Scintillating insights into PSR~J0737$-$3039A and the interstellar plasma of the Gum Nebula from MeerKAT

TL;DR

Using 18 months of MeerKAT scintillation data for PSR J0737$-$3039A/B, the study identifies a single dominant, moderately anisotropic scattering screen located near the Gum Nebula edge with $D_s = 360^{+30}_{-40}$ pc and $A_R = 2.4 \,\pm\, 0.2$, and constrains the orbital orientation to $Ω = 40^{\circ} \pm 3^{\circ}$ and $i > 90^{\circ}$. The measured frequency scaling of the scintillation bandwidth is $\\alpha' = 3.61 \,\pm\, 0.05$, indicating non-Kolmogorov turbulence, while a velocity analysis yields a screen expansion speed of $V_s = 35 \,\pm\, 5$ km s$^{-1}$ and an SNR age of $t \,\approx\, 1$ Myr. Distance modelling shows consistency with the VLBI parallax distance of $D = 770 \,\pm\, 70$ pc, favoring outer-scale or Kolmogorov turbulence for the turbulence spectrum; timing distances are only compatible with outer-scale models. The work demonstrates that scintillation can provide an independent distance diagnostic and illuminate IISM structure along complex lines of sight, with potential extensions to other pulsars and future facilities like the SKA.

Abstract

The double pulsar system PSR~J0737$-$3039A/B has enabled some of the most precise tests of strong-field gravity to date. Here, we present a scintillation analysis of the system based on an 18-month observation campaign with the MeerKAT radio telescope. We characterise this interference pattern to infer properties of scattering plasma and the orbital geometry of the system. Our preferred model supports a scattering screen located at a distance of $D_s = 360^{+30}_{-40}$ pc. This moderately anisotropic screen of ionized gas (axial ratio $A_R = 2.4 \pm 0.2$) lies near the edge of the Gum Nebula, which is believed to be a supernova remnant (SNR) or an \HII\, region. We estimate the expansion velocity of the nebula to be $V_{\textrm{s}} = 35 \pm 5$ km s$^{-1}$, implying a SNR age of $t \approx 1$ Myr. We also constrain the orbital orientation and inclination sense of the double pulsar to be $Ω= 40^{\circ} \pm 3^{\circ}$ and $i > 90^{\circ}$, respectively. Assuming standard scattering geometry, our model yields a distance estimate consistent with the parallax-derived value of $D = 770 \pm 70$ pc from very long baseline interferometry. We conclude by discussing how future models of pulsar scintillation can enhance our understanding of the IISM and the properties of pulsars embedded within or lying behind such intervening structures.

Figures (12)

• Figure 1: H$\alpha$ emission towards the Gum Nebula and PSR J0737$-$3039. The colour bar shows the intensity for a given Galactic latitude (y-axis) and longitude (x-axis) in units of ergs cm$^{-2}$ sec$^{-1}$. The white 36$^{\circ}$-wide dashed circle is centred on the putative centre of the nebula. The cyan ellipse and arrow highlight the star $\zeta$ Puppis and its proper motion. The orange ellipse and vector show the location and proper motion of the double pulsar. The green vectors show the screen velocity from our model. The semi-transparent vectors are drawn from the posterior probability distribution to show the uncertainty in the velocity. The emission-line map was produced from the Southern H$\alpha$ Sky Survey Atlas Gaustad2001.
• Figure 2: Dynamic spectra of PSR J0737$-$3039A in S-band (top), L-band (middle), and UHF-band (bottom). Two eclipses, where the beam of pulsar A passes through the magnetosphere of pulsar B, can be seen, marked by the red arrows. The colour scales are all independent across each panel and use linear scaling. The data presented in each panel show a 100 MHz subband. Throughout the work we used the full bandwidth of each observation.
• Figure 3: Measurements of, and model for, $\Delta\tau_d^{-1}$. Top panel: Reciprocal of scintillation timescales for UHF (circles), L-band (squares), and S-band (triangles). The solid lines indicate the best-fitting model for $\textbf{V}_{\textrm{eff}}$ each epoch (16 in total). The maximum a-posteriori parameters for this model are presented in Table \ref{['tab:table_model']}. Lower panel: Normalised residuals for the best-fitting model. These are the differences between the observations and the model, divided by measured uncertainties. We discuss the whiteness of the residuals in Appendix \ref{['chapt:AppendixB']}. The dashed vertical line indicates the epoch of superior conjunction. Both panels are plotted against the orbital phase of the double pulsar, determined from the true anomaly expressed in degrees Reardon2019.
• Figure 4: Frequency dependence of scintillation for the double pulsar. The scintillation bandwidth measurements are shown at each epoch in the UHF (circles), L-band (squares), and S-band (triangles) observations. At each epoch, we infer a value for $\Delta\nu_{\textrm{d, 1GHz}}$. We rescale each epoch with the same median value to account for epoch-to-epoch variations in the scintillation bandwidth. The solid (red) line indicates the globally inferred value for $\alpha^{\prime}$ and its uncertainty accounting for the variation of $\Delta\nu_{\textrm{d, 1GHz}}$ between epochs. We also show a global model assuming Kolmogorov turbulence in black. The dashed lines show the channel bandwidth in each observing band. The second panel shows the normalised residuals of this scaled bandwidth (data - model / uncertainty).
• Figure 5: Time series of spatial scale and flux density. The top panel shows the spatial scale variations using two models that depend on the epoch spatial scale (red) and the inferred spatial scale from the scintillation bandwidth measurements (blue), respectively. The middle panel shows the calibrated flux density taken for each observation. Measurements taken at UHF and L-band are shown with circles and squares, respectively. The flux density during late 2022 (MJD 59900) correlated ($\approx 0.66$) with the spatial scale from our scintillation measurements. The S-band data have not yet been flux-density calibrated.