  Breaks in the radio spectra of supernova remnants (SNRs) reflect the maximum
energy of either shock-accelerated electrons or - in the case of pulsar wind
nebulae - of electrons injected by the central pulsar. Otherwise, the break may
result from energy losses due to synchrotron aging or it is caused by
energy-dependent diffusion. A spectral steepening of the plerionic SNR CTB87 at
around 11 GHz was observed in the eighties, but a recent analysis of CTB87's
energetic properties based on new radio data raised doubt on it. CTB87 consists
of a central compact component surrounded by a diffuse centrally peaked almost
circular halo. Missing faint halo emission due to insufficient sensitivity of
early high-frequency observations may be be the reason for the reported
spectral break. We intend to clarify the high-frequency spectrum of CTB87 by
new sensitive observations. We used the broad-band 2-cm receiver at the
Effelsberg 100-m telescope for sensitive continuum observations of CTB87 and
its halo in two frequency bands. The new 2-cm maps of CTB87 show halo emission
with a diameter of about 17' or 30 pc for a distance of 6.1 kpc in agreement
with lower-frequency data. The measured flux densities are significantly higher
than those reported earlier. The new 2-cm data establish the high-frequency
continuation of CTB87's low-frequency spectrum. Any significant high-frequency
spectral bend or break is constrained to frequencies well above about 18 GHz.
The extended halo of CTB87 has a faint counterpart in gamma-rays (VER J2016+37)
and thus indicates a common origin of the emitting electrons.
