This sequence from the X-ray Telescope aboard
NASA’s Swift shows changes in the central region of the Milky Way galaxy from
2006 through 2013. Watch for flares from binary systems containing a neutron
star or black hole and the changing brightness of Sgr A* (center), the galaxy’s
monster black hole.
Image Credit: NASA/Swift/N. Degenaar (Univ. of
Michigan)
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Recent observations by NASA's
Swift spacecraft have provided scientists a unique glimpse into the activity at
the center of our galaxy and led to the discovery of a rare celestial entity
that may help them test predictions of Albert Einstein's theory of general
relativity.
This week, at the annual meeting of the American Astronomical Society in
National Harbor, Md., scientists presented their research into images captured
by Swift, explaining how these images will help decipher the physical nature of
X-ray flares and enabled their discovery of a rare subclass of neutron star.
This X-ray image of the galactic center merges
Swift XRT observations through 2013. Sgr A* is at center. Low-energy X-rays (300
to 1,500 electron volts) are shown in red, medium-energy (1,500 to 3,000 eV) in
green, and high-energy (3,000 to 10,000 eV) in blue. The total exposure time is
12.6 days.
Image Credit: NASA/Swift/N. Degenaar (Univ. of
Michigan)
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Swift's seven-year campaign to monitor the center of the Milky Way has
doubled the number of images available to scientists of bright X-ray flares
occurring at the galaxy's central black hole, dubbed Sagittarius A* (Sgr
A*).
Sgr A* sits in the center of the Milky Way's innermost region, 26,000
light-years away in the direction of the constellation Sagittarius. Its mass is
at least 4 million times that of the sun. Despite its considerable size, it is
not nearly as bright as it could be if it was more active, according to one
expert.
"Given its size, this supermassive black hole is about a billion times
fainter than it could be," said 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."
To better understand the black hole's behavior over time, the Swift team
began making regular observations of the Milky Way's center in February 2006.
Every few days, the Swift spacecraft turns toward the innermost region of the
galaxy and takes a 17-minute-long snapshot with its X-ray Telescope (XRT).
This simulation shows the future behavior of the G2
gas cloud now approaching Sgr A*, the supermassive black hole at the center of
the Milky Way. X-ray emission from the cloud’s tidal interaction with the black
hole is expected sometime this spring.
Image Credit: ESO/MPE/M.Schartmann
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To date, Swift's XRT 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 enabled the team to estimate that similar flares occur
every five to 10 days. Scientists will look at differences between the outbursts
to decipher their physical nature.
The Swift XRT team expects 2014 to be a banner year for the campaign. A cold
gas cloud named G2, about three times the mass of Earth, will pass near Sgr A*
and already is being affected by tides from the black hole's powerful
gravitational field. Astronomers expect G2 will swing so close to the black hole
during the second quarter of the year that it will heat up to the point where it
produces X-rays.
If some of the cloud's gas actually reaches Sgr A*, astronomers may witness a
significant increase in activity from the black hole. The event will unfold over
the next few years, giving scientists a front-row seat to study the
phenomena.
"Astronomers around the world are eagerly awaiting the first sign that this
interaction has begun," said Jamie Kennea, a team member at Pennsylvania State
University in University Park, Pa. "With the invaluable help of Swift, our
monitoring program may well provide that indicator."
Scientists saw what they thought was a sign in April, when Swift detected a
powerful high-energy burst and a dramatic rise in the X-ray brightness of the
Sgr A* region. They were excited to discover the activity came from separate
source very near the black hole: a rare subclass of neutron star.
A neutron star is the crushed core of a star destroyed by a supernova
explosion, packing the equivalent mass of a half-million Earths into a sphere no
wider than Washington. The neutron star, named SGR J1745-29, is a magnetar,
meaning its magnetic field is thousands of times stronger than an average
neutron star. Only 26 magnetars have been identified to date.
The discovery of SGR J1745-29 may aid scientists in their exploration of
important properties of the Sgr A* black hole. As it spins, the magnetar emits
regular X-ray and radio pulses. As it orbits Sgr A*, astronomers could detect
subtle changes in the pulse timing because of the black hole's gravitational
field, 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," 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. Goddard operates
the spacecraft in collaboration with Pennsylvania State University, the Los
Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles,
Va. International collaborators are located in the United Kingdom and Italy. The
mission includes contributions from Germany and Japan.
For more about the Swift mission, visit:
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NASA
Guillermo Gonzalo Sánchez Achutegui
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