Gigahertz-peaked spectra pulsars in Pulsar Wind Nebulae

TL;DR

This study investigates the gigahertz-peaked spectra (GPS) phenomenon in pulsars within Pulsar Wind Nebulae, suggesting thermal absorption in surrounding media as a key cause, supported by interferometric observations and spectral modeling.

Contribution

It provides the first detailed interferometric analysis of GPS pulsars in PWN, confirming thermal absorption as a primary mechanism and exploring the properties of the absorbing medium.

Findings

Four out of six pulsars showed GPS behavior.

Thermal free-free absorption models fit the spectra well.

Pulsar J1747-2958 exhibits GPS and possible scattering effects.

Abstract

We have carried out a detailed study of the spectral nature of six pulsars surrounded by Pulsar wind nebulae (PWN). The pulsar flux density were estimated using the interferometric imaging technique of the Giant Metrewave Radio Telescope at three frequencies 325 MHz, 610 MHz and 1280 MHz. The spectra showed a turnover around gigahertz frequency in four out of six pulsars. It has been suggested that the gigahertz peaked spectra (GPS) in pulsars arises due to thermal absorption of the pulsar emission in surrounding medium like PWN, HII regions, Supernova remnants, etc. The relatively high incidence of GPS behaviour in pulsars surrounded by PWN impart further credence to this view. The pulsar J1747$-$2958 associated with the well known Mouse nebula was also observed in our sample and exhibited GPS behaviour. The pulsar was detected as a point source in the high resolution images. However, the pulsed emission was not seen in the phased array mode. It is possible that the pulsed emission was affected by extreme scattering causing considerable smearing of the emission at low radio frequencies. The GPS spectra were modeled using the thermal free-free absorption and the estimated absorber properties were largely consistent with PWN. The spatial resolution of the images made it unlikely that the point source associated with J1747$-$2958 was the compact head of the PWN, but the synchrotron self-absorption seen in such sources was a better fit to the estimated spectral shape.