High-Resolution Measurements with the CTAO Southern Array: The Case for Pulsar Wind Nebulae

TL;DR

The study argues that PWNe offer rich, arcminute-scale structure at multi-TeV energies that can be exploited by the CTAO southern array. By building X-ray–driven TeV morphology models for HESS J1813-178 and MSH 15-52 and simulating CTAO observations under multiple IRFs, the authors quantify the improved capability to constrain magnetic-field and high-energy electron distributions. They show that angular-resolution gains from advanced reconstruction (e.g., Free-PACT) substantially enhance model discrimination, though photon statistics in the multi-TeV regime remain a limiting factor and increasing exposure remains important. The work highlights the value of joint X-ray–gamma-ray morphologies and outlines next steps toward more physically motivated, energy-dependent modeling and joint analyses, with broader implications for PWNe and other small, structured TeV sources.

Abstract

The advent of the Cherenkov Telescope Array Observatory (CTAO) and recent advances in reconstruction of gamma-ray photons with Cherenkov telescopes are bound to push the limit of angular resolution to an unprecedented precision of less than one arcminute at tens of TeV. Naturally, such instrumental improvements open up possibilities for new and interesting scientific studies. We aim to show that the study of pulsar wind nebulae (PWNe) in particular is bound to profit from these high-resolution measurements. This is because PWNe are the dominant Galactic source population at TeV energies, exhibit hard spectra up to hundreds of TeV and from X-ray observations are known to possess plentiful structure on arcminute scales. Using HESS J1813-178 and MSH 15-52 as examples, we create simple leptonic models of the TeV morphology of these sources based on X-ray observations and existing gamma-ray measurements. Then, assuming different models for the exposure and point spread function of the observatory, we create mock observations with the future CTAO southern array. We use these to assess the ability of these observations to differentiate between models and study the physics of these sources, in particular to infer the structure of the magnetic field and electron distributions. We find that future observations with the CTAO southern array at multi-TeV energies - in combination with existing X-ray measurements - will likely be able to constrain the distributions of magnetic field and high-energy electrons in these sources. We demonstrate that the sensitivity of these measurements can be significantly enhanced with the improved angular resolution achievable with novel reconstruction algorithms. However, we also show that in the relevant multi-TeV regime, signal-photon statistics remain a limitation and trading event statistics for improved angular resolution is not necessarily advantageous.

Figures (18)

• Figure 1: 1D profile of our (empirical) analytical model of the X-ray morphology of HESS$\,$J1813$-$178 in comparison to the XMM-Newton measurements from Funk:2006xk.
• Figure 2: Illustration of the steps to produce a model of MSH$\,$15$-$52 from the raw, exposure-corrected eROSITA counts map of MSH$\,$15$-$52. In the first step, the emission from the pulsar is removed by replacing the $\approx 100$ brightest pixels close to the pulsar with values sampled from the brightness distribution of the pixels in the two neighboring rows. Then, in the second step, the background is estimated from the area marked by the orange rectangle and removed throughout the image. Furthermore, the thermal-emission from the north-eastern shoulder of MSH$\,$15$-$52 is masked out. Finally, the resulting image is smoothed with a $12"$ gaussian kernel and a hysteresis thresholding is applied.
• Figure 3: Plot of the different normalised morphological model hypotheses for HESS$\,$J1813$-$178. The first column shows the X-ray morphology, the second column the magnetic field and the third column the high-energy electrons for three different models assumptions: The second row shows our baseline fixed ratio assumption, in the first row the magnetic field energy density is capped at $\eta=0.25$, in the third row the electron energy density is capped at $\eta=0.25$.
• Figure 4: Plot of the different normalised morphological model hypotheses for MSH$\,$15$-$52. The first column shows the X-ray morphology, the second column the magnetic field and the third column the high-energy electrons for three different models assumptions: The second row shows our baseline fixed ratio assumption, in the first row the magnetic field energy density is capped at $\eta=0.5$, in the third row the electron energy density is capped at $\eta=0.5$.
• Figure 5: Effective area and angular resolution as a function of true gamma-ray energy for on-axis photons at $20^{\circ}$ zenith angle for the four sets of IRFs used in this work. Shown in dark green are the H.E.S.S. IRFs taken from observation ID 26964 from the public data release HESS:2018zix. In lime green, we show the CTAO Prod5 IRFs cherenkov_telescope_array_observatory_2021_5499840. The Free-PACT resolution curves are shown in orange. The light blue curve shows the Free-PACT-Event Type IRFs for which only the better half of events in terms of predicted angular resolution are considered. The IRFs are used for the study of HESS$\,$J1813$-$178.