Monster Lies Camouflaged Inside Nebula’s Heart

Most galaxies, including the Milky Way, are filled with
giant clouds of gas and dust called nebulae that appear as dark silhouettes
against the starry background. Nebulae shine only when illuminated or
excited by nearby energy sources.

Usually, the energy source is one or more stars. But today at the 204th
meeting of the American Astronomical Society in Denver, Colorado,
Smithsonian astrophysicist Philip Kaaret (Harvard-Smithsonian Center for
Astrophysics) announced that one nebula is illuminated by X-rays from a
black hole. Moreover, the brightness of the nebula suggests that the X-ray
source may be an intermediate-mass black hole many times larger than
most stellar black holes.

This surprising find offers only the second known example of a black
hole-illuminated nebula, after LMC X-1 in the Large Magellanic Cloud,
and the first example of a nebula powered by an intermediate-mass black
hole.

"Astronomers always get excited about new things, and this nebula is
certainly something new. Finding it is like getting a royal flush the first
time you play poker — it’s that rare," said Kaaret.

Initially discovered by Manfred Pakull and Laurent Mirioni (University of
Strasbourg), the nebula is located 10 million light-years away in the dwarf
irregular galaxy Holmberg II. Two years ago, Pakull and Mirioni noted
that it seemed to be associated with an ultraluminous X-ray source.

By combining observations from NASA’s Hubble Space Telescope and
Chandra X-ray Observatory with those from ESA’s XMM-Newton
spacecraft, Kaaret and his colleagues, Martin Ward (University of
Leicester) and Andreas Zezas (CfA), pinpointed the X-ray source at the
center of the nebula. Moreover, the mystery source is pouring out X-rays
at a tremendous rate, shining one million times brighter in X-rays than the
Sun shines at all wavelengths of light combined.

The observations by Kaaret and his associates indicate that those X-rays
are generated by a black hole gobbling matter from a young, massive
companion star at a rate of about one Earth mass every four years. That
modest accretion rate is sufficient to ionize and light up a huge 100-light-
year-wide swath of the surrounding nebula.

The X-ray emissions provide an important clue to the nature of the black
hole. Some astronomers have suggested that X-rays from the source in
Holmberg II and similar bright sources are beamed in the Earth’s direction
like a searchlight. Such beaming would make the X-ray source appear
brighter than it really is, thereby making the black hole appear more
massive than it really is.

Kaaret’s data contradict that view, showing instead that the black hole in
Holmberg II sends out X-rays evenly in all directions. Therefore, its
brightness suggests that it must be more massive than any stellar black
hole in our own Galaxy, weighing in at more than 25 times the mass of the
Sun and likely more than 40 solar masses. That would rank it as an
"intermediate-mass" black hole.

"It’s not easy to explain how intermediate-mass black holes form. Since
we only have a few examples to study, every new find is important," said
Kaaret.

This research will be published in a paper co-authored by Kaaret, Ward
and Zezas in an upcoming issue of the Monthly Notices of the Royal
Astronomical Society.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin, evolution
and ultimate fate of the universe.

Note to editors: A high-resolution image to accompany this release is
available online at:
http://www.cfa.harvard.edu/press/pr0422image.html