Many X-ray flares and rare 'magnetar' observed in black hole

A seven-year campaign to monitor the center of our galaxy with NASA's Swift spacecraft has more than doubled the number of bright X-ray flares observed from our galaxy's central black hole and has led to the discovery of a rare type of neutron star called a "magnetar." Jamie Kennea, a Penn State astronomer, presented the findings during a press conference on Jan. 8 at a meeting of the American Astronomical Society. The science and flight operations of the Swift observatory are controlled by Penn State from the Mission Operations Center on the University Park campus.

A behemoth black hole containing at least 4 million times the mass of the Sun lurks in the innermost region of our galaxy, 26,000 light-years from Earth. Located in the direction of the constellation Sagittarius as seen from Earth, this black-hole heart of our galaxy is called Sgr A* (pronounced "saj a-star"). The discovery of a magnetar near Sgr A* may allow scientists to explore important properties of this black hole and to test predictions of Einstein’s theory of general relativity.

To better understand the black hole's long-term behavior, as well as to catch other fleeting events in the region, the Swift team began regular observations of the galactic center in February 2006. "Every few days, the spacecraft turns toward the inmost galaxy and takes a 17-minute-long "snapshot" with its X-Ray Telescope," Kennea said.

Prior to this campaign, astronomers had observed four bright X-ray flares from Sgr A*, two each with NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton spacecraft. Now, Kennea reports, Swift's X-Ray Telescope has detected an additional six bright flares, during which the black hole's X-ray emission brightened by up to 150 times for a couple of hours. These new detections have enabled the team to estimate that similar flares occur every five to 10 days. Differences between the outbursts will help scientists decipher their physical nature.

The Swift XRT team expects 2014 to be a banner year for the campaign. A cold gas cloud named G2, with about three times Earth's mass, will pass near Sgr A* and is already being affected by tides from its powerful gravitational field. Sometime this spring, astronomers expect G2 will swing so close that its gas will heat up to the point where it produces X-rays.

"Astronomers around the world are eagerly awaiting the first sign this interaction has begun, and our monitoring program may well provide it," Kennea said. The event will unfold over the next few years, giving scientists a front-row seat for studying the tidal disruption of a gas cloud. Should some of the cloud's gas actually reach Sgr A*, astronomers also may witness extensive activation of the Milky Way's central black hole.

On April 24, 2013, the team experienced its first false alarm. Swift detected a dramatic rise in the X-ray brightness of the region near Sgr A*, as well as a powerful high-energy burst. Initial excitement that the event heralded a strong outburst from the central black hole -- one possibly related to the G2 cloud -- continued even as the team realized the emission came from a different source: a rare subclass of neutron star. A neutron star is the crushed core of a star destroyed by a supernova explosion, packing the equivalent of half a million Earths into a sphere the size of Washington, D.C. The Swift discovery, named SGR J1745-29, is a neutron star with a magnetic field thousands of times stronger than normal -- a magnetar. Astronomers have identified only 26 magnetars so far.

"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," said Neil Gehrels, the mission's principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Md. Goddard manages Swift, which was launched in November 2004. The spacecraft is operated in collaboration with Penn State University, the Los Alamos National Laboratory, and Orbital Sciences Corporation. International collaborators are located in the United Kingdom and Italy, and the mission includes contributions from Germany and Japan.