January 9, 2014
Milky Way’s Black Hole Helps To Test Einstein Theory Of General Relativity
[ Watch the Video: Swift Sees Changes in the Milky Way 2006 – 2013 ]
Lee Rannals for redOrbit.com - Your Universe OnlineScientists have discovered a rare celestial entity that could help test predictions of Albert Einstein’s theory of general relativity.
A team at the annual meeting of the American Astronomical Society in National Harbor, Maryland, explained how images obtained through NASA’s Swift spacecraft could help decipher the physical nature of X-ray flares and enable the discovery of a rare subclass of neutron star.
Swift has helped scientists monitor the supermassive black hole that sits in the center of the Milky Way known as Sagittarius A* (Sgr A*). This black hole is at least four million times more massive than the sun, and is 26,000 light-years away in the direction of the constellation Sagittarius. However, despite its size, scientists say that it is not nearly as bright as it could be if it were more active.
"Given its size, this supermassive black hole is about a billion times fainter than it could be,"stated Nathalie Degenaar, principal investigator on the Swift galactic center campaign and an astronomer at the University of Michigan in Ann Arbor. "Though it's sedate now, it was quite active in the past and still regularly produces brief X-ray flares today."
NASA said Swift has detected six bright flares during which the black hole’s X-ray emission was as much as 150 times brighter for a couple of hours. These new detections have enabled scientists to estimate that similar flares occur every five to 10 days.
The Swift team believes 2014 could be a big year for the campaign, because a cold gas cloud named G2 will be passing by the black hole and is already being affected by tides from its powerful gravitational field. Astronomers believe this gas cloud will be so close to the black hole that it will heat up to the point where it produces X-rays.
"Astronomers around the world are eagerly awaiting the first sign that this interaction has begun," Jamie Kennea, a team member at Pennsylvania State University in University Park, Pennsylvania, said in a statement. "With the invaluable help of Swift, our monitoring program may well provide that indicator.”
In April last year, Swift detected a powerful high-energy burst and a big rise in the X-ray brightness in the black hole’s region. They determined that this activity came from a separate source near the black hole, which was a rare subclass of neutron star named SGR J1745-29.
This neutron star has a magnetic field thousands of times stronger than an average neutron star. Most neutron stars pack the equivalent mass of a half-million Earths into a sphere no wider than Washington, but SGR J1745-29 is a little different.
The discovery of SGR J1745-29 may help scientists explore the properties of Sgr A*. As the neutron star orbits the black hole, astronomers will detect subtle changes in the pulse timing of the black hole’s gravitational field, which is a prediction of Einstein’s theory of general relativity.
"This long-term program has reaped many scientific rewards, and due to a combination of the spacecraft's flexibility and the sensitivity of its XRT, Swift is the only satellite that can carry out such a campaign,"stated Neil Gehrels, the mission's principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland.