826-34's long intervals of "null" pulses are in fact a weak Q-like mode, which has a very different profile from that of the strong, drifting mode (see their Fig.…”

Pulsar bi-drifting: implications for polar cap geometry

“…The emission of the complex pulsar B0826-34 (Biggs et al 1985, Gupta et al 2004, Esamdin et al 2005, Bhattacharrya et al 2008, van Leeuwen & Timokhin 2012) extends throughout most of its pulse. It is possible to discern up to 13 distinct driftbands across the profile and the behaviour of this pulsar is undoubtedly strong evidence for the existence of polar cap carousels.…”

Investigation of the unique nulling properties of PSR B0818−41

“…So, to explain 5% slow-down of drift in B0818−41 just before the onset of the nulls, one would have to invoke about 0.1 % polar cap surface temperature variation. Based on the PSG model Gupta et al (2004) argued (see their section 4.4) that only about 0.14 % change in the surface polar cap temperature (i.e. a change of about 4000 K around 2×10…”