paper

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

arXiv:astro-ph/0612753 · doi:10.1093/pasj/59.1.167

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 the IGM 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 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 significance, even though the background uncertainties and the IGM modeling errors are carefully accounted. A hard excess could be also present in NGC 1399. The excess was successfully explained by a power-law model with a photon 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 to be 5.5E41 erg/s at 2--10 keV, which is ~ 30% of the thermal IGM luminosity in 0.7--2.5 keV. Non-thermal and thermal interpretations of this excess components are discussed.

18 pages, 10 figures; published with minor updates in proof procedure

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