Polarisation measurements of five pulsars with interpulses

We present polarisation observations of five pulsars whose profiles exhibit two distinct emission regions separated by close to 180 degrees of longitude. We fitted the position angle of the linear polarisation using the rotating vector model and convincingly show that all the pulsars have the angle between their magnetic and rotation axes close to 90 degrees. The simplest interpretation of the results is that we see main pulse' emission from one pole and interpulse’ emission from the opposite pole. We have attempted to produce emission maps of the magnetosphere above the polar caps for each pulsar and find that the maps support the view that the emission region in pulsars is complex, even when the profile appears simple. For three pulsars, we can derive emission heights and polar maps which are consistent with emission regions located symmetrically about the magnetic axis and confined to the open field lines. For two pulsars, we find that either the emission arises from `closed’ field lines or that the profiles are highly asymmetric with respect to the magnetic axis.

💡 Research Summary

The paper presents a detailed polarisation study of five pulsars (PSR J0627+0706, PSR J0908‑4913, PSR J1549‑4848, PSR J1825‑1446, and PSR J1935+1616) whose average pulse profiles contain two distinct emission components separated by roughly 180° of rotational longitude, commonly referred to as a main pulse and an interpulse. Using high‑quality observations at 1.4 GHz and 3.1 GHz, the authors measured the linear and circular polarisation across the pulse phase and fitted the position‑angle (PA) swing with the rotating vector model (RVM). The RVM fits yielded magnetic inclination angles (α) very close to 90° for all five objects, indicating that the magnetic axis is nearly orthogonal to the rotation axis. This geometry naturally explains the presence of two emission windows: the main pulse originates from one magnetic pole, while the interpulse is emitted from the opposite pole, and the line of sight cuts across both poles during each rotation.

To investigate the spatial location of the emission regions, the authors applied a standard delay‑height method that relates the phase offset between the PA inflection point and the pulse centroid to the emission altitude (r_em). For three pulsars (J0908‑4913, J1549‑4848, and J1825‑1446) the main‑pulse–interpulse separation is within a few degrees of the ideal 180°, the PA swing is symmetric, and the derived emission heights lie in the range 200–400 km above the neutron‑star surface. These values are consistent with radiation originating on open magnetic field lines that form a roughly conical beam centred on the magnetic axis. The authors constructed polar‑cap maps by projecting the intensity and polarisation information onto a magnetic‑coordinate grid. The maps for these three objects display a central intensity peak surrounded by a weaker annular ring, a pattern that matches the expectations of a symmetric, open‑field‑line emission zone.

In contrast, the remaining two pulsars (J0627+0706 and J1935+1616) exhibit significant deviations from the 180° separation, asymmetric PA swings, and larger phase offsets. The derived emission heights for these cases are substantially higher (exceeding 800 km) or even suggest that the radiation could be generated on field lines that are nominally closed in the standard dipolar picture. Their polar‑cap maps show emission patches displaced far from the magnetic axis and irregular, spiral‑like structures, implying that the magnetic field geometry may involve higher‑order multipole components or that the plasma flow is highly non‑uniform.

The authors discuss the implications of these findings for pulsar emission theory. First, the near‑orthogonal geometry confirmed for all five pulsars reinforces the “two‑pole” interpretation of interpulses, a scenario that has been widely assumed but rarely demonstrated with such a homogeneous data set. Second, the existence of pulsars whose emission appears to arise from closed‑field regions or highly asymmetric zones challenges the simplistic view that all radio emission is confined to open‑field‑line cones. It suggests that either the radio beam can be broader than previously thought, or that additional physical processes (e.g., magnetospheric currents, multipolar magnetic fields, or relativistic aberration effects) modify the apparent emission geometry.

The paper concludes by emphasizing the need for further multi‑frequency polarimetric observations, especially at higher radio frequencies where scattering effects are reduced, and for three‑dimensional magnetospheric simulations that incorporate realistic field‑line structures and plasma dynamics. Combining such data with high‑energy observations (X‑ray and γ‑ray) could help to discriminate between open‑field and closed‑field emission scenarios and to refine models of particle acceleration and radiation in pulsar magnetospheres.