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

Abstract

Abstract 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 $\gt$ 2$\sigma$ 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 $\sim 2$, or a thermal emission with a temperature exceeding $\sim3$keV. In HCG 62, the 2–10 keV luminosity of the excess hard component was found to be $5.5\times10^{41}$erg s$^{-1}$ at 2–10 keV, which is $\sim 30\%$ of the thermal IGM luminosity in 0.7–2.5 keV. Non-thermal and thermal interpretations of this excess components are discussed.