A Comprehensive X-ray Characterization of the Pulsar Wind Nebula CTB 87: Evidence for Reverse-Shock Interaction from Its X-ray and Broadband Emissions

We present a comprehensive X-ray study of the pulsar wind nebula (PWN) CTB 87, utilizing new NuSTAR observations and archival Chandra and XMM-Newton data. Our joint analysis reveals that the PWN's X-ray spectrum exhibits significant curvature, well described by a broken or cutoff power law. The broken power-law fit yields photon indices $Γ_{1}=1.89\pm 0.05$ and $Γ_{2}=2.73 \pm 0.10$ below and above a break energy of $E_{\rm b}=5.12 \pm 0.26$ keV, respectively, while the cutoff power-law fit gives $Γ=1.23 \pm 0.10$ and $E_{\rm c} = 5.29 \pm 0.67$ keV. Spatially resolved spectroscopy confirms a previously observed radial softening of $Γ$, indicating significant synchrotron cooling as electrons move outward from the pulsar. We employed leptonic emission scenarios to model the broadband emission from CTB 87, focusing on the measured X-ray spectrum and the radial profiles. While an evolutionary one-zone model can explain the general features of the broadband spectral energy distribution (SED), its predictions do not fully reproduce the observed X-ray spectrum. Our multi-zone modeling demonstrates that the hard electron distribution required to explain the X-ray spectrum cannot account for the observed gamma-ray SED. This discrepancy suggests the presence of an additional electron population, possibly linked to the reverse shock interaction, and highlights CTB 87 as an evolved PWN whose broadband emission cannot be captured by these simple SED models. We discuss the potential origins for this second electron population and its implications for understanding the evolution of PWNe.