Harvard-Smithsonian Center for Astrophysics|
The CfA Almanac Vol. XIII No. 1, March 2000
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The voracious black hole at the center of our galaxy may be dining on a steady stream of dust and gas pouring into the Milky Way's inner regions. Robin S. McGary and Paul T.P. Ho of the CfA report that narrow "streamers" of ammonia gas appear to be flowing from giant clouds of gas toward the center of the galaxy.
Astronomers have long known that a supermassive black hole, more than 2 million times more massive than our Sun, lies at the center of the galaxy some 27,000 light-years from Earth. A point-like source of radio emission called Sagittarius A* (pronounced "A-star") marks the location of this black hole. The black hole is surrounded by a ring of dust and gas orbiting Sagittarius A* (Sgr A*) at a radius of about 6.5 light-years from the black hole. This "circumnuclear disk" revolves around the black hole at a velocity of 110 km/s. Gas and dust are stripped from the disk by the strong gravitational pull of the black hole and spiral towards Sgr A*.
The formation and dynamics of this circumnuclear disk are unclear, so the astronomers have been looking for clues to the origin of the dust and gas seen in the circumnuclear disk. Their most recent results come from the observation of ammonia molecules in the region around the disk. Emission from the ammonia molecules traces the dense, hot gas known to exist near the center of the galaxy and can be used to determine the location and motion of gas near the circumnuclear disk.
Since some ammonia emission has a wavelength of 1.3 cm, the National Science Foundation's Very Large Array radio telescope at Socorro, NM, was used for the observations. The array is composed of 27 radio antennas, each 25 meters (82 ft) in diameter. The antennas were arranged in a "Y" shape, 3.4 km (2.1 mi) across, allowing the resolution of details as small as 0.13 light-years in diameter at the galactic center.
Using the High Resolution Camera (HRC) built by SAO for the Chandra X-ray Observatory, an international team of astronomers has made an energy bar code of hot gas in the vicinity of a giant black hole. These measurements, the most precise of their kind ever made with an X-ray telescope, demonstrate the existence of a blanket of warm gas that is expanding rapidly away from the black hole.
A team led by scientists from the Space Research Organization of the Netherlands (SRON) used the Low Energy Transmission Grating in conjunction with the HRC--constructed under the direction of SAO's Steve Murray--to measure the relative intensity of X rays present at each energy wavelength. With this information they constructed an X-ray spectrum of the source.
Their target was the central region, or nucleus of the galaxy NGC 5548, which they observed for 24 hours. This galaxy is one of a class of galaxies known to have unusually bright nuclei that are associated with gas flowing around and into giant black holes. This inflow produces an enormous outpouring of energy that blows some of the matter away from the vicinity of the black hole.
As the high-energy X rays stream away from the vicinity of the black hole, they heat the surrounding blanket of gas to temperatures of a few million degrees, and the blanket absorbs some of the X rays from the central source. This produces dark stripes, or absorption lines, in the X-ray spectrum. Bright stripes, or emission lines due to emission from the blanketing gas, are also present. Since each element has its own unique structure, these lines can be read like a cosmic bar code to take inventory of the gas. The team was able to determine what atoms the gas contains and how many of each kind, the number of electrons each atom has retained in the hostile environment of the black hole, and how the gas is moving there. They found lines from eight different elements, including carbon, nitrogen, oxygen, and iron. The amount of this gas was found to be about 100 times greater than that found with optical and ultraviolet observations.
A forthcoming issue of Smithsonian's Research Reports will feature the stunning success of the Chandra Observatory. Of course, like all major space science projects, that success is due to the contributions of literally hundreds of individuals at NASA, at MIT and Penn State, at TRW and at dozens of other academic, industrial, and commercial partners. At the CfA alone, scores of people--administrators, secretaries, contract specialists, computer programmers, purchasing agents, and even public affairs writers provided behind-the-scenes support of the scientific endeavor that would lead to Chandra's new visions of the Universe. Perhaps the largest single group was the Central Engineering Department, headed by Bruce Dias, which had the task of transforming the visionary concepts of space scientists into concrete forms of flight hardware. (See Research Reports, No. 97, Summer 1999).
Space limitations would not allow citing every member of CE in Research Reports, so here is a list of (hopefully, all) those contributors from Central Engineering: Draftsmen/Designers Robert Cook, Arthur Gentile, Edward Imbier, James Logan, and Dale Noll; Technicians William Brymer, Florine Collette, Frank DeFreze, Leslie Frazier, Francisco Rivera, Patricia Riddle, Richard Scovel, and Peter Warren; and, Engineers Vaman Bawdekar, John Boczenowski, David Boyd, Lester Cohen, William Davis, Edward Dennis, Roger Eng, Mark Freeman, Richard Goddard, Jack Gomes, Russell Ingram, Everett Johnston, Michael Maresco, Warren Martell, Timothy Norton, Paul Okun, Mark Ordway, Paul Ouellette, William Podgorski, John Polizotti, Joel Rosenberg, Adrian Roy, David Weaver, and Ying Zhou. Among the former members of Central Engineering are: Edward McLaughlin, Fred Robinson, Noel Lanham (deceased), Elizabeth Whitbeck, and Randall Moore. In addition, members of the Chandra Program Office should also be mentioned, including Gerry Austin, Kathy Daigle, Lauren Deknis-Bortolami, Sandra Field-Daly, Carol Oliphant, Joseph Swider, Joseph Webber, and Janice Wilson, and recently retired members of the office, including Frank Cocuzzo, Phil McKinnon, and Dave Schultz.
What started out as a routine observing project from a small campus telescope has been turned into an exciting discovery of a possible black hole in an unusual collaboration between undergraduate students and astronomers at the CfA. The black hole candidate is part of a binary star system in which the two components revolve around each other every 91 days.
If confirmed, this will be the longest-known orbital period for a black hole binary by a factor of about 10 and the first system known to undergo eclipses from Earth's viewpoint. This fortuitous combination makes it an excellent laboratory for learning more about how black holes are fueled.
"When we started this project, I never dreamed that a black hole might be lurking in this star system," says Priscilla Benson, a professor of astronomy at Wellesley College, who presented the results at the American Astronomical Society meeting in Atlanta in January. "What makes this discovery even more exciting is that much of the work was done by undergraduates using relatively small telescopes."
The investigation of this system, a variable star of unknown type and period named BG Geminorium (known as BG Gem), began in 1992 when Benson asked her undergraduate students to monitor its brightness changes with Wellesley College's 0.6-meter (24-inch) telescope at the school's Whitin Observatory on the campus west of Boston. Five years' worth of data were then compiled by student observer Alceste Bonanos, now a Wellesley College senior, who determined BG Gem is a long-period ellipsoidal binary system.
The resulting light curve piqued the interest of binary star expert Scott Kenyon of the CfA, who recognized this object as an interesting binary in which the light from the "secondary star" is being stretched out by the strong gravity of the more massive "primary star." Further, material from the secondary seems to be flowing towards the primary.
To probe this system further, Kenyon obtained visible spectra of BG Gem with the 1.5-meter (60-inch) telescope of the Fred L. Whipple Observatory. "I was very excited by our first spectrum showing emission from hot gas orbiting the primary star," says Kenyon. "Then, I was sure we had an interesting and rare type of binary."
The spectra, analyzed by Swarthmore College junior Allyn Dullighan along with Benson, Kenyon, and Kim McLeod, assistant professor of astronomy at Wellesley, yielded some surprises. Hot gas around the primary appears to be eclipsed by the cooler secondary once in each orbital period. Likewise, the secondary, estimated from its spectrum to contain half the mass of the Sun, is eclipsed half a cycle later. The eclipses imply that the binary system orbits in a plane nearly lying in our line of sight--a rare arrangement. The eclipses allow estimation of the sizes of the star and the disk.
But the big surprise was yet to come. Dullighan determined that the secondary star races around the primary at 75 km/s (170,000 mph), but the hot gas shows that the primary itself stands nearly still. Like a small dancer being whipped around by a big, strong partner, the secondary in BG Gem is being pulled around at high speed by the gravity of a more massive primary. Using Kepler's Laws of Motion, the students estimated the mass of the primary to be about 4.5 times the mass of the Sun. Kenyon refined this basic result with detailed computer modeling and determined that such a massive primary should be easily visible. While the spectra do indicate hot gas swirling in an accretion disk around the primary, they fail to show the light of the primary itself.
"At this point, we wondered what could be hiding this massive object," says Benson. "One possibility is that the primary is a normal hot star which is emitting most of its light in the ultraviolet part of the spectrum. The alternative is that the primary star is a black hole." To test the first possibility, the astronomers have applied for observing time to obtain ultraviolet spectra with NASA's Hubble Space Telescope. However, such a star would have a hard time heating the surrounding gas to the high observed temperature. If the ultraviolet does not show a hot star spectrum, the best alternative is that the primary star is a black hole whose gravity fuels the hot gas. Final determination of the nature of this hidden object will require more observations by the authors and confirmation by other astronomers.
The CfA's John G. Wolbach Library has reinstated a long tradition of mounting temporary exhibits on topics related to astronomy and its history. A recent display focused on "Women at the Harvard College Observatory before 1920." It featured Williamina Paton Fleming (1857-1911), who in 1898 was appointed Curator of Astronomical Photographs, thus, becoming the first woman to receive a corporation appointment at Harvard.
Fleming had become a permanent staff member in 1881, and a few years later became involved with the Henry Draper Memorial program. She assumed responsibility for the examination of photographic spectra, development of an empirical classification system, and general care of the rapidly growing number of photographic plates. While examining thousands of stellar spectra, Fleming discovered more than 300 variable stars, 10 novae, and 59 gaseous nebulae, and compiled lists of thousands of stars with peculiar spectra.
A group of women staff members directed by Mrs. Williamina Fleming, circa 1890. Standing--Fleming and Edward C. Pickering, sitting in front--Evelyn Leland, at rear left--Antonio Maura. (Courtesy Curator of Astronomical Photographs at Harvard College Observatory)
Fleming was widely recognized for her iportant contributions to astronomical research. She was only the fifth woman, and the first American woman, admitted to the American Astronomical Society. Wellesley College named her an honorary fellow in astronomy.
Williamina Fleming was remembered by her friends and colleagues as a person with a highly magnetic personality and attractive countenance. Her lively, independent spirit shines through the pages of her diary (also in the display, courtesy Harvard University Archives). A copy of that diary, written in March 1900 as part of a University-wide project involving many members of faculty and staff, is available for browsing at the Library Circulation Desk.
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