A HARD X-RAY HALO AROUND CYG X-3

Ginga observations have confirmed the extended hard X-ray component ar ound Cyg X-3 which has been detected by EXOSAT. Scanning observations of Cyg X-3 show that this component extends up to 2-degrees from Cyg X -3 along the Galactic plane. The surface brightness observed by Ginga is consistent with that observed by the EXOSAT ME. The ratio of the co unt rates in the 3-6 and 6-10 keV bands shows that the extended compon ent has either a thermal bremsstrahlung spectrum with a temperature of 6 +/- 2 keV or a power-law spectrum with a photon index of 2.3 +/- 0. 3. If this is associated with. Cyg X-3, the X-ray luminosity in the 2- 10 keV band is estimated to be about 1 X 10(36) erg s-1. We discuss tw o alternative explanations for the origin of the X-ray halo: thermal e mission from a hot gas or synchrotron radiation of the ultra-high-ener gy electrons which are expected to be produced along with ultra-high-e nergy gamma-rays.