Retracing the Cold Plasma Dispersion Law in Pulsar B0329+54: New Insights into Frequency-Dependent Dispersion Measures

TL;DR

This study uses the upgraded GMRT to observe pulsar B0329+54 across 300-1460 MHz, confirming the cold-plasma dispersion law and tightly constraining the emission region size, revealing frequency-dependent dispersion measures.

Contribution

It provides the first comprehensive broadband observation confirming the dispersion law and constrains the pulsar's emission region to within 204 km, improving previous height estimates by at least five times.

Findings

Confirmed the cold-plasma dispersion law across 300-1460 MHz.

Constrained the emission region to within 204 km from the pulsar surface.

Revealed frequency-dependent DMs linked to emission geometry.

Abstract

Multiple studies have investigated potential frequency-dependent dispersion measures (DM) in PSR B0329+54, with sensitivities at levels of $10^{-3} \, \text{pc} \, \text{cm}^{-3}$ or higher, using frequencies below 1 GHz. Utilizing the extensive bandwidth of the upgraded Giant Meterwave Radio Telescope, we conducted simultaneous observations of this pulsar across a frequency range of 300 to 1460 MHz. Our observations reveal a distinct point in the pulse profile of PSR B0329+54 that appears to align remarkably well with the cold-plasma dispersion law, resulting in a unique measured DM across the entire frequency range. In contrast, using times of arrival (ToAs) from widely adopted pulsar timing techniques (e.g., FFTFIT)-leads to frequency-dependent DMs. We investigated the potential causes of these frequency-dependent DMs in this pulsar and their relationship with the underlying magnetic field geometry corresponding to the radio emission. Our study reveals that all frequencies in the range 300-1460 MHz originate from a region no larger than 204 km, and the dipolar magnetic-field geometry model indicates that the emission region is centered at $\sim$800 km from the star. This is the tightest constraint on the size of the emission region reported so far for PSR B0329+54 at the given frequencies, and it is at least five times more stringent than the existing emission height constraints based on the dipolar geometry model.