A Radio Continuum and Polarisation Study of the pulsar wind nebula CTB87 (G74.9+1.2)

We present radio continuum and linear polarisation observations of the pulsar wind nebula CTB87 (G74.9+1.2) with the Effelsberg 100-m radio telescope between 4.75 and 32 GHz. An analysis of these new data including archived low-frequency observations at 1420 MHz and 408 MHz from the Canadian Galactic Plane Survey shows that CTB87 consists of two distinct emission components: a compact kidney-shaped component, 14 pc x 8.5 pc (7.8' x 4.8') in size and a larger diffuse, spherical and centrally peaked component of about 30 pc (17') in diameter. The kidney-shaped component with a much steeper radio continuum spectrum is highly linearly polarised and likely represents a relic pulsar wind nebula. The diffuse component represents the undisturbed part of the PWN expanding inside a cavity or stellar wind bubble. The previously reported spectral break above 10 GHz is likely the result of missing large-scale emission and insufficient sensitivity of the high-frequency radio continuum observations. The simulation of the system's evolution yields an age of about 18,000 years as the result of a type II supernova explosion with an ejecta mass of about 12 solar masses and an explosion energy of about 7 x 10^50 erg. We also found evidence for a radio shell in our polarisation data which represents the blast wave that entered the molecular cloud complex at a radius of about 13 pc.