NASA's Swift Satellite Discovers A New Black Hole In Our Galaxy

Oct. 5, 2012

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
301-286-4044
robert.j.gutro@nasa.gov

RELEASE: 12-353

NASA'S SWIFT SATELLITE DISCOVERS A NEW BLACK HOLE IN OUR GALAXY

WASHINGTON -- NASA's Swift satellite recently detected a rising tide
of high-energy X-rays from a source toward the center of our Milky
Way galaxy. The outburst, produced by a rare X-ray nova, announced
the presence of a previously unknown stellar-mass black hole.

"Bright X-ray novae are so rare that they're essentially
once-a-mission events and this is the first one Swift has seen," said
Neil Gehrels, the mission's principal investigator, at NASA's Goddard
Space Flight Center in Greenbelt, Md. "This is really something we've
been waiting for."

An X-ray nova is a short-lived X-ray source that appears suddenly,
reaches its emission peak in a few days and then fades out over a
period of months. The outburst arises when a torrent of stored gas
suddenly rushes toward one of the most compact objects known, either
a neutron star or a black hole.

The rapidly brightening source triggered Swift's Burst Alert Telescope
twice on the morning of Sept. 16, and once again the next day.

Named Swift J1745-26 after the coordinates of its sky position, the
nova is located a few degrees from the center of our galaxy toward
the constellation Sagittarius. While astronomers do not know its
precise distance, they think the object resides about 20,000 to
30,000 light-years away in the galaxy's inner region.

Ground-based observatories detected infrared and radio emissions, but
thick clouds of obscuring dust have prevented astronomers from
catching Swift J1745-26 in visible light.

The nova peaked in hard X-rays -- energies above 10,000 electron
volts, or several thousand times that of visible light -- on Sept.
18, when it reached an intensity equivalent to that of the famous
Crab Nebula, a supernova remnant that serves as a calibration target
for high-energy observatories and is considered one of the brightest
sources beyond the solar system at these energies.

Even as it dimmed at higher energies, the nova brightened in the
lower-energy, or softer, emissions detected by Swift's X-ray
Telescope, a behavior typical of X-ray novae. By Wednesday, Swift
J1745-26 was 30 times brighter in soft X-rays than when it was
discovered and it continued to brighten.

"The pattern we're seeing is observed in X-ray novae where the central
object is a black hole. Once the X-rays fade away, we hope to measure
its mass and confirm its black hole status," said Boris Sbarufatti,
an astrophysicist at Brera Observatory in Milan, Italy, who currently
is working with other Swift team members at Penn State in University
Park, Pa.

The black hole must be a member of a low-mass X-ray binary (LMXB)
system, which includes a normal, sun-like star. A stream of gas flows
from the normal star and enters into a storage disk around the black
hole. In most LMXBs, the gas in the disk spirals inward, heats up as
it heads toward the black hole, and produces a steady stream of
X-rays.

But under certain conditions, stable flow within the disk depends on
the rate of matter flowing into it from the companion star. At
certain rates, the disk fails to maintain a steady internal flow and
instead flips between two dramatically different conditions -- a
cooler, less ionized state where gas simply collects in the outer
portion of the disk like water behind a dam, and a hotter, more
ionized state that sends a tidal wave of gas surging toward the
center.

"Each outburst clears out the inner disk, and with little or no matter
falling toward the black hole, the system ceases to be a bright
source of X-rays," said John Cannizzo, a Goddard astrophysicist.
"Decades later, after enough gas has accumulated in the outer disk,
it switches again to its hot state and sends a deluge of gas toward
the black hole, resulting in a new X-ray outburst."

This phenomenon, called the thermal-viscous limit cycle, helps
astronomers explain transient outbursts across a wide range of
systems, from protoplanetary disks around young stars, to dwarf novae
-- where the central object is a white dwarf star -- and even bright
emission from supermassive black holes in the hearts of distant
galaxies.

Swift, launched in November 2004, is managed by Goddard Space Flight
Center. It is operated in collaboration with Penn State, the Los
Alamos National Laboratory in New Mexico and Orbital Sciences Corp.
in Dulles, Va., with international collaborators in the United
Kingdom and Italy and including contributions from Germany and Japan.


For images related to this discovery and more information about Swift,
visit:

http://go.nasa.gov/QLlkCH


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