Steve Sampsell
Penn State, University Park, PA
Phone: 814-865-1390
sws102@psu.edu

Steve Roy
Marshall Space Flight Center, Huntsville, AL
Phone: 256-544-6535

Megan Watzke
Chandra X-ray Observatory Center, CFA, Cambridge, MA
Phone: 617-496-7998
cxcpress@cfa.harvard.edu

Science Contacts:
Jeffrey McClintock, 617-495-7136, jmcclintock@cfa.harvard.edu
Ann Esin, 626-395-4259, aidle@tapir.caltech.edu

May 14, 2001

CXC PR: 01-07

Chandra Examines Black Holes Large and Small in Nearby Galaxy

Probing a large, nearby galaxy in the constellation of Circinus, NASA's
Chandra X-ray Observatory presents a new view of both the galaxy's
supermassive black hole and a host of potential smaller black holes sprinkled
throughout its spiral arms. The results include the first detection of a
black hole's periodic variability in X-rays outside our galactic neighborhood.

Astronomers from Penn State University used Chandra to discover a variable
object within the dozen or so X-ray emitting sources sprinkled throughout the
Circinus galaxy. The intensity of X-rays from this source changes on a cycle
of 7.5 hours -- the first time this "periodic variability" has been detected
at X-ray wavelengths in an object outside the "Local Group" of galaxies. And,
along with its brightness, this evidence strongly suggests that the system
contains a black hole some 50 times the mass of the Sun.

"Extremely luminous X-ray sources such as this one appear to be common among
other galaxies," said Franz Bauer, a postdoctoral scholar at Penn State and
lead author of a July 2001 paper in The Astronomical Journal. "But until
Chandra, we have never had an instrument that could clearly identify whether
they were simply massive X-ray binary systems, or if they represented a new
class of objects"

"The periodic variability in the Chandra data of Circinus provides us with a
key signature that these objects are indeed X-ray binary systems," continued
Bauer. "This is important because black holes with masses much larger than
10 times the mass of the Sun such as this one are difficult to explain under
current theories of star formation and destruction. Definitively finding a
periodic signal in one allows us to test some of our past assumptions."

The X-ray data acquired by two independent teams -- one at Penn State and
George Mason University and the other at the University of Maryland -- also
provide evidence that strongly supports the "unified model," a theory in
which a large doughnut-shaped ring is thought to obscure the central black
hole.

In the case of Circinus, this ring of material obscures a direct line of
sight to the central black hole. Its existence is inferred from a detailed
analysis of the gas distribution and physical conditions near the center
of the galaxy by the two teams. At least two different gas components are
identified near the nucleus. One, a warm gas that is heated and ionized by
the radiation field from the black hole, contains strong emissions of highly
ionized elements such as argon, calcium, iron, magnesium, neon, silicon,
and sulfur. The second gas component is cooler and features a strong iron
emission line. In addition, the astronomers were able to show that the two
gas components have different distributions, with the warmer gas being
spread over a much larger region around the black hole than the cooler gas.

"The Chandra observations of Circinus show us how complex the gaseous
environment of supermassive black holes can be," said Rita Sambruna,
assistant professor of physics and astronomy at George Mason University.
"Because it is close and thus easy to study, Circinus provides an important
testbed for what might be happening in other, more distant so-called active
galaxies."

The Chandra images allowed the astronomers to determine that the X-ray
emission associated with the central black hole can be resolved into a
number of distinct components. A bright, compact emission source is present
at the center of the image, and that nuclear source is surrounded by a
diffuse X-ray halo that extends out several hundred light years. The
researchers also detected a cone of hot gas extending out from the galaxy's
core to a distance of 2000 light years to the northwest.

"The large region of X-ray emitting gas extending out of the galaxy disk
looks very similar to the gas seen in optical observations," said David
Smith, research associate at the University of Maryland. "This may imply
the X-ray gas is heated by material close to the galaxy's supermassive
black hole."

On astronomical scales, this galaxy is considered quite close, a mere 13
million light-years from Earth. Since it's located near the plane of our
own Milky Way Galaxy, the Circinus galaxy is partially hidden by intervening
dust along our line of sight. As a result, the galaxy went unnoticed until
about 25 years ago.

Along with Sambruna and Bauer, the first research group included Hagai
Netzer of Tel-Aviv University and the following collaborators from Penn
State: Niel Brandt, George Chartas, Gordon Garmire, John Nousek, and Shai
Kaspi. The University of Maryland team also included Andrew Wilson. The
results of theses Chandra observations appear in recent and upcoming papers
in The Astrophysical and Astronomical Journals.

Observations with Chandra, using the Advanced CCD Imaging Spectrometer
(ACIS) and the High Energy Transmission Grating Spectrometer (HETGS), were
made on June 6-7, 2000 and observations without the HETGS were made on
March 14, 2000.

The HETG and ACIS instruments were built for NASA by the Massachusetts
Institute of Technology, Cambridge, MA, and Pennsylvania State University,
University Park. NASA's Marshall Space Flight Center, Huntsville, AL,
manages the Chandra program for the Office of Space Science, Washington,
DC. TRW, Inc., Redondo Beach, California, is the prime contractor for the
spacecraft. The Smithsonian's Chandra X-ray Center controls science and
flight operations from Cambridge, MA.

Images associated with this release are available on the World Wide Web at:

http://chandra.harvard.edu

AND

http://chandra.nasa.gov

IMAGE CAPTION:
[http://chandra.harvard.edu/photo/cycle1/0167/index.html
or
http://www1.msfc.nasa.gov/NEWSROOM/news/photos/2001/photos01-171.htm]
This Chandra x-ray image shows the inner portion of the Circinus Galaxy, with
north at the top of the image and east to the left. In terms of X-ray energies,
red represents low energy, green intermediate and blue the highest observed
energies. The emission is resolved into a number of distinct components, many
of which are associated with a central black hole. A bright, compact emission
source is present at the center of the image. That nuclear source is
surrounded by a diffuse X-ray halo that extends out several hundred light
years. The X-rays directly to the northwest of the nucleus appear red,
indicating predominantly soft energies, while the X-rays to the southeast are
blue, indicating only hard energies.

Because low X-ray energies are absorbed by gas more easily than higher
energies, the sharp contrast suggests that the red emission to the northwest
originates from the near side of the disk of the Circinus Galaxy. And, the
blue emission is more highly absorbed and must come from the gas within the
disk or on the far side. Such geometry corresponds to the disk of the galaxy
as seen in optical and radio images. A bright, soft X-ray plume of emission
extends approximately 1,200 light years (380 parsecs) to the northwest and
coincides with an optical region containing gas ionized by the nucleus.
There is a very strong correlation between the X-ray emission and the
high-excitation ionized gas seen in emission-line images obtained by the
Hubble Space Telescope and ground-based telescopes. The x-ray image was
made with NASA's Chandra X-ray Observatory and the Advanced CCD Imaging
Spectrometer (ACIS) from 67,000 seconds of exposure time on June 6-7, 2000.

Scale: 80 arcsec per side

Credit: NASA/Penn State/F. Bauer et al.