Yoshitomo Maeda

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

The nuclei of most galaxies are now believed to harbour supermassive black holes. The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 × 106u2009solar masses (refs 2, 3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgru2009A*) at the dynamical centre of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. Sgru2009A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently. Here we report the discovery of rapid X-ray flaring from the direction of Sgru2009A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Centre.

A Chandra Study of Sagittarius A East: A Supernova Remnant Regulating the Activity of Our Galactic Center?

We report on the X-ray emission from the shell-like, nonthermal radio source Sgr A East (SNR 000.0+00.0), located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-Ray Observatory. This is the first time Sgr A East has been clearly resolved from other complex structures in the region. The X-ray-emitting region is concentrated within the central 2 pc of the larger radio shell. The spectrum shows strong Kα lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature ~2 keV, absorption column ~1 × 1023 H cm-2, luminosity ~8 × 1034 ergs s-1 in the 2-10 keV band, and gas mass ~2η1/2 M☉ with a filling factor η. The plasma appears to be rich in heavy elements, overabundant by roughly a factor of 4 with respect to solar abundances, and shows a spatial gradient of elemental abundance; the spatial distribution of iron is more compact than that of the lighter elements. The gas mass and elemental abundance of the X-ray emission support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR), perhaps produced by the Type II supernova explosion of a massive star with a main-sequence mass of 13-20 M☉. The combination of the radio and X-ray morphologies classifies Sgr A East as a new metal-rich mixed morphology (MM) SNR. The size of the Sgr A East radio shell is the smallest of the known MM SNRs, which strongly suggests that the ejecta have expanded into a very dense interstellar medium. The ejecta-dominated chemical compositions of the plasma indicate that the ambient materials should be highly homogeneous. We thus evaluate a simplified dynamical evolution model where an SNR was formed about 10,000 yr ago and expanded into an ambient medium with a homogeneous density of 103 cm-3. The model roughly reproduces most of the observed properties in the X-ray and radio wavelengths. A comparison with the radio observations requires the dense ambient medium to be ionized, but a luminous X-ray irradiator with an expected X-ray luminosity of ~1040 ergs s-1 is not currently present. The presence of the ionized gas may be explained if the massive black hole (MBH) associated with the compact, nonthermal radio source Sgr A* was bright in X-rays about 300 yr ago but is presently dim. It is possible that the dust/molecular ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past and may have removed material from the black hole vicinity, leading to its present quiescent state. This may be a specific example of the intimate relationship between an SNR and MBH accretion activity in galactic nuclei.

ASCA Observations of the Sagittarius B2 Cloud: An X-Ray Reflection Nebula

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 cutoU 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 1@¨2@. 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 re—ection nebula. Possible implications of the Galactic center activity related to this unique source are presented. Subject headings: Galaxy: abundancesGalaxy: centerISM: clouds ¨ ISM: individual (Sagittarius B2) ¨ X-rays: ISM

Chandra Observations of Diffuse X-Rays from the Sagittarius B2 Cloud

We present the first Chandra results of the giant molecular cloud Sagittarius B2 (Sgr B2), located about 100 pc away from the Galactic center. Diffuse X-rays are clearly separated from one-and-a-half-dozen resolved point sources. The X-ray spectrum exhibits pronounced iron K-shell transition lines at 6.40 keV (Kα) and 7.06 keV (Kβ), deep iron K-edge at 7.11 keV, and large photoelectric absorption at low energy. The absorption-corrected X-ray luminosity is ~1 × 1035 ergs s-1, 2 orders of magnitude larger than the integrated luminosity of all the resolved point sources. The diffuse X-rays come mainly from the southwest half of the cloud with a concave shape pointing to the Galactic center direction. These results strongly support the ASCA model that Sgr B2 is irradiated by an X-ray source at the Galactic center side.

Discovery of X-rays from the protostellar outflow object HH2

Herbig–Haro (HH) objects have been known for 50 years to be luminous condensations of gas in star-forming regions, but their underlying physical nature is still being elucidated. Previously suggested models encompass newborn stars, stellar winds clashing with nebular material, dense pockets of interstellar gas excited by shocks from outflows, and interstellar ‘bullets’ (ref. 6). Recent progress has been made with the jet-induced shock model, in which material streams out of young stellar objects and collides with the surrounding interstellar medium. A clear prediction of this model is that the most energetic Herbig–Haro objects will emit X-rays, although they have not hitherto been detected. Here we report the discovery of X-ray emission from one of the brightest and closest Herbig–Haro objects, HH2, at a level consistent with the model predictions. We conclude that this Herbig–Haro object contains shock-heated material located at or near its leading edge with a temperature of about 106u2009K.

Origin of the hard x-ray emission from the Galactic plane.

The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17′ by 17′ field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space.

ASCA Discovery of Diffuse 6.4 keV Emission near the Sagittarius C Complex: A New X-Ray Reflection Nebula

We present an ASCA discovery of diffuse hard X-ray emission from the Sgr C complex with its peak in the vicinity of the molecular cloud core. The X-ray spectrum is characterized by a strong 6.4 keV line and large absorption. These properties suggest that Sgr C is a new X-ray reflection nebula that emits fluorescent and scattered X-rays via irradiation from an external X-ray source. We found no adequately bright source in the immediate Sgr C vicinity to fully account for the fluorescence. The irradiating source may be the Galactic nucleus Sgr A*, which was brighter in the past than it is now, as is suggested from observations of the first X-ray reflection nebula Sgr B2.

X-Ray Evidence for Wind-Wind Collision in the Wolf-Rayet Binary V444 Cygni

We report the results of three ASCA observations of the eclipsing Wolf-Rayet binary V444 Cyg (WN5+O6). These observations were obtained at orbital phases 0.0, 0.25, and 0.5, with the WN5 star in front at phase 0.0 and the O6 star in front at phase 0.5. Acceptable fits of the X-ray spectra using optically thin plasma models require at least two different temperature components, with a soft component at kT1≈0.6 keV and a harder component at kT2≈2 keV. The absorption of the hard component varies with orbital phase and is largest when the WN5 star is in front, whereas the X-ray luminosity of the hard component is at a minimum when the O6 star is in front. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the WN5 and O6 stars, with the shock most likely located near the surface of the O6 star. On the other hand, the soft-component emission at kT1≈0.6 keV has nearly constant absorption and X-ray luminosity. The soft-component luminosity is LX, 1=(6-11) × 1032 ergs s-1 (0.2-4 keV), implying LX, 1/Lbol~10−6 to 10-7. This luminosity ratio and the soft-component temperature are similar to those of single massive stars, leading us to attribute the soft emission to the individual O6 and WN5 components.

ASCA Observations of GRO J1744–28

We report the ASCA results of the bursting X-ray pulsar GRO J1744-28, which was observed in 1996 February and 1997 March. The source flux in the 2-10 keV band was 2.0 × 10-8 ergs s-1 cm2 in 1996 and 5.0 × 10-9 ergs s-1 cm2 in 1997. We detected 12 and 17 type II bursts during the two observations, with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6-7 keV. The constant absorption column, (5-6) × 1022 cm-2, independent of the observation dates and emission phases (persistent, burst, and dip) is interpreted as an interstellar absorption. The source may be actually located near the Galactic center, at a distance of 8.5 kpc. The structure in the energy spectrum at 6-7 keV is most probably due to iron and may be reproduced by a disk line model with additional broadening mechanism.

Unusual Properties of X-Ray Emission near the Galactic Center

The X-ray spectrum in a