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Kazuhiro Nakazawa, Kazuo Makishima, Yasushi Fukazawa, Hard X-Ray Properties of Groups of Galaxies as Observed with ASCA, Publications of the Astronomical Society of Japan, Volume 59, Issue 1, 25 February 2007, Pages 167–183, https://doi.org/10.1093/pasj/59.1.167
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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.
