Detection of VHE $\gamma$-Rays from HESS J0632+057 during the 2011 February X-Ray Outburst with the MAGIC Telescopes}

The very high energy (VHE) {$\gamma$}-ray source HESS J0632+057 has recently been confirmed to be a {$\gamma$}-ray binary. The optical counterpart is the Be star MWC 148, and a compact object of unknown nature orbits it every \~{}321 days with a high eccentricity of \~{}0.8. We monitored HESS J0632+057 with the stereoscopic MAGIC telescopes from 2010 October to 2011 March and detected significant VHE {$\gamma$}-ray emission during 2011 February, when the system exhibited an X-ray outburst. We find no {$\gamma$}-ray signal in the other observation periods when the system did not show increased X-ray flux. Thus, HESS J0632+057 exhibits {$\gamma$}-ray variability on timescales of the order of one to two months possibly linked to the X-ray outburst that takes place about 100 days after the periastron passage. Furthermore, our measurements provide for the first time the {$\gamma$}-ray spectrum down to about 140 GeV and indicate no turnover of the spectrum at low energies. We compare the properties of HESS J0632+057 with the similar {$\gamma$}-ray binary LS I +61{\deg}303 and discuss the possible origin of the multi-wavelength emission of the source.