X-Ray Obervation of the Normal Spiral Galaxies NGC2903 and NGC628 with ASCA

Abstract

X-ray emission from spiral galaxies without activity is thought to consist of low-mass X-ray binaries (LMXBs), and the Einstein observations established the relation between X-ray (0.2-4 keV) and optical luminosities as $\log (\frac{{{{L}_{X}}}}{{{{L}_{B}}}})\sim - 4$ (Fabbiano, 1992). This relation has been used when discussing the activity other than LMXBs (Iyomoto, 1996). However, spectral information of Einstein Observatory was rather poor above 3 keV, where LMXBs would emit significant energy flux. Therefore we performed ASCA observations of two normal spirals, NGC2903 and NGC628, in order to better calibrate the L _x -L _B relation.