X-ray Imaging Spectroscopy of the Galactic Center Region

Yoshitomo Maeda

Paper: PhD thesis, Dept.of Physics, Kyoto University


We present the ASCA results of the first high quality X-ray images and spectra of the Galactic Center in the wide X-ray band of 0.7 - 10 keV. We found a new X-ray source AX J1745.6-2901 about 1'.3 south-west from the nuclear supermassive black hole candidate Sgr A*. We discovered a type-I X-ray burst and eclipses from AX J1745.6-2901, hence established AX J1745.6-2901 to be an eclipsing low-mass X-ray binary. Excess soft X-rays during the eclipse were detected, which is well explained by the scattering process of interstellar dust-grains.

From the entire ~1 square degree field near the Galactic Center, we found a diffuse emission with K-shell transition lines from highly ionized elements (silicon, sulfur, argon, calcium: here "the lighter elements"), in addition to that of highly ionized iron, which has been previously reported . The K alpha flux from highly ionized iron is symmetrically extended along the Galactic plane, while those of the lighter elements are more irregularly distributed, both with the peaks at a few arcmin-radius region near Sgr A*. K alpha lines from neutral or low ionized iron are also found from molecular clouds. The morphology, spectrum and iron line flux are fully consistent with reflected X-rays from the cold molecular clouds, but we found no apparent bright source to irradiate the clouds.

The presence of the K alpha line from highly ionized iron gives firm evidence for a diffuse high-temperature plasma of kT~10 keV, while those of the lighter elements indicate the presence of much less temperature plasma of ~1 keV.

The X-rays in the brightest region near Sgr A* would come from a hot gas filling the interstellar space with the thermal energy of about 2*1050 erg.

In the entire ~1 square degree field, the lower temperature plasma is interpreted to be attributable to a superposition of 101-2 supernova remnants, while the origin of the higher temperature plasma is more debatable.

The higher temperature plasma with the total kinetic energy as large as 1054 ergs, may come from a diffuse hot gas. We suggest that the Galactic center exhibited intermittent activities with a time-averaged energy generation rate ( ~1041-42 erg s-1) comparable to the Seyfert nuclei, a class of active galactic nuclei. The intermittent activity is also one possibility to explain the reflected X-rays with no apparent irradiating sources; the clouds has been irradiated by X-rays from the Galactic center which was bright in the past but is dim at present.

A superposition of faint discrete sources such as 105 of cataclysmic variables (CV), would be another possibility. This requires an extremely high density of CVs near the center, and also predicts a high rate of type-Ia supernova explosion.

We also carried out the Chris-cross mapping up to 10 degree around the Galactic Center with ASCA and detected the Galactic bulge and ridge emissions around the Galactic Center but with much reduced flux level. The observed Galactic ridge spectra often show the K alpha -lines from highly-ionized silicon, sulfur, argon, calcium, and iron. The bulge emission shows a high emissivity for the silicon K alpha -line, which may be another remain of the past activities in the Galactic Center.

Preprints available from the authors at maeda@astro.psu.edu .

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