Broad band properties of medium and low Lx/Lb Early Type Galaxies

Abstract

We have measured the spectral properties of five galaxies of low to intermediate Lx/Lb ratios with BeppoSAX and ASCA. A hard component (kT ~4-10 keV) is observed in all galaxies. In NGC1553 the BeppoSAX data show that this component is extended, and suggest an origin for the emission in the evolved stellar population. In NGC3115, a point-like source appears embedded in an extended component morphologically similar to the stellar body, suggesting an almost equal contribution from the nuclear region and the binary population. A large central mass concentration and low level optical activity in NGC3379 argue for a contribution from the nucleus also in this object. However, for both NGC3115 and NGC3379 the nuclear emission is at a level well below that observed in other galaxies who host similar nuclear black holes. A second soft (kT ~0.3-0.7 keV) spectral component is needed to fit the data of NGC1407, NGC1553 and NGC4125 over the entire energy range probed (about 0.2-10 keV), best represented by a thermal component with line emission. We discuss possible interpretations of the origin of this component, which however will be better defined only with higher quality data.