Low-frequency spectra and scattering of pulsars towards the Gum Nebula with the MWA

Context. The Gum Nebula is a nearby ionised complex with sight-line-dependent pulsar propagation. Aims. We test whether a smooth ionised structure can explain low-frequency spectral turnovers and pulse broadening, or whether smaller-scale material is required. Methods. We analysed Murchison Widefield Array observations of six pulsars at 80–240 MHz, combining sub-band flux densities with literature data. We compared power-law, free–free absorption (FFA), and turnover-constrained broken-power-law (BPL) spectra, and modelled scattering through pulse-profile convolution. PSR J0820-4114 and the Vela pulsar were fitted jointly across frequency. Results. All six spectra depart from a single power law. FFA is favoured for PSRs J0737-3039A and J0820-4114, and BPL for J0742-2822 and J0835-4510; the other two classifications depend on assumed error floors. Conditional FFA solutions imply emission measures of ${10}^4$–${10}^5$ pc cm${}^{-6}$ for electron temperatures of $3\times {10}^3$–${10}^4$ K, far exceeding smooth-shell expectations. PSRs J0737-3039A and J0837-4135 have scattering laws consistent with ${\tau }_{\mathrm{s}\mathrm{c}}\propto {\nu }^{-4}$. We obtain ${\alpha }_{\mathrm{s}\mathrm{c}}=-3.07\pm 0.11$ for J0742-2822 and $-1.59\pm 0.44$ for J0820-4114 using a joint two-component model. The Vela pulsar is best described by an effective thick-screen model with ${\alpha }_{\mathrm{s}\mathrm{c}}\simeq -2.74$. Neither tested Galactic electron-density model reproduces both independent distances and low-frequency scattering uniformly. Conclusions. The results favour a structured Gum–Vela magneto-ionic environment in which neighbouring sight lines probe different absorbing and scattering material; these effects need not arise in the same plasma component.