Hess J 0632 + 057 : a New Gamma - Ray Binary ?

The H.E.S.S. survey of the Galactic plane has established the existence of a substantial number (∼40) of Galactic TeV γ-ray sources, a large fraction of which remain unidentified. HESS J0632+057 is one of a small fraction of these objects which is point-like in nature (< 2 ′ rms), and is one of only two point-like sources that remain unidentified. Follow-up observations of this object with XMM-Newton have revealed an X-ray source coincident with the TeV source and with the massive star MWC 148, of spectral type B0pe. This source exhibits a hard spectrum, consistent with an absorbed power-law with Γ = 1.26 ± 0.04, and shows significant variability on hour timescales. We discuss this spatial coincidence and the implied spectral energy distribution of this object and argue that it is likely a new γ-ray binary system with a close resemblance to the three known members of this class, and in particular to LS I +61 303. Further X-ray, radio and optical observations of this system are needed to firmly establish HESS J0632+057 as a new member of this rare class of Galactic objects.