Probing the morphology of the Gum Nebula through pulsar observables and a novel distance estimation method

TL;DR

This work tackles uncertainties in Gum Nebula electron density models (GEDMs) and their impact on pulsar distance estimates along Gum Nebula sightlines. It combines a two-component scattering framework with a novel joint DM–temporal-broadening approach to constrain the Gum Nebula frontal edge using the Vela pulsar and to refine Gum Nebula parameters within the YMW16 GEDM by leveraging ten pulsars with independent distances. Key results include a Vela-line fractional scatterer distance of $x = 0.89 \pm 0.01$ and a scatterer distance of $D_s = 254 \pm 16$ pc, a frontal-edge distance of $196 \pm 16$ pc from the Nebula centre with angular radius $24^\circ \pm 2^\circ$, and a Gum Nebula strength of $\kappa_\nu^{GN} \approx 384$ at 1 GHz; the updated GN model (GUM25) places the Vela pulsar behind the frontal shell, aligning pulsar distances more consistently with independent measures. The analysis also reveals non-monotonic temporal broadening as a function of distance due to density enhancements, indicating that DM-based scaling alone is insufficient to capture scattering. Overall, the study provides a refined, testable GEDM for Gum Nebula directions and presents a generalizable method for improving pulsar-distance estimates across complex ionized structures.

Abstract

Various existing models of the Gum Nebula differ significantly in their parameters and suggested origins, which can be independently tested for consistency with data on some key observables of pulsars in the direction of the nebula. Our analysis of such data on the Vela pulsar, assuming a dominant scattering region in its foreground, suggests that the fractional distance of the scatterer is $0.89 \pm 0.01$, and for the given distance of the Vela pulsar, it translates to $254 \pm 16$ pc. Using independent distances of ten pulsars, we suggest a refined description of the Gum Nebula electron density model with its basic morphology similar to that used in the YMW16 model, which now provides better estimates of pulsar distances in these directions. In our new Gum Nebula model, as expected, the Vela pulsar would be behind the frontal edge of the Gum shell, which was intriguingly located in front of the nebula in the YMW16 model. We also present a new technique to better constrain the pulsar distances using their dispersion measure and temporal broadening simultaneously, and find that it is less affected by the uncertainties in the Galactic electron density distribution models. Notably, the new approach shows that the expected temporal broadening as a function of trial distance does not follow a monotonic increasing trend, but instead exhibits oscillations at regions of enhanced electron density. This behaviour is expected, as the method employs the integral form of temporal broadening with the appropriate weighting kernel, leading to more reliable estimates.

Figures (6)

• Figure 1: Left: Both roots of the quadratic equation (\ref{['eq:1']}) are plotted against the fractional scatterer distance, shown in blue and red. The green curve represents $D\ =\ d_s/x$, where the scatterer distance is 254 pc. The physical solution for the fractional scatterer distance corresponds to the intersection of blue and green curves at $x$ = 0.89. The red curve reaches unrealistic distance estimates for a range of fractional scatterer distance values. Right: The variation of $R_\theta$ and $R_\text{v}$ with the fractional scatterer distance and scattering strength factor ($Y$) is shown. The black points with error bars indicate the estimated values for the Vela pulsar, with $r_{\theta}$ = 0.49 kpc$^{-1/2}$, $r_\text{v}$ = 0.95 kpc$^{-1/2}$. The estimated fractional scatterer distance and scattering strength factor for the Vela pulsar are $0.89 \pm 0.01$, 384, respectively.
• Figure 2: The plot shows a comparison between the model-predicted and measured temporal broadening values of the pulsars in the direction of the Gum Nebula at 1 GHz. The measured temporal broadening values are scaled to 1 GHz using the frequency scaling spectral index, $\alpha$ = -4. The values on or close to the green line ($y = x$) have less deviation from the model predictions.
• Figure 3: The plot illustrates the electron density profile along the line of sight to the Vela pulsar. The dashed-dotted vertical line marks the location of the Vela pulsar. The first peak, around 100 pc, corresponds to the electron density enhancement in the Local Bubble, followed by two peaks associated with the Gum Nebula. In the YMW16 model, the Vela pulsar appears in the foreground of the Gum Nebula shell, whereas in the NE2001 and modified model (GUM25), it is positioned just behind the frontal edge of the nebula.
• Figure 4: The figure presents the H $\alpha$ image of the Gum Nebula overlaid with pulsars in this region, and model ellipses from YMW16 (blue) and our modified nebula model (red), with their respective centres marked by filled squares. The solid red ellipse delineates the peak density in the shell of the GUM25 model, while the dashed red ellipses denote (inner and outer) extents, where electron density described by Gaussian shell profile falls to $1/e$ of the peak density.
• Figure 5: The plot shows the variation of the $\kappa_\nu^{\rm GN}$ factor with angular separation of pulsars from the centre of the Gum Nebula. The dot sizes are scaled inversely with distance, such that nearby pulsars appear larger. It is clearly seen that the $\kappa_\nu$ factor values vary significantly across different lines of sight through the nebula.