Anomalous scattering of pulsars towards the Gum Nebula

TL;DR

This study investigates anomalous scattering toward the Gum Nebula by performing wideband ($250$–$500$ MHz) pulsar observations with the uGMRT Band-3. By modeling scatter-broadening with a thin-screen transfer function and applying MCMC to infer the scattering index $\alpha$ and timescale $\tau_{sc}$, the authors quadruple the number of $\alpha$ estimates in the region and reveal an inverse relation between scattering strength and distance. The Vela pulsar presents a notably flatter $\alpha$ ($= 2.9 \pm 0.2$), indicating that its scattering is likely dominated by the Vela SNR rather than the Gum Nebula. The results show that nearby lines of sight are more affected by nebular density turbulence, while distant pulsars experience little Gum Nebula influence, highlighting the need for broader frequency coverage and a larger sample to fully characterize the turbulence across the nebula and refine pulsar distance estimates.

Abstract

We report wideband scatter-broadening estimates of 14 pulsars towards the Gum nebula region using the Band-3 of the upgraded GMRT. This work increases the measurements of frequency scaling index of scatter-broadening ($α$) across the nebula by more than 3 times. A strong correlation between the distance and the scattering strength is observed for pulsars behind the nebula. It is also observed that for distant pulsars ($> 2 kpc$), the effect of the Gum nebula in DM and scattering strength is not substantial. We also report a much flatter $α$ for the Vela pulsar and argue that its scattering is not caused by the Gum nebula, but the Vela supernova remnant.

Figures (18)

• Figure 1: The left panel shows the profile at each sub-band of the uGMRT band3 for PSR J0809$-$4753. The black curves show the observed profile and the red curves show the best fit model. The template (intrinsic) profile used for fitting the band-3 profiles is shown at the top of the panel. The right side panel shows the measured $\tau_{sc}$ in black filled circles as a function of observing frequency in log-log plot. The black continuous line shows the best fit model and the dotted line shows the expected Kolmogorov turbulence model curve ($\tau_{sc} \propto \nu^{-4.4}$) extrapolated from the lowest frequency bin in the dataset. The best fit $\alpha$ measurement is given at the top of the panel.
• Figure 2: The distribution of the $\tau_{sc}$ of pulsars across the Gum nebula is shown. The black filled circles show the pulsars in front of the nebula (by distance), the green filled circles show the pulsar positions that are behind the nebula. The red filled circles represent the pulsar behind the nebula that did not show any scatter-broadening. The white circles are the $\tau_{sc}$ estimates from literature scaled to 325 MHz and lw2kmnjm15. The diameter of the circles indicate the $\tau_{sc}$ of each pulsar at 325 MHz. The background image is taken from the H$\alpha$ images produced by fink03.
• Figure 3: Same as Figure \ref{['fig2']}. The diameter of the circles indicate the scattering strength in that line of sight, instead of $\tau_{sc}$ as in Figure \ref{['fig2']}. The colour code is also the same.
• Figure 4: Distribution of the scattering strength of the 17 pulsars under study is plotted as a function of DM and distance in left and right hand side panels. A trend of decrease in scattering strength as a function of DM and distance is clearly visible.
• Figure A1: PSR J0738$-$4042. The left side panel shows the observed profile in each sub-band (black curves) and the best fit model (red curves). The template used to fit the profile is shown at the top. The center frequency of each sub-band is noted for the profiles. For some profiles, only the leading edge component is fitted as was done in kmnjm15. The right side panel shows the plot of $\tau_{sc}$ as a function of frequency in log-log scale. The best fit power law model is shown as a continuous curve and the Kolmogorov model as dotted curves. The final value of $\alpha$ is printed at the top of the panel. All the figures in this appendix are in the same format.