Hard X-Ray Properties of Groups of Galaxies as Observed with ASCA

X-ray spectra of groups of galaxies, obtained with the GIS instrument onboardASCA, were investigated for diffuse hard X-rays in excess of the soft thermalemission from their inter-galactic medium (IGM). In total, 18 objects with theIGM temperature of 0.7--1.7 keV were studied, including HCG 62 in particular.Non X-ray backgrounds in the GIS spectra were carefully estimated andsubtracted. The IGM emission was represented by up to two temperature thermalmodels, which was determined in a soft energy band below 2.5 keV mainly by theSIS data. When extrapolated to a higher energy range of 4--8 keV, this thermalmodel under-predicted the background-subtracted GIS counts in HCG 62 and RGH 80by > 2 sigma significance, even though the background uncertainties and the IGMmodeling errors are carefully accounted. A hard excess could be also present inNGC 1399. The excess was successfully explained by a power-law model with aphoton index ~ 2, or a thermal emission with a temperature exceeding ~ 3 keV.In HCG 62, the 2--10 keV luminosity of the excess hard component was found tobe 5.5E41 erg/s at 2--10 keV, which is ~ 30% of the thermal IGM luminosity in0.7--2.5 keV. Non-thermal and thermal interpretations of this excess componentsare discussed.Comment: 18 pages, 10 figures; published with minor updates in proof procedur