On the Plerionic Supernova Remnant CTB 87 (G74.9+1.2) and Its Powering Engine: Insights from the Chandra X-ray Observatory

Pulsar Wind Nebulae (PWNe) offer a valuable astrophysical laboratory to study the physics of pulsar winds and their interaction with the ISM, and to search for missed pulsars. While the Crab nebula has been known for decades to represent the prototype of PWNe, there are several PWNe whose properties differ from the Crab, leading to a class dubbed as `plerions of a second kind'. CTB 87 (G74.9+1.2) belongs to this class and is one of the least studied members in X-rays. In the radio, it has a low-frequency spectral break and an unusually steep spectral index hinting to an evolved PWN. We present an archival ASCA observation of this object and a new 70 ksec ACIS-I Chandra observation dedicated to resolve the putative pulsar and to perform a high-resolution imaging and spectroscopic study of the PWN, including the search for structures associated with the deposition of the neutron star's energy into its surroundings. The peak of X-ray emission is clearly offset from the radio peak. For the ASCA data, a power-law model fit yields a column density of (1.14±0.22)x10²² cm^-2, a photon index of 1.77±0.15, and a luminosity of 1.4x10³⁴ erg s^-1 (at the revised distance of 6.1kpc). Thanks to Chandra, the source powering the nebula is resolved and its spectrum is studied separately from the PWN. Both are well described by a power law model with a hard photon index. The Chandra source-the putative pulsar- is found at the south-eastern edge of the bright radio nebula, with jet-like and diffuse emission from the PWN extending to the northwest. We discuss our X-ray study in correlation with recent sensitive radio continuum and polarization measurements obtained with the CGPS and Effelsberg. Finally, we compare G74.9+1.2 to other PWNe likely in a similar stage of their evolution, including G63.7+1.1, G65.7+1.2, and G76.9+1.0.