Radio study of HESS J1857+026. Gamma-rays from a superbubble?
  Aims. We provide new insights into the nature of HESS J1857+026, a
very-high-energy {\gamma}-ray source whose complex morphology in the TeV band
was attributed to the superposition of two distinct sources. Methods. We
performed radio continuum observations to look for the pulsar wind nebula and
the supernova remnant associated with the pulsar PSR J1856+0245, which might be
powering part of the {\gamma}-ray emission. We observed HESS J1857+026 with the
Karl G. Jansky Very Large Array (VLA) at 1.5 GHz in the C configuration. In
addition, using the same array configuration, we observed a region of
$0.4^\circ \times 0.4^\circ$ towards PSR J1856+0245 at 6.0 GHz. We obtained
complementary data for the neutral hydrogen and molecular gas emission from
public surveys in order to investigate the properties of the interstellar
medium in the direction of HESS J1857+026. Results. The new observations at
1.5GHz do not show evidence of emission above the noise level of
$0.7\,\rm{mJy\,beam^{-1}}$ that could be associated with either HESS J1857+026
or PSR J1856+0245. Also, in the new image at 6.0GHz we do not detect radio
emission from a pulsar wind nebula powered by PSR J1856+0245. The neutral gas
analysis shows the existence of a superbubble in the direction of the
{\gamma}-ray source. We suggest that this structure is located at
$\sim5.5\,\rm{kpc}$, compatible with the distance to the pulsar PSR J1856+0245.
Conclusions. We conclude that TeV emission from HESS J1857+026 originates in a
superbubble, arguing in favour of a single {\gamma}-ray source rather than the
superposition of two distinct sources. The pulsar PSR J1856+0245 could also be
contributing as a source of {\gamma}-rays within the bubble.
