Context :

Aims : HESS J1857+026 is an extended TeV gamma-ray source that was discovered by H.E.S.S .
as part of its Galactic plane survey .
Given its broadband spectral energy distribution and its spatial coincidence with the young energetic pulsar PSR J1856+0245 , the source has been put forward as a pulsar wind nebula ( PWN ) candidate .
MAGIC has performed follow-up observations aimed at mapping the source down to energies approaching 100 GeV in order to better understand its complex morphology .

Methods : HESS J1857+026 was observed by MAGIC in 2010 , yielding 29 hours of good quality stereoscopic data that allowed us to map the source region in two separate ranges of energy .

Results : We detected very-high-energy gamma-ray emission from HESS J1857+026 with a significance of 12 \sigma above 150 GeV .
The differential energy spectrum between 100 GeV and 13 TeV is well described by a power law function dN / dE = N _ { 0 } ( E / 1 \textrm { TeV } ) ^ { - \Gamma } with N _ { 0 } = ( 5.37 \pm 0.44 _ { stat } \pm 1.5 _ { sys } ) \times 10 ^ { -12 } ( \textrm { TeV } ^ { -1 } % \textrm { cm } ^ { -2 } \textrm { s } ^ { -1 } ) and \Gamma = 2.16 \pm 0.07 _ { stat } \pm 0.15 _ { sys } , which bridges the gap between the GeV emission measured by Fermi -LAT and the multi-TeV emission measured by H.E.S.S..
In addition , we present a detailed analysis of the energy-dependent morphology of this region .
We couple these results with archival multi-wavelength data and outline evidence in favor of a two-source scenario , whereby one source is associated with a PWN , while the other could be linked with a molecular cloud complex containing an H ii region and a possible gas cavity .

Conclusions :