**New X-ray Telescope Technology Propels Virtual Journey to Black Hole**

**Background information for September 14 Nature article**

In one of its most spectacular attempts yet to capture the black hole
phenomenon, the Hubble Space Telescope peered into the core of galaxy
M87 and found a jet of particles racing out a nearly the speed of light.
This jet of electrons and sub-atomic particles stretches for 5,000 light
years \-- just out to beyond the galaxy\'s borders \-- and is powered by
a supermassive black hole.

But don\'t let that term \"supermassive\" fool you. A black hole is tiny
compared its host galaxy. That bright core that you see is about 200
light years across. This is roughly 1/50th the size of the entire
galaxy. The supermassive black hole is buried in this core, and it is
about 1/3000th of a light year across or about the size of our solar
system. So that\'s nearly a half a million times smaller than the core
of the galaxy.

It took Hubble\'s fantastic resolution to capture such a clear image of
the M87 galaxy core. Can we go a million times deeper to capture a
picture of the black hole?

Yes, we can. This is the primary goal of the MAXIM mission, a proposed
NASA X-ray observatory that will have the resolution to image a black
hole. Such a feat is comparable to seeing the details of a dinner plate
on the surface of the sun.

To attain this caliber of resolution, MAXIM will employ an emerging
technology called X-ray interferometry. Think of interferometry as
telescope teamwork. Essentially, MAXIM will utilize a multitude of
separate X-ray reflecting mirrors orbiting Earth in unison and teaming
up to create the effect of one massive X-ray telescope. A description of
interferometry is below.

**What Does the Invisible Look Like?**

One might ask how we plan to go about imaging a black hole, an object
that is by definition invisible. What we hope to image, actually, is the
region bordering the black hole, called the event horizon. This region
forms a halo of radiation around the invisible black hole.

Once light or matter crosses the event horizon into the black hole, it
is lost forever. The force of gravity is so great that even a photon
moving at light speed cannot escape the pull. Think of the event horizon
as the point of no return. Light can escape [near]{.underline} the event
horizon. Yes, gravity is fantastically powerful. This gravity may
distort the light, but nonetheless, light can escape from very close to
a black hole and travel across space to our telescopes to tell the tale.

Imaging this very region, where matter and energy are put to the
ultimate tests of gravity, will be a windfall for the fields of
astronomy and physics. For example, at the event horizon, we can test
the predictions put forward in Albert Einstein\'s Theory of General
Relativity: Does time crawl to a standstill, and is the fabric of space
distorted by gravity? Gravity is a force that is all around us, yet
scientists know surprisingly little about what produces this force. A
black hole is the ultimate gravity laboratory.

What will the black hole look like? We envision it to be a black void
surrounded by gas glowing brightly in X rays, distorted by gravity in
bizarre ways to look like an image in a funhouse mirror. To image a
black hole, therefore, we must turn to the X-ray band of the
electromagnetic spectrum.

Matter falling into a black hole emits energy in a variety of forms,
including radio waves, optical light and ultraviolet radiation. X-ray
photons, however, detail the action closest to the black hole. Further
away from the black hole, lower-energy ultraviolet radiation or radio
waves may dominate. But at the event horizon, where gravity is
strongest, temperatures are the highest and X rays dominate.

X rays also have the unique ability to pierce through a galaxy\'s worth
of dust and travel great distances through space en route to our
orbiting X-ray detectors. Even if a next-generation Hubble-type optical
telescope had a million-fold increase in resolution, it still could not
peer into the heart of a galaxy to see a black hole. Galaxy cores are
simply too dusty, and optical light cannot penetrate this dust \-- just
as a flashlight cannot shine through a wall. X rays, as you may know,
can pass through walls. \[see spectrum \"sidebar\" below\]

**A Journey Through Resolution\...**

As telescopes become more and more powerful, we essentially travel
closer and closer to the stars. Galileo first turned an optical
telescope toward the sky in 1610. Before this, humans relied solely on
their eyes to observe the heavens. The unaided eye can see detail as
fine as approximately 100 arcseconds. Galileo was able to resolve 10
arcseconds features. This mere 10-fold increase in optical resolution
allowed Galileo to see moons around Jupiter and craters on our own Moon.
The Hubble Space Telescope sees at about 0.1 arcseconds, a hundred time
improvement over Galileo\'s scope. \[see arc second \"sidebar\" below\]

Mind you, MAXIM\'s goal of imaging a black hole requires 0.000001
arcsecond resolution! Building an X-ray detector with such superior
resolution is a great undertaking. Currently, optical telescopes achieve
greater resolution than X-ray telescopes. There are some physical
reasons for this, based on the nature of optical light waves compared to
X-ray photons. There is also the fact that optical telescopes have
enjoyed a 400-year head start.

Cosmic X-ray detectors didn\'t come about until 1960. We had the
technology to detect X rays for about 60 years before this, but we had
no cosmic X rays to detect. Our atmosphere shields us, fortunately, from
these X-rays. We needed to go to space to \"see\" the X-ray realm of the
universe. The first X-ray detectors were carried on sounding rockets
that flew an arch-shaped path in a near-space environment.

In a mere 40 years, X-ray astronomy has progressed rapidly. The Chandra
X-ray Observatory, launched in 1999, is a marvel. In many regards,
Chandra is a sister to the Hubble mission, imaging star explosions and
extremely distant galaxies with unprecedented resolution. At about 0.5
arcsecond resolution, Chandra cannot quite achieve the clarity of
Hubble. But Chandra is seeing a huge slice of the universe that Hubble
cannot see due to the limitations of optical light. In fact, after just
six months in space, Chandra was detecting new types of galaxies and
black hole activity so distant and faint that Hubble and Keck, a
powerful ground-based telescope, couldn\'t even follow-up with optical
observations.

Chandra is a massive telescope, one of the heaviest ever to be lifted
into orbit by the space shuttle. Building a bigger X-ray telescope \--
that is, with greater collecting area like the huge telescopes we see on
earth \-- would simply be too expensive and too impractical to launch
into space. But X-ray astronomy is not licked. The future lies in the
MAXIM mission and X-ray interferometry.

**Interfermetry \-- Telescope Teamwork\...**

Interferometry is the process of coupling two or more telescopes
together to synthetically build an aperture equal to the separation of
the telescopes. Sounds too good to be true, but it is a real phenomenon
and we use this technique today.

Radio astronomers were the first to capitalize on interferometry. In the
1970s, the National Science Foundation funded the construction of the
Very Long Array (VLA), a 40-kilometer chain of 29 radio dishes that
essentially serves as one single 40-kilometer wide radio telescope. With
such integrated collecting power capable of 0.04 arcsecond resolution,
the VLA opened up a world of quasars and particle jets never before
resolved with even the largest individual radio telescopes. In the late
1990s, the Very Long Baseline Array (VLBA) opened for business. This
string of ten 25-meter radio telescopes scattered from Hawaii to St.
Croix and as far north as Washington and New Hampshire achieves a
resolution 400 times better than Hubble \-- over 0.001 arcsecond
resolution.

Optical interferometry is now coming of age as well. The Very Large
Telescope (VLT), over 2.5 kilometers atop Chile\'s Cerro Paranal,
comprises four 8.2-meter telescopes that can act independently or
together. The Keck Observatory atop Hawaii\'s Mauna Kea comprises two
10-meter telescopes that also employ interferometry.

**X-ray Interferometry: Difficult but Achievable\...**

Dr. Webster Cash at University of Colorado and his colleagues have
achieved 0.1 arcsecond resolution in the laboratory with their design of
an X-ray interferometer. This resolution is comparable to Hubble. NASA
is now considering this design for the MAXIM mission. MAXIM, of course,
must fly in space. An X-ray interferometer large enough to image a black
hole would entail a fleet of up to 33 optics spacecraft flying in
formation, a challenging task but possible with today\'s technology.

In general, X-ray telescopes are difficult to build because, to obtain a
true focus, X-ray photons must reflect twice from very precisely figured
hyperbolic and parabolic surfaces. These surfaces are nested cylinders
that are very expensive to shape to the required precision. In this
regard, the Chandra X-ray Observatory is a technological marvel and is
at the forefront X-ray technology. Chandra achieve 0.5 arcsecond
resolution.

This disadvantage for X-ray astronomy \-- the need for smooth mirrors
\-- becomes an advantage when it comes to interferometry. Instead of
precisely focusing X-rays with expensive mirrors onto a detector, the
University of Colorado team used readily-made flat mirrors to mix the
X-ray wavefronts. This technique can produce an even sharper image,
similar to the way sound waves can be combined to either cancel each
other out (resulting in silence) or amplify the sound. The very nature
of X rays being difficult to focus means that they can be mixed more
easily than visible light and radio waves.

**Two Steps: A Pathfinder, then MAXIM**

Flying the various parts of an X-ray interferometer in unison is tricky
business. NASA is considering to first test the X-ray interferometer
technique in space with a \"pathfinder\" mission. The MAXIM Pathfinder
would have a one-meter separation between the mirrors so that all the
X-ray optics are on one spacecraft. The MAXIM Pathfinder would provide
100 microarcsecond resolution.

A larger separation between mirrors provides greater resolution. The
MAXIM mission \-- an X-ray interferometer large enough to image a black
hole \-- would entail a fleet of up to 33 optics spacecraft flying in
formation with a precision of 20 nano-meter, plus a detector spacecraft
500 kilometers behind the mirrors. MAXIM would achieve 100 nanoarcsecond
resolution.

With 100 microarcsecond resolution, astronomers could image the coronae
of nearby stars, seeing the actual disks of other stars which appear now
only as points of light. With 300 nanoarcsecond resolution, astronomers
could attain one of astronomy\'s ultimate goals \-- imaging a black
hole.

(In comparison, Hubble and Chandra attain around 100 and 500
milliarcsecond resolution, respectively. The lower the number, the
higher the resolution. It is a thousand-fold increase in each jump from
\"milli\" to \"micro\" to \"nano\".)

Through the power of ultra-high resolution, we could journey to distant
places without need for a warp drive. We could see stars as clearly as
we now see our own Sun. This is the promise of MAXIM and the MAXIM
Pathfinder.

\[end\]

**The spectrum:** X rays are by no means \"superior\" to other forms of
energy. Scientists value every part of the electromagnetic spectrum. Low
energy microwaves were the first light waves emitted in the Universe,
300,000 years after the Big Bang. High-energy gamma-rays chronicle the
most energetic events in the Universe, such as neutron star collisions.
X rays are ideal for viewing black hole activity because they are
produced close to the source and can escape the region without being
absorbed by intervening dust.

Radio telescopes can provide in-depth observations of black holes as
well as a variety of other astronomical phenomena. With advance radio
interferometry, one team of astronomers hopes to image the region
surrounding the suspected supermassive black hole at the center of our
own galaxy, called Sagittarius A\*. This black hole is no longer active:
that is, it is no longer pulling in matter with such plentitude. With a
lower flow of matter, radio waves may be the dominant form of radiation.

**What is an Arc Second:** Astronomers view the sky as a circle with the
Earth in the center. In geometry, a circle is divided into 360 degrees.
An arc minute is 1/60th of a degree, and an arc second is 1/60th of an
arc minute. So, an arc second is an angular measure equal to 1/3600 of a
degree \-- or a sliver of a birthday cake that\'s 1/3600th of the whole
cake.

The full moon is about 30 arc minutes in diameter when viewed from
Earth. That is, the angle formed from the Earth to opposite edges of the
moon is a half degree \-- a sliver that\'s 1/180th of the whole cake.
Stars, which are much bigger than the moon, are farther away and appear
from Earth to be only about an arc second across or less. The human eye
can resolve objects in detail as fine as about 1 arc-minute (60
arc-seconds). By the mid-twentieth century, advanced telescopes were
resolving images to 1 arc-second \-- about the size of a dime seen from
two miles away. Hubble increased that resolution by 10-fold, to about .1
arc-second. The advent of radio telescopes has allowed astronomers to
resolve images as fine as .001 arc-seconds \-- as if the observer were
100,000 times closer to the object. X-ray interferometry may be about
3,000 times better than the most powerful radio telescopes.