On the aberration-retardation effects in pulsars

The magnetospheric locations of pulsar radio emission region are not well known. The actual form of the so–called radius–to–frequency mapping should be reflected in the aberration–retardation (A/R) effects that shift and/or delay the photons depending on the emission height in the magnetosphere. Recent studies suggest that in a handful of pulsars the A/R effect can be discerned w.r.t the peak of the central core emission region. To verify these effects in an ensemble of pulsars we launched a project analysing multi–frequency total intensity pulsar profiles obtained from the new observations from the Giant Meterwave Radio Telescope (GMRT), Arecibo Observatory (AO) and archival European Pulsar Network (EPN) data. For all these profiles we measure the shift of the outer cone components with respect to the core component which is necessary for establishing the A/R effect. Within our sample of 23 pulsars 7 show the A/R effects, 12 of them (doubtful cases) show a tendency towards this effect, while the remaining 4 are obvious counter examples. The counter–examples and doubtful cases may arise from uncertainties in determination of the location of the meridional plane and/or the core emission component. It hence appears that the A/R effects are likely to operate in most pulsars from our sample. We conclude that in cases where those effects are present the core emission has to originate below the conal emission region.

💡 Research Summary

The paper investigates the presence of aberration‑retardation (A/R) effects in the radio emission of pulsars, aiming to use these effects as a diagnostic of emission height and to test the radius‑to‑frequency mapping (RFM) hypothesis. The authors assembled a sample of 23 pulsars for which high‑quality, multi‑frequency total‑intensity profiles are available from new observations with the Giant Metrewave Radio Telescope (GMRT), the Arecibo Observatory (AO), and archival data from the European Pulsar Network (EPN). All profiles have sufficient time resolution and signal‑to‑noise ratio to allow reliable component decomposition.

The methodology follows the approach pioneered by Blaskiewicz, Cordes & Wasserman (1991) and later refined by Gangadhara & Gupta (2001, 2003). First, the authors identify the central “core” component, assumed to originate close to the neutron‑star surface, and the outer “cone” components, presumed to be emitted at higher altitudes. The core is located by a combination of criteria: central position in the profile, steep swing of the linear polarisation position angle (PA) near the magnetic meridian, a sign‑changing circular polarisation, and a steeper spectral index than the cones. When possible, multi‑frequency polarisation data (e.g., from Gould & Lyne 1998) and sub‑pulse drift analysis are used to confirm the core identification.

With the core set as phase zero, the leading and trailing cone peaks are measured at phases φ_l and φ_t. The phase separation Δφ = φ_t – |φ_l| quantifies the asymmetry between the two cones relative to the core. According to the relativistic A/R model, this asymmetry directly yields an emission height:

r_AR = – (Δφ / 360°) × (R_LC / 2π),

where R_LC = cP / (2π) is the light‑cylinder radius and P is the pulsar period. Positive Δφ (leading cone ahead of the core) indicates that the cone emission occurs at a higher altitude than the core, consistent with the A/R picture.

The authors discuss three major sources of systematic uncertainty. (1) The exact location of the magnetic meridian (the meridional plane) is not directly observable; the PA inflection point is displaced by A/R itself, so the meridian lies somewhere between the profile midpoint and the PA inflection. (2) Misidentification of the core component can invert the sign of Δφ, especially in cases where a bright central component is actually a conal “partial cone” or where the PA swing is flat. (3) Partial‑cone pulsars, where only one side of the emission cone is visible, lead to an incorrect estimate of the profile midpoint, biasing Δφ.

Applying this procedure, the authors find that 7 of the 23 pulsars exhibit a clear, positive Δφ consistent with A/R. Twelve additional pulsars show a positive Δφ but with lower signal‑to‑noise or ambiguous core identification; these are classified as “possible” A/R cases. The remaining four pulsars display a negative Δφ, i.e., the cones appear to lag the core, contrary to the simple A/R expectation. The authors argue that these counter‑examples can be explained by the uncertainties listed above rather than by a failure of the A/R effect itself.

The paper concludes that A/R effects are likely present in the majority of normal pulsars. When the effect is observed, it implies that the core emission originates at a lower altitude than the conal emission, supporting a hierarchical emission geometry where nested cones are emitted higher in the magnetosphere. The authors suggest that future work should focus on higher‑resolution polarisation studies and single‑pulse analyses to refine core‑cone discrimination, which will in turn tighten constraints on emission heights and the RFM relationship.