Clusters and groups of galaxies are one of the most energetic objects in the universe. We analyzed the ASCA data of near-by 18 groups of galaxies, and found that about half the sample show evidence for an excess hard emission, in addition to the ~ 1 keV thermal emission from the hot intra-group matter (IGM). We present the detailed analysis of HCG 62, which shows the most significant hard excess, as well as the results from the systematical analyses of other 17 groups. With these results, we for the first time study statistical properties of the hard excess X-rays in galaxy groups.
The hard component in HCG 62 is clearly extended; its radial profile is similar to or rather wider than that of the IGM. Its spectra are well fitted by a power-law with photon index ~ 2 or a thermal emission with temperature > 5.7 keV. The 2--10 keV luminosity of the hard component is derived as 4.2 x 1041 h75-2 erg s-1, which is ~ 20% of the IGM luminosity in the 0.5--10 keV band. For the other groups, 9 out of 17 sample show evidence of a hard component, with the luminosity in the range 1 ~ 18 x 1041 h75-2 erg s-1, which is 10 ~ 40% of that of the IGM. On the other hand, the ramaining 8 groups do not exhibit statistically significant hard emission, with an upper limit of ~ 5% of the IGM.
The hard X-rays suggest the existence of high energy particles widely distributed in the inter-galactic space. We searched many parameters for correlation with the strength of the hard X-rays, and found that most of the groups with significant hard excess emission host a few bright galaxies in their central regions, while those without hard emission predominantly host a single dominant galaxy. Using all these results, we derive constraints on the emission mechanisms proposed for the hard X-rays, and discuss possible acceleration mechanisms of the high energy particles.