Frequency Dependent Dispersion measures in Pulsar B0329+54

Shyam Sunder^1*, Tetsuya Hashimoto¹, Tomo Goto², Shotaro Yamasaki¹, Simon C. C. Ho⁴

¹Department of Science, National Chung Hsing University, Taichung, Taiwan
²Department of Physics, NTHU, Hsinchu, Taiwan
³Astronomy Department, ASIAA, Taipei, Taiwan
⁴Astronomy and Astrophysics, , The Australian National University, Canberra, Australia

* Presenter:Shyam Sunder, email:sssharmatifr@gmail.com

Pulsars are rotating neutron stars that emit periodic radio pulses. Pulse arrival times at different radio frequencies are generally well described by the cold-plasma dispersion law of the interstellar medium (ISM). Fitting this law yields the dispersion measure (DM), which quantifies the integrated electron density along the line of sight. A single, frequency-independent DM implies that the pulse delays strictly follow the cold-plasma dispersion law. However, several pulsars exhibit apparent deviations that manifest as frequency-dependent DMs. For example, previous studies of PSR B0329+54 at frequencies below 1 GHz reported DM variations exceeding 10⁻³ pc cm⁻³, though it remained unclear whether these arose from the ISM or from intrinsic pulsar emission geometry.

We present results from simultaneous wideband observations of PSR B0329+54 with the upgraded GMRT, spanning 300–1460 MHz. Our analysis identifies a distinct pulse feature whose arrival times closely follow the cold-plasma dispersion law, yielding a single DM that remains constant across the entire band. In contrast, standard timing techniques such as FFTFIT produce frequency-dependent DMs. We investigate the origin of this discrepancy and demonstrate that it arises from geometric effects within the pulsar magnetosphere rather than from propagation through the ISM. Our results imply that the radio emission originates from a compact region of extent ≤204 km, located approximately 800 km above the neutron star surface. This provides the most stringent constraint to date on the emission height of PSR B0329+54, significantly improving upon previous estimates based on dipolar magnetic field models. We discuss the implications of these findings for pulsar emission physics and magnetic field geometry.

Keywords: Pulsars, Neutron Stars, Interstellar medium, Radio Astronomy