H. Awaki, K. Koyama, H. Kunieda, S. Takano, Y. Tawara, and T. Ohashi

Abstract

Three X-ray luminous early-type galaxies NGC 4472, NGC 4636, and NGC 3998 were oserved with the Ginga satellite. The X-pray 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 + or - 0.2 keV and 0.9 + or - 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) x 10 to the 12th solar masses and (0.5-2.2) x 10 to the 12th solar masses, respectively. The X-ray spectrum of NGC 3998 cannot be fitted by a thin thermal model but is well fitted by a power-law model having a photon index of 2.0 + or - 0.1, the same sort of spectrum typical of AGNs.