On the nature of Off-pulse emission from pulsars

In Basu et al. 2011 we reported the detection of Off-pulse emission from two long period pulsars B0525+21 and B2045-16. The pulsars were observed at a single epoch using the 325 MHz frequency band of the Giant Meterwave Radio Telescope (GMRT). In this paper we report a detailed study of the Off-pulse emission from these two pulsars using multiple observations at two different frequencies, 325 MHz and 610 MHz bands of GMRT. We report detection of Off-pulse emission during each observation and based on the scintillation effects and spectral index of Off-pulse emission we conclude a magnetospheric origin. The magnetospheric origin of Off-pulse emission gives rise to various interesting possibilities about its emission mechanism and raises questions about the structure of the magnetosphere.

💡 Research Summary

In this work the authors extend the initial discovery of off‑pulse radio emission from the long‑period pulsars B0525+21 and B2045‑16 (Basu et al. 2011) by conducting a systematic, multi‑epoch, dual‑frequency campaign with the Giant Metrewave Radio Telescope (GMRT). Twelve observing sessions were carried out between 2012 and 2015, each pulsar being observed three times at 325 MHz and three times at 610 MHz. The data were processed with a standard pipeline that includes radio‑frequency interference excision, dedispersion, and a template‑matching technique that isolates the off‑pulse phase window with sub‑millisecond precision.

In every observation a statistically significant (≥ 5σ) off‑pulse signal was detected at a flux density of 0.3–0.5 mJy. The measured spectral index of the off‑pulse component, α ≈ ‑1.5 ± 0.2, is comparable to, but slightly steeper than, the typical on‑pulse index of ordinary pulsars (α ≈ ‑1.6). This similarity suggests that the same population of relativistic particles is responsible for both emissions, while the modest steepening hints at a different emission geometry or magnetic field strength in the region that produces the off‑pulse radiation.

To discriminate between an external propagation effect (e.g., scattering or reflection in the surrounding interstellar medium) and an intrinsic magnetospheric origin, the authors performed a scintillation analysis. They computed the dynamic spectra for both on‑pulse and off‑pulse windows and derived the corresponding scintillation bandwidths and timescales. Both components exhibit identical scintillation parameters (scintillation bandwidth ≈ 10 MHz, timescale ≈ 10–30 min) and a high correlation coefficient (≈ 0.78). Because scintillation is imposed by the interstellar plasma on the line‑of‑sight signal, the identical behavior of the two components strongly indicates that they traverse the same propagation path and therefore share a common origin within the pulsar system.

The paper then explores plausible magnetospheric mechanisms. The classic polar‑cap model can, in principle, generate weak emission at phases far from the main pulse if small‑scale current fluctuations extend beyond the open‑field line region. More compelling, however, are models that invoke high‑altitude acceleration zones such as the outer gap or the current sheet that forms near the light cylinder. Recent three‑dimensional particle‑in‑cell simulations show that reconnection in the current sheet can produce broadband, low‑intensity radio bursts over a wide range of rotational phase, naturally accounting for the observed off‑pulse emission without requiring a separate emission cone.

An important observational clue is that both pulsars are long‑period (P ≈ 2–4 s) and otherwise ordinary in their timing and polarization properties. Off‑pulse emission has previously been reported mainly for younger, high‑energy pulsars or for millisecond pulsars with complex magnetospheres. The detection in these two slow rotators suggests that the ability to generate off‑pulse radiation may be linked to the geometry of the magnetic field and the location of the null charge surface rather than to spin‑down power alone.

In summary, the authors provide robust, repeatable evidence that off‑pulse radio emission from B0525+21 and B2045‑16 originates inside the pulsar magnetosphere. The combination of multi‑frequency detection, consistent spectral index, and identical scintillation signatures rules out external scattering as the primary cause. This finding expands the phenomenology of pulsar radio emission, challenges the traditional view that coherent radio output is confined to the narrow on‑pulse window, and motivates further high‑sensitivity, high‑time‑resolution studies (e.g., with the Square Kilometre Array) and detailed magnetospheric simulations to pinpoint the exact location and physical process responsible for the off‑pulse component.