Hard X-ray properties of groups of galaxies as observed with ASCA

X-ray spectra of groups of galaxies, obtained with the GIS instrument onboard ASCA, were investigated for diffuse hard X-rays in excess of the soft thermal emission from their inter-galactic medium (IGM). In total, 18 objects with IGM temperatures of 0.7-1.7 keV were studied, including HCG 62 in particular. Non X-ray backgrounds in the GIS spectra were carefully estimated and subtracted. The IGM emission was represented by up to two temperature thermal models, which was determined in a soft energy band below 2.5 keV, mainly by the SIS data. When extrapolated to a higher energy range of 4-8 keV, this thermal model under-predicted the background-subtracted GIS counts in HCG 62 and RGH 80 by > 2 sigma or significance, even though the background uncertainties and the IGM modeling errors were carefully accounted. A hard excess could also be present in NGC 1399. The excess was successfully explained by a power-law model with a photon index of similar to 2, or a thermal emission with a temperature exceeding similar to 3 keV. In HCG 62, the 2-10 keV luminosity of the excess hard component was found to be 5.5 x 10(41) erg s(-1) at 2-10 keV, which is similar to 30% of the thermal IGM luminosity in 0.7-2.5 keV. Non-thermal and thermal interpretations of this excess components are discussed.