Summer 2001 -- Vol. 18, No. 2

Astronomers Utilize Chandra to Craft Unprecedented Detail of Nearby Galaxy

Astronomers using the Chandra X-ray Observatory have made unprecedented observations of the inner regions and general structure of the nearby Circinus Galaxy, enabling them to study both the supermassive black hole in the center of the galaxy and numerous potential smaller black holes or neutron stars sprinkled throughout the spiral disk of the galaxy.

For the first time, they have detected the periodic variability of one of the emission sources associated with one of those black holes or neutron stars. They also have gained a better understanding of the complexity, composition, and structure of the matter nearby the supermassive black hole.

The astronomers from Penn State and George Mason University—whose results have been published in three papers in The Astronomical Journal —have determined that numerous X-ray emitting sources are sprinkled throughout the galaxy, and that those sources are probably the result of systems in which a neutron star or small black hole closely orbits an evolving normal star. Those systems are known as X-ray binary systems.

“Thanks to Chandra, we were able to show the periodicity of that source, and that is the key signature to an X-ray binary system,” said Franz Bauer, a postdoctoral scholar from Penn State who detailed the group’s results in one of the papers. “Usually we can do that only with the closest of sources, but for the first time we were able to get a glimpse at such a system outside our galaxy.”

Chandra’s detailed X-ray spectrum allowed astronomers to detect emissions and gather information not possible with other X-ray observatories.

Results associated with the Chandra images included an analysis of 16 emission points, known as “point sources,” and a determination that nearly half of the sources coincided with the star-forming features of the galaxy established by previous optical and radio images.

Earlier studies have established that Circinus, located 13 million light years from Earth, contains a supermassive black hole and that matter accreting onto that black hole radiates with such intensity that it outshines the rest of the galaxy. Such galaxies are often called “active” galaxies, and their centrally located black holes are called “active galactic nuclei” or AGN, because the intensity of the radiation is much greater than that of “passive” galaxies such as our own Milky Way. Because Circinus is the closest active galaxy to Earth, further study of it might provide important information about regions close to the centers of other active galaxies.

Chandra’s detailed images and spatial resolution of the galaxy allowed the astronomers to determine that its emission is resolved into a number of distinct components that are associated with a central black hole. A bright, compact emission source is present at the center of the image and that nuclear source is surrounded by a diffuse X-ray halo that extends out several hundred light years.

In addition, there is a strong correlation between the X-ray emission and the high-excitation ionized gas seen in emission-line images obtained by the Hubble Space Telescope and ground-based telescopes.

The astronomers also conducted a detailed analysis of the gas distribution and physical conditions near the center of the galaxy and identified at least two different gas components. One, a warm gas that is photoionized by the radiation field from the black hole, contains strong emissions of highly ionized elements such as argon, calcium, iron, magnesium, neon, silicon, and sulfur. The second gas component is cooler and features a strong iron emission line.

In addition, the astronomers were able to show that the two gas components have different spatial extents, with the warmer gas being spread over a much larger region around the black hole than the cooler gas.

“The Chandra observations of Circinus are showing us that the gaseous environment of supermassive black holes can be very complex,” said Rita Sambruna, assistant professor of physics and astronomy at George Mason University. “Because it is close and thus easy to study, Circinus provides and important testbed for what might be happening in other, more distant AGNs.”

Along with Sambruna and Bauer, the research group included Hagai Netzer of Tel-Aviv University and the following collaborators from Penn State: Neil Brandt, assistant professor of astronomy and astrophysics; George Chartas, senior research associate; Gordon Garmire, Evan Pugh Professor of Astronomy and Astrophysics; John Nousek, professor of astronomy and astrophysics; and Shai Kaspi, a postdoctoral researcher. Observations with Chandra, using the Advanced CCD Imaging Spectrometer (ACIS) and the High Energy Transmission Grating Spectrometer (HETGS), were made on 6 and 7 June 2000.

(Images and captions available at http://www.science.psu.edu/alert/Bauer5-2001-2.htm#images)

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