INTERMEDIATE MASS BLACK HOLES in IXOs
Black Holes
When a compact object becomes very massive, the gravitational field becomes
very strong, and the space near the object becomes severely distorted. This
effect is similar to when a trampoline becomes distorted when someone jumps
up and down on it. The large mass bends the fabric of the space.
Eventually, when the gravitational field is very, very
strong, a black hole can form.
No light can escape from a black hole once it has
passed through the
This means that in order to really "prove" that an object is a black hole, you need to show that all of the mass is located within a very small space. Even though this has not been done, most astronomers truly believe that black holes exist and use them regularly to explain observations of many astrophysical phenomena.
Although, theoretically, black holes could come in any size, until
recently there has only been evidence for
two types of black holes:
supermassive ones
that are a million to a billion times the mass of the
sun (and therefore 3 million to 3 billion kilometers in "size"), and
stellar-size black holes, which are from one to twenty times the mass
of the sun.
Have a look at an animation of matter around a supermassive black hole.
Both of these types of black holes are believed to exist based on
observations of the radio, optical, ultraviolet and X-ray light that
is emitted as matter falls into (is accreted onto) the black hole, or matter
that is in orbit around the black hole. There are probably also many
"dormant" black holes waiting to be discovered which do not have accreting
matter around them, but of course we can't see them!
Supermassive black holes are found at the center of
Stellar-size black holes also have accretion disks surrounding the black
hole, but the matter falling into the black hole normally comes from a star
that is orbiting around the black hole, not from gas.
A cartoon of one of these so-called
The X-ray spectra of X-ray binaries are more complex. They often vary in
brightness and are known to change "state." In simple terms, X-ray
binaries have a "high-luminosity" or "soft" state in which the photon
power law index is about 2.5 to 3.5 and a "low-luminosity" of "hard"
state in which the photon power law index is about 1.8 or so. An
example of what a Gamma=2.5 spectrum looks like is shown in the picture
to the right. It is the steeper of the two spectra.
IXOs are present in all types of galaxies. The 21 galaxies with compact
X-ray sources from our ROSAT survey included
dwarf galaxies, elliptical galaxies, and spiral galaxies.
On average, we are finding one IXO in every other galaxy, so about 0.5 IXOs
per galaxy. Multiply that by the total number of galaxies in the Universe,
and that is a lot of IXOs, and a lot of potential intermediate mass black
holes!
The spectacular Chandra X-ray
satellite is now finding IXOs in all types of galaxies.
We should soon learn much more about the nature of these fascinating objects!
To learn more about our ROSAT IXOs, we looked at X-ray spectra
of three elliptical galaxy sources and three spiral galaxy sources.
X-ray spectra are a measure of the power coming from the X-ray source as
a function of the energy of the X-ray photon. For example, red light
from a stoplight has more red photons coming out than blue or green
photons. We can similarly examine the X-ray light to see if more
soft (low energy) photons are coming out rather than hard
photons. There are certain models for the spectrum that describe an AGN
(an accreting supermassive black hole) and and X-ray binary (an
accreting stellar-mass black hole). So we tried both models on the
data. The data were taken with the
ASCA X-ray satellite.
The data for the elliptical galaxies were better fit by the AGN models,
so their X-ray emission is probably dominated by accreting supermassive black
holes. However, the data for the spiral galaxies was best fit by the
X-ray binary model. In particular, as mentioned above, the spectrum was
steep, like an X-ray binary in its "soft" state.
For the IXO in one galaxy (NGC1313),
the two different ASCA spectra taken 2.4 years apart had different
spectral slopes, changing from a shallow "hard" state to a steep "soft"
state. This is similar to what is observed in X-ray binaries, so we
assumed that the best working model was that the objects were similar to
X-ray binaries, mostly in their soft state.
We then fit the ASCA specta with a model for soft-state X-ray binaries
in order to obtain estimates on the central mass of the black hole.
This model predicted central masses on the order of a few hundred times
the mass of the sun. So we then
found evidence for intermediate-mass black holes!
Another theory of the formation of supermassive black holes is that they
formed from the continual merging of smaller mass black holes. Take a
peek at a movie of two black holes merging.
Simulations have shown, however, that in a galaxy, black holes formed in this
way can only get as large as a few hundred to a few thousand solar masses.
This is because, after a certain amount of time, they have eaten up all of
the smaller black holes and stars nearby. Thereafter, they grow in mass and
size by accreting gas. So, there actually is a possible explanation
for the formation of intermediate mass black holes. It's just that we are
only
now starting to see evidence for them, and it will be some time before we
figure out how they formed.
"Stellar mass" black holes are, by definition, black holes with masses about
the same
as a star, so will be on the order of 1-10 times that of our sun.
Stars get to be about as massive as 50-100 solar masses, and then
when those stars expode (as a supernova), maybe 50-90 percent of
the mass is lost
in the gas of the exposion, leaving the rest in a neutron star or black hole.
The theoretical largest mass for a neutron star is 1.4 solar masses, and
the approximate maximum size black hole you can get from a supernova is about
20 solar masses. If IXOs were 20 solar mass black holes, they would be
emitting at over 5 times their Eddington luminosity.
This may be possible if they are beaming light towards us like a flashlight,
instead of shining at all angles, like the sun. So, it's possible,
but probably not likely.
On the other hand, IXOs are off-center from the nucleus, so they can't be
more massive than about 100 thousand solar masses or they would have sunk
into the nucleus. The only way around this is if they were formed recently.
So, there you have it, they have masses between around 100 and 100 thousand
solar masses, intermediate between X-ray binaries and AGNs.
Unfortunately, there are none in our Galaxy that
we know of. There is one in M33, though, which is fairly close to the
Milky Way. Browse around on the web, though. These objects are pretty
popular right now. We are having a conference in Pasadena in June 2001,
so hopefully there will be some exciting, new results then!
X-ray Emission from Black Holes
"Light from black holes" sounds like a contradiction since we know that
no light escapes from black holes! In truth, the
light comes from the accretion disk that surrounds the black hole, or
from pulvarized matter near the black hole.
The accretion disks surrounding supermassive black holes are located
at larger radii than those in X-ray binaries, so they emit at cooler
wavelengths, in the ultraviolet wave band.
The ultraviolet light is scattered by an electron corona that
is located above the disk and is converted into X-ray light. So, the
X-ray light is not coming from the black hole! The characteristic
signature of a supermassive black hole in an AGN is a
Intermediate-luminosity X-ray Objects (IXOs) in Normal Galaxies
Since the days of the Einstein X-ray satellite in the early
1980s, astronomers
have known about X-ray sources with intermediate luminosities
(between those of X-ray binaries and AGNs)
in the centers of
Our Research
We decided to investigate these questions and
try to understand what these intermediate range X-ray sources were.
Using archival imaging data taken with the ROSAT High Resolution
Imager (HRI), we surveyed a "complete" sample of 39 nearby
How do Intermediate-Mass Black Holes Form?
At right is an artist's conception of what an intermediate-mass accreting
black hole would look like.
Now, the whole reason why these objects are so exciting is that most of the
objects that we know to contain black holes are thought to
have either stellar mass black holes
or supermassive black holes. There is a clear explanation of how these
two types of black holes formed. Supernovae explosions leave behind
neutron stars and black holes and the black holes formed in this way can
be as large as about 20 solar masses. It is not really known how
supermassive black holes formed, but one theory is that they formed in
the early Universe when primordial gas clouds collapsed. When such
large gas clouds collapse the black hole formation becomes inefficient
for forming black holes of masses smaller than about a million solar
masses. So, neither of these two scenarios explains how to form
an intermediate mass black holes.
How do we know IXOs have intermediate masses of about 100-1000 solar masses and are different from stellar mass and supermassive black holes?
Well, regardless of how they form, the primary characteristic of a black hole is
its mass, otherwise they are pretty much all the same. The mass determines
its size (Schwarzchild radius). Some black holes may be spinning, though.
So the black hole itself is not much different, except for its size. But the
matter around it (the stuff that we see with our astronomical instruments)
is probably much different.
Let's go out and have a look at some!
Questions or comments? Contact
Edward J. M. Colbert
(colbert@pha.jhu.edu)
at the JHU Physics and Astronomy Department.
Last update: 2001 Apr 2
