Journal Article

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

Kazuhiro Nakazawa, Kazuo Makishima and Yasushi Fukazawa

in Publications of the Astronomical Society of Japan

Volume 59, issue 1, pages 167-183
Published in print February 2007 | ISSN: 0004-6264
Published online February 2007 | e-ISSN: 2053-051X | DOI: http://dx.doi.org/10.1093/pasj/59.1.167
Hard X-Ray Properties of Groups of Galaxies as Observed with ASCA

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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 [math] 2[math] 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 [math], or a thermal emission with a temperature exceeding [math]keV. In HCG 62, the 2–10 keV luminosity of the excess hard component was found to be [math]erg s[math] at 2–10 keV, which is [math] of the thermal IGM luminosity in 0.7–2.5 keV. Non-thermal and thermal interpretations of this excess components are discussed.

Keywords: intergalactic medium; galaxies: clusters: general; X-rays: galaxies: clusters

Journal Article.  10702 words.  Illustrated.

Subjects: Galaxies ; Astronomy and Astrophysics

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