Early XRay Observations

Early X-ray observation in this region with rocket and satellite instrumentation have revealed many variable, but few steady, sources. Uhuru observations showed the first evidence for an extended source, although not distinguishable from a source complex. Subsequent observations with other satellite instruments have led to the detection of several new sources, including bursters. But not earlier than with the Einstein Observatory in 1979, X-ray imaging of the Galactic Center could be performed [27] (Fig. 19.2).

In addition to a large, diffuse emission extending over more than 30' in NE-SW direction (i.e., along the galactic plane), enhanced flux could be measured from the direction of Sgr A. Although the angular resolution was only of the order of 1'

ROSAT PSPC

-28°40' □—■—1—1—1—1—I—1—1—'—'—'—I—'—'—'—'—'—q

ROSAT PSPC

-28°40' □—■—1—1—1—1—I—1—1—'—'—'—I—'—'—'—'—'—q

17h47m 17h46m 17h45m 17h44m

Right Ascension [2000]

17h47m 17h46m 17h45m 17h44m

Right Ascension [2000]

Fig. 19.2 Early X-ray images of the Galactic Center region observed with the Einstein Observatory (top, [27]) and ROSAT (bottom, [19])

Rigth ascension (1950.0)

Fig. 19.3 ART-P observations, spring 1990 [17]. Image of a 2.3° x 2.3° region near the Galactic Center (3-17 keV). Map contours correspond to a statistical significance of source detection at the 3.5, 5, 6.5 standard deviation levels. Each pixel is 1.3' on a side

Rigth ascension (1950.0)

Fig. 19.3 ART-P observations, spring 1990 [17]. Image of a 2.3° x 2.3° region near the Galactic Center (3-17 keV). Map contours correspond to a statistical significance of source detection at the 3.5, 5, 6.5 standard deviation levels. Each pixel is 1.3' on a side with the imaging proportional counter (IPC), 12 point sources could be resolved, among them also one coincident with the Center itself. Its nature as well as the nature of the apparently diffuse emission remained unclear. Twelve years later, the same region was observed with ROSAT, and a few more sources could be resolved, although the energy bandpass of ROSAT was a bit too soft compared with the large column density toward the Galactic Center and the resulting low energy cut-off due to the interstellar absorption. The source at the Center found by Einstein (1E1742.5-2859) could be resolved into three individual objects (Fig. 19.2), one out of which is coincident with Sgr A* within 8" [19].

The luminosity of this source (or source complex given the prosumed density in this region) was derived to be Lx = 1.1 x 1035ergs_1. At higher energies, instruments like ART-P on the Russian Granat satellite could detect a few point sources, most of them variable but also some diffuse emission on a larger scale [17] (Fig. 19.3). Brightest source in this field is a burster (A1742-294) and the "Einstein-source" 1E1740.7-2942, a source that exhibits a 511 keV line and is, therefore, often referred as the "Great Annihilator." Another bright source is the double source SLX1744-299/300 slightly south of the Galactic Center.

Fig. 19.4 Chandra "true color" X-ray mosaic of the central 2° x 0.8° band of the GC region [26]. The three energy bands are 1-3 keV (shown in red), 3-5 keV (green), and 5-8 keV (blue). The spatial resolution ranges from 0.5'' on-axis to 5'' at the near edge of the CCD and to 10'' at the diagonal edge. This image is adaptively smoothed with a signal-to-noise ratio of 3 and is plotted logarithmically to emphasize low surface brightness emission. The saw-shaped boundaries of the map, plotted in Galactic coordinates, results from a specific roll angle of the observations

Fig. 19.4 Chandra "true color" X-ray mosaic of the central 2° x 0.8° band of the GC region [26]. The three energy bands are 1-3 keV (shown in red), 3-5 keV (green), and 5-8 keV (blue). The spatial resolution ranges from 0.5'' on-axis to 5'' at the near edge of the CCD and to 10'' at the diagonal edge. This image is adaptively smoothed with a signal-to-noise ratio of 3 and is plotted logarithmically to emphasize low surface brightness emission. The saw-shaped boundaries of the map, plotted in Galactic coordinates, results from a specific roll angle of the observations

Fig. 19.5 Galactic Center region seen with XMM-Newton (left, [20]). Red color represents photon energies below 2.5 keV, dark blue color energies above 6 keV, green color is for intermediate energies between 2.5 and 6 keV. The bright supernova remnant Sgr A East is at the center of the image. There is diffuse emission along the galactic plane into NE direction and filaments of 6.4 keV emission (neutral iron Ka line, light blue color). The corresponding Chandra Observatory image (right, smaller field of view, higher resolution) shows the detailed structure of Sgr A East and, across to the galactic plane, various lobes of hot gas, probably remnants of past explosions at the center [13]

Fig. 19.5 Galactic Center region seen with XMM-Newton (left, [20]). Red color represents photon energies below 2.5 keV, dark blue color energies above 6 keV, green color is for intermediate energies between 2.5 and 6 keV. The bright supernova remnant Sgr A East is at the center of the image. There is diffuse emission along the galactic plane into NE direction and filaments of 6.4 keV emission (neutral iron Ka line, light blue color). The corresponding Chandra Observatory image (right, smaller field of view, higher resolution) shows the detailed structure of Sgr A East and, across to the galactic plane, various lobes of hot gas, probably remnants of past explosions at the center [13]

At higher energies, the center of the Galaxy itself (Sgr A*) belongs to the less striking sources in the field. These results made it clear that our Galactic Center is a rather weak representative of galactic nuclei (Fig. 19.4).

Meanwhile, with the use of high resolution and high sensitivity X-ray telescopes (Chandra Observatory and XMM-Newton) Sgr A* could be established as an X-ray source, primarily by its flaring behavior. At the same time also in near infrared flares could be detected from this compact radio source. Thus the large variety of theoretical models for the Galactic Center source could be somewhat constrained.

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