Ancient Black Hole Speeds Through Sun’s Galactic Neighborhood, Devouring Companion Star

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Data from the Space Telescope Science Institute’s Digital
Sky Survey has played an important supporting role in helping radio and
X-ray astronomers discover an ancient black hole speeding through the
Sun’s galactic neighborhood. The rogue black hole is devouring a small
companion star as the pair travels in an eccentric orbit looping to the
outer reaches of our Milky Way galaxy.

The scientists believe the black hole is the remnant of
a massive star that lived out its brief life billions of years ago and
later was gravitationally kicked from its home star cluster to wander
the Galaxy with its companion.

The discovery was made with observations from the National
Science Foundation’s Very Long Baseline Array (VLBA) radio telescope and
the Rossi X-ray satellite.

Important supporting evidence came from studying optical
images made for the Palomar Observatory Sky Survey (POSS) taken 43 years
apart. The POSS images were digitized by the Space Telescope Science Institute
to support the Hubble Space Telescope observing programs and also as a
service to the astronomical community. This huge database, called the
Digital Sky Survey, allows astronomers to quickly and easily measure stellar
motion across the sky. The DSS scans confirmed the motion of the black
hole and companion star. The DSS scans, combined with data from both the
radio and optical images, allowed the astronomers to calculate the object’s
orbital path around the galactic center.

“This discovery is the first step toward filling in a missing
chapter in the history of our Galaxy,” said Felix Mirabel, an astrophysicist
at the Institute for Astronomy and Space Physics of Argentina and the
French Atomic Energy Commission. “We believe that hundreds of thousands
of very massive stars formed early in the history of our Galaxy, but this
is the first black hole remnant of one of those huge primeval stars that
we’ve found.”

“This also is the first time that a black hole’s motion
through space has been measured,” Mirabel added. A black hole is a dense
concentration of mass with a gravitational pull so strong that not even
light can escape it. The research is reported in the September 13, 2001
issue of the scientific journal Nature.

The object is called XTE J1118+480 and was discovered by
the Rossi X- Ray satellite on March 29, 2000. Later observations with
optical and radio telescopes showed that it is about 6,000 light-years
from Earth, and it is a “microquasar” in which material sucked by the
black hole from its companion star forms a hot, spinning disk that spits
out “jets” of subatomic particles that emit radio waves.

Most of the stars in our Milky Way galaxy are within a thin
disk, called the plane of the Galaxy. However, there also are globular
clusters, each containing hundreds of thousands of the oldest stars in
the Galaxy, which orbit the Galaxy’s center in paths that take them far
from the Galaxy’s plane. XTE J1118+480 orbits the Galaxy’s center in a
path similar to those of the globular clusters, moving at 300,000 miles
per hour (145 kilometers per second) relative to the Earth.

How did it get into such an orbit? “There are two possibilities:
either it formed in the Galaxy’s plane and was somehow kicked out of the
plane, or it formed in a globular cluster and was kicked out of the cluster,”
said Vivek Dhawan, an astronomer at the National Radio Astronomy Observatory
(NRAO) in Socorro, New Mexico.

A massive star ends its life by exploding as a supernova,
leaving either a neutron star or a black hole as a remnant. Some neutron
stars show rapid motion, thought to result from a sideways “kick” during
the supernova explosion. “This black hole has much more mass — about
seven times the mass of our Sun — than any neutron star,” said Dhawan.
“To accelerate it to its present speed would require a kick from the supernova
that we consider improbable,” Dhawan added.

“We think it’s more likely that it was gravitationally ejected
from the globular cluster,” Dhawan said. Simulations of the gravitational
interactions in globular clusters have shown that the black holes resulting
from the collapse of the most massive stars should eventually be ejected
from the cluster.

“The star that preceded this black hole probably formed
in a globular cluster even before our Galaxy’s disk was formed,” Mirabel
said. “What we’re doing here is the astronomical equivalent of archaeology,
seeing traces of the intense burst of star formation that took place during
an early stage of our Galaxy’s development.”

The black hole has consumed so much of its companion star
that the inner layers of the smaller star — only about one-third the
mass of the Sun — now are exposed. The scientists believe the black hole
captured the companion before being ejected from the globular cluster,
as if it were grabbing a snack for the road.

“Because this microquasar happened to be relatively close
to the Earth, we were able to track its motion with the VLBA even though
it’s normally faint,” said Mirabel. “Now, we want to find more of these
ancient black holes. There must be hundreds of thousands swirling around
in our Galaxy.”

The astronomers used the VLBA to observe XTE J1118+480 in
May and July of 2000, using the VLBA’s great resolving power, or ability
to see fine detail, to precisely measure the object’s movement against
the backdrop of more-distant celestial bodies.

“With the VLBA, we could start observing soon after this
object was discovered and get extremely precise information on its position.
Then, we were able to use the digitized data from the Palomar surveys
to extend backward the time span of our information. This is a great example
of applying multiple tools of modern astronomy — telescopes covering
different wavelengths and digital databases — to a single problem,” said
Dhawan.

In addition to Mirabel and Dhawan, the research was performed
by Roberto Mignani of the European Southern Observatory; Irapuan Rodrigues,
who is a fellow of the Brazilian National Research Council at the French
Atomic Energy Commission; and Fabrizia Guglielmetti of the Space Telescope
Science Institute in Baltimore, MD.

The National Radio Astronomy Observatory is a facility of
the National Science Foundation, operated under cooperative agreement
by Associated Universities, Inc.

CONTACT:
Dave Finley
National Radio Astronomy Observatory, Socorro, NM
(Phone: 505-835-7302; E-mail: dfinley@nrao.edu)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4514; E-mail: villard@stsci.edu)