ASCA SPECTRA OF THE X-RAY FAINT S0 GALAXY NGC 4382

摘要

NGC 4382 is one of the E and S0 galaxies detected with the lowest X-ray-to-optical luminosity ratio. These galaxies have a peculiar X-ray (0.1-3 keV) spectrum, with a significant excess of counts in the lowest spectral channels ( < 1 keV) relative to the spectral count distributions of X-ray brighter E and S0 galaxies. Analyzing the ROSAT Position Sensitive Proportional Counter observations of NGC 4382, it was unclear whether this soft excess were due to a real very soft component in a multicomponent spectrum or reflected an extremely low metal abundance in a isothermal hot gas. Our ASCA observations show that the low-abundance single-temperature model does not fit well the X-ray spectrum, in agreement with our previous suggestions. A better explanation is a composite spectrum with a very soft component (～0.3 keV) in addition to a hard, likely stellar, component (～5 keV). In this model, the abundance cannot be constrained. However, other more complex spectral models cannot be excluded.