August 2007
Recently, the Milagro collaboration, operating a ground-based air shower detector based on the water Cherenkov technique, announced the detection of several new sources of very-high-energy gamma rays in the Galactic Plane (A. Abdo et al., 2007). Compared to Cherenkov telescopes, Milagro has a higher energy threshold – the median energy of detected events is about 20 TeV – and a reduced angular resolution of about 1 degr. Located in the northern hemisphere, Milagro coverage of the Galactic Plane extends from about 30 degr. longitude towards higher longitudes. Four sources are detected at sufficient significance, the Crab Nebula and the new sources MGRO 2019+37, MGRO 1908+06 and MGRO 2031+41. Of these, MGO 1908+06, with a flux of 80% of the Crab flux and a diameter of up to 2.6 degr., is within the field of view of the H.E.S.S. Galactic Plane Survey, at a longitude around 40 degr.
Fig. 1 shows the H.E.S.S. sky map of the corresponding region, based on 10 h of data. Confirming the Milagro result, a gamma-ray source is detected with up to 9 sigma significance. The H.E.S.S. source is located at l=40.45 degr. and b=-0.80 degr., consistent with the Milagro position of l=40.4 degr. and b=-1.0 degr., with an error radius of 0.24 degr. The rms size of the H.E.S.S. source is about 0.2 degr. The energy spectrum measured by H.E.S.S. is shown in Fig. 2; the source exhibits a rather flat spectrum with a spectral index of 2.1. The spectrum measured with H.E.S.S agrees well with the Milagro data point at 20 TeV. The nature of the source is unclear; it coincides within errors with the (also unidentified) EGRET GeV source GRO J1908+0556 / GEV J1907+0557 (Lamb and Macomb 1997, Reimer et al. 1997) and with the shell-type supernova remnant G40.5-0.5 (Fig. 3), which is however smaller than the TeV source.
