Observation of γ rays > 1015 eV from Cygnus X-3

The X-ray binary system Cygnus X-3 is a source of particular interest. As well as emitting X-rays which are modulated with a 4.8-h period, it has been observed in 30-100 MeV γ rays by the SAS II satellite and several groups have detected γ rays in the TeV range showing the same period. Most recently, using the small extensive air shower array at Kiel, workers have found that the γ-ray spectrum of Cygnus X-3 extends above 2 x 1015 eV, with an integral flux of (7.4 +- 3.2) x 10-14 cm-2 s-1. This paper confirms the Kiel observations and presents evidence that the γ-ray spectrum of Cygnus X-3 steepens above 1016 eV. (author)