Masahiro Tsujimoto

We present the ASCA results of imaging spectroscopy of the giant molecular cloud Sgr B2. The X-ray spectrum is found to be very peculiar; it exhibits a strong emission line at 6.4 keV, a low energy cutoff below 4 keV and a pronounced edge-structure at 7.1 keV. The X-ray image is extended and its peak position is shifted in the direction of the Galactic center by about 1-2 arcminute from the core of the molecular cloud. The X-ray spectrum and morphology are well reproduced by a scenario that X-rays from a source located at the Galactic center side are scattered by the molecular cloud Sgr B2, and come into our line of sight. Thus Sgr B2 may be called an X-ray reflection nebula. © 2000 COSPAR. Published by Elsevier Science Ltd.

We present the ASCA results of imaging spectroscopy of the giant molecular cloud Sgr B2. The X-ray spectrum is found to be very peculiar; it exhibits a strong emission line at 6.4 keV, a low-energy cutoff below about 4 keV, and a pronounced edge structure at 7.1 keV. The X-ray image is extended, and its peak position is shifted from the core of the molecular cloud toward the Galactic center by about 1́-2́. The X-ray spectrum and the morphology are well reproduced by a scenario that X-rays from an external source located in the Galactic center direction are scattered by the molecular cloud Sgr B2 and come into our line of sight. Thus, Sgr B2 may be called an X-ray reflection nebula. Possible implications of the Galactic center activity related to this unique source are presented.

We report on the discovery and analysis of an X-ray pulsar, AX J0058-7203, in the Small Magellanic Cloud. This pulsar exhibits coherent pulsations at P = 280.3 s with a double-peak structure. The X-ray spectrum is well fitted with a simple power-law model of photon index Γ ∼ 0.7. No significant change of the pulsation period over the observation was found. A comparison with ROSAT observations in the same field reveals that AX J0058-7203 is highly variable, and is most likely a Be star binary pulsar.

ASCA observed the central region of the Small Magellanic Cloud, and found a hard X-ray source, AX J0051-733, at the position of the ROSAT source RX J0050.8-7316, which has an optical counterpart of a Be star. Coherent X-ray pulsations of 323.1 ± 0.3 s were discovered from AX J0051-733. The pulse profile shows several sub-peaks in the soft (0.7-2.0 keV) X-ray band, but becomes nearly sinusoidal in the harder (2.0-7.0 keV) X-ray band. The X-ray spectrum was found to be hard, and is well fitted by a power-law model with a photon index of 1.0 ± 0.4. The long-term flux history was examined with the archival data of Einstein observatory and ROSAT; a flux variability with a factor ≳ 10 was found.

We report on the results of two ASCA observations on an X-ray source, AX J0049-729 (= RX J0049.1-7250), in the Small Magellanic Cloud. A sinusoidal pulsation with a barycentric period of 74.676 ± 0.022 s was found. The pulse fraction was ∼ 70%, independent of the X-ray energy. The spectrum was hard with an absorbed power-law having a photon index of ∼ 1. The X-ray flux was 1.1 x 10-11 erg s-1 cm-2, and then declined to 1.6 x 10-12 erg s-1 cm-2 from the first to the second observations, separated by about one month. Together with the previous results of the Einstein and ROSAT satellites, we found that AX J0049-729 has been in quiescent states, but occasionally showing flare activities. The flux variability, long-pulse period, and hard spectrum support the idea that AX J0049-729 is an X-ray binary pulsar, most likely a Be star binary.

With ASCA, we obtained detailed X-ray images and spectra of the Galactic Center. X-rays in the central parsecs from the dynamical center of the Galaxy, Sgr A*, were dominated by extended emission. Since Sgr A* was not spatially resolved from the extended structure, an upper limit to its luminosity was found to be 3 × 1035 erg s-1. This result indicates that the current AGN activity of the Galactic nucleus is very low. In the central hundreds of parsecs, X-ray stars, thin thermal diffuse plasma, and iron fluorescent emission from molecular clouds were seen. Based on the analyses of the X-ray stars and both of the diffuse components, we discuss some features which are possibly related to past nuclear activities.

We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained with five pointing observations in a time span of 3.5 yr. The X-ray spectrum for each period is well fitted with a single power law absorbed by a high column of gas. The X-ray flux changes by a factor of 2 from period to period, but the other spectral parameters show no significant change. The photon index is flat with T = 0.9-1.3. The column density of hydrogen NH is ∼ 1.0 × 1023 H cm-2 and that of iron NFe is ∼ 1019 Fe cm-2. These large column densities indicate that 1E 1740.7-2942 is near the Galactic center. The column density ratio leads the iron abundance to be 2 times larger than the other elements in a unit of the solar ratio. The equivalent width of the Kα line from a neutral iron is less than 15 eV in 90% confidence. This indicates that the iron column density within several parsecs from 1E 1740.7 -2942 is less than 5 × 1017 Fe cm-2. In addition, the derived hydrogen column density is about one-sixth of that of giant molecular clouds in the line of sight. All these facts support the fact that 1E 1740.7-2942 is not in a molecular cloud, but possibly in front of it; the X-rays are not powered by accretion from a molecular cloud, but from a companion star like ordinary X-ray binaries.