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

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.