Media Contact:
Dr. Kazushi Iwasawa
Institute of Astronomy, University of Cambridge
ki@ast.cam.ac.uk

issued by the University of Cambridge

September 9, 2004, NEW ORLEANS -- Scientists at the Institute of Astronomy in Cambridge, England, have pieced together the journey of a bundle of doomed matter as it orbited a black hole four times, an observational first. Their technique provides a new method to measure the mass of a black hole; and this may enable the testing of Einstein's theory of gravity to a degree few thought possible.

The scientists followed the trail of hot gas over the course of a day as it whipped around the supermassive black hole roughly at the same distance the Earth orbits the Sun. Quickened by the extreme gravity of the black hole, however, the orbit took about a quarter of a day instead of a year. The science team calculated the mass of the black hole by plugging in the measurements for the energy of the light, its distance from the black hole, and the time it took to orbit the black hole -- a marriage of Einstein's general relativity and good old-fashioned Keplerian physics.

Drs. Kazushi Iwasawa and Giovanni Miniutti present this result today during a Web-based press conference in New Orleans at the meeting of the High Energy Astrophysics Division of the American Astronomical Society. Dr. Andrew Fabian of the IoA joins them on an article appearing in an upcoming issue of the Monthly Notices of the Royal Astronomical Society. The data is from the European Space Agency's XMM-Newton observatory.

"Einstein's theories have held up well in recent years, but they may begin to falter near the border of a black hole," said Iwasawa. "Understanding the physics of time, matter and energy near a black hole is crucial to testing Einstein's math. But the data can be a real mess. Gas is pulled in all directions as it whips around a black hole at near light speed. The X-ray data reflects all these forces acting on the gas. This new analysis technique may help us get a handle on the black hole phenomenon."

The team studied a galaxy named NGC 3516, which is thought to harbor a supermassive black hole in its core. Gas in this central region glows in X-ray radiation as it is heated to temperatures in the millions of degrees under the force of the black hole's extreme gravity. NGC 3516, classified as a Seyfert I galaxy because of its black hole activity, is about 100 million light years away in the constellation Ursa Major, home to the Big Dipper (also known as the Plow).

XMM-Newton captured spectral features from light around the black hole, displayed on a spectrograph with spikes indicating certain energy levels, similar in appearance to the jagged lines of a cardiograph. In a laboratory, a spectrograph of iron gas bombarded with X rays produces neat spikes at specific energies. In space around a black hole, these spikes are distorted.

Hot gas orbiting any object, for example, has a double-horned profile due to the Doppler effect. That is, some gas is moving towards us, slightly boosting the energy of its X-ray emission; and other gas is moving away, slightly reducing the energy of its X rays. This results in a "widened" spectral line with two peaks, one for the boosted X rays and one for the weakened ones. Around a black hole, Einstein's general relativity kicks in, and the entire widened profile is gravitationally redshifted, or reduced in energy, as a result of strong gravity tugging at light particles trying to escape the black hole's pull.

Iwasawa's team took advantage of XMM-Newton's distant orbit, which is far away from Earth and allows for long, uninterrupted observations of many celestial objects without the Earth blocking the view. The XMM-Newton observation of NGC 3516 lasted about one day. During this time, XMM captured a flare from excited gas orbiting the black hole as it whipped around four times. This was the crucial bit of information needed to measure the black hole mass.

Earlier analysis revealed a gravitationally redshifted spectral line that indicated the small distance from the black hole of the hot gas emitting this light -- that is, the extent of the redshifting is related to how close an object is to a black hole. With information about the orbital time of the flare combined with location of the flare, the scientists could pin down a mass measurement. The mass is between 10 million and 50 million solar masses, in agreement with values obtained with other techniques.

While the calculation is straightforward, the analysis to understand the orbital period of an X-ray flare is new and intricate. Essentially, the scientists detected a cycle repeated four times: a modulation in the light's intensity accompanied by an oscillation in the light's energy. Taking into account the Doppler effect (stealing and boasting energy) and gravitational redshifting (just plain stealing energy), the scientists interpreted this cycle (ranging in energy from 5.7 to 6.5 kilo-electron-volts ) as a signal of excited gas orbiting around and around. Knowing the peak energy and flux in the cycle also strengthened the estimation of the gas' distance from the black hole.

The analysis technique implies, to this science team's surprise, that the current generation of X-ray observatories can make significant gains in measuring black hole mass, albeit with long observations and black hole systems with long-lasting flares. Building upon this information, proposed missions such as Constellation-X or XEUS can make deeper inroads to testing Einstein's math in the laboratory of extreme gravity.Ê

The Institute of Astronomy is a department of the University of Cambridge and is engaged in teaching and research in the fields of theoretical and observational Astronomy.Ê