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"Two things affect us most deeply' the stars above
and the conscience within."-Folk wisdom
by Alexander Mitrofanov
HIS ARTICLE IS DEVOTED TO
a modest but still glorious
event. Thirty years ago-or/ to
be exact, on |une 18,1962-the
iirst nonsolar source of X rays was
discovered. This source is in the
constellation of Scorpio, and according
to the accepted terminology is
now called Sco X-1.
The discovery was made cluite
unexpectedly during a rocket experiment
by the American scientists
Bruno Rossi and George Clark
of the Massachusetts Institute of
Technology and Ricardo Giacconi,
F. Paolini, and Herb Gursky of
American Science and Engineering,
Inc. The experimenters had planned
to do research in the X-ray band of
the spectrum (in the region 0.2 nm
< 1" < 0.8 nm) on the fluorescence of
the lunar sur{ace induced by the
flow of fast particles coming from
the hot solar corona (the solar
wind). So-called "so[t" X rays can't
be detected at the Earth's surface because
they are absorbed in the atmosphere.
So devices were installed
onboard the Aerobee-150 rocket,
which was capable of lifting scienti{ic
ecluipment to an altitude of 200 km
or more.
At that time the existence of detectable
celestial X-ray sources other
than the Sun and Moon was consid-
1X-ray solar radiation was discovered
in the Iate 1940s by American
scientists. In near-Earth orbit the flow
of X-ray radiation {rom the "calm" Sun
is about 106 photons/cm2 ' s. During
ered improbable.l Indeed, the distances
to the stars, even the nearest
ones, are so large that the 1/R2 decrease
in the flux of X rays from a star
at a distance R would negate all attempts
at detecting X-ray radiation
from stars like the Sun and those that
are even hotter and bigger.2
It was estimated that the X-ray
flux from hot stars should not be
more than about 10r photon/cm2 ' s
too faint to be detected by devices
-far
available at that time. Nevertheless,
tn 1962 two of the three
photon detectors in the X-ray experiment
showed a sharp increase in the
photon counting rate.
During that historic flight the
rocket rotated about its longitudinal
axis, and the readings of the detectors
were correlated with this rotation.
Thus, despite the large angular
view of the detectors, it was clear
that the source o{ the X rays was
located somewhere near the center
of our galaxy. The exact direction to
the source could not be established
in that experiment, although evidently
neither the Sun nor the Moon
was connected in arry way with the
increase in X-rayphotons. That very
first experiment also showed the
existence of a cosmic background of
solar bursts it increases many times
over.
2Modern technology is capable of
recording the X-ray radiation o{ the
stellar coronas in ordinary stars-for
example, Alpha Centauri, which is a
star similar to our Sun.
comparatively bright, continuous X-
ray radiation.
One year later the scientists again
launched Aerobee, but this time
with new equipment. The angular
view of the X-ray detectors was decreasedby
means of a Roentgen collimator
whose walls were impervious
to X rays. The same source
discovered in 1952 was detected
again! But this time its coordinates
on the celestial sphere were determined.
In another part of the sky a
second bright X-ray source was
found, in the Crab Nebula (TauX-1).
There could no longer be any doubt:
unusually bright X-ray sources exist
outside our solar system (see figures
I and2 on page 12). This discovery
revolutionized our view of the structure
of the universe and stimulated
the development of a new experimental
science: X-ray astronomy.
You can read more about this in
many fine books on popular astronomy.
As for our story, we leave the sky
along with the discovery that transformed
astrophysics in our century
andretumto Earth, to the experimental
equipment that makes such discoveries
possible. Let's take a closer
look at one of the tools of X-ray optics-the
collimator, which is a component
of modern X-ray or gammaray
telescopes (figures 3 and 4). Its
design is rather simple: it consists of
a system of parallel metal plates/
masks, slits, and little identical tubes
working together to limit the angular
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10 JUI.Y/AUOUSI 1SS4