GINGA OBSERVATIONS OF 3 X-RAY-LUMINOUS EARLY-TYPE GALAXIES - NGC-4472, NGC-4636, AND NGC-3998

Three X-ray luminous early-type galaxies, NGC 4472, NGC 4636, and NGC 3998, were observed with the Ginga satellite. The X-ray spectra of NGC 4472 and NGC 4636 can be described as thermal emission from an optically thin plasma. The temperatures of the hot gas are determined more accurately than with the Einstein imaging proportional counter (IPC) observations and are 1.9±0.2 keV and 0.9±0.2 keV for NGC 4472 and NGC 4636, respectively. Assuming hydrostatic equilibrium, the binding masses of NGC 4472 and NGC 4636 are estimated to be (0.6-4.6)×1012 M⊙ and (0.5-2.2) ×1012 M⊙, respectively. The X-ray spectrum of NGC 3998 cannot be fitted by a thin thermal model but is well fitted by a powerlaw model having a photon index of 2.0±0.1, the same sort of spectrum typical of AGNs. Although the X-ray flux of NGC 3998 qualifies it as a low-luminosity Seyfert galaxy, no time variability in less 3×104 s could be observed.