216
the X-ray signals, by contrast,
radiated a thousand times more
X-rays than light. Although a
small, barely visible point in the
sky, the source was pumping out
one thousand times more X-rays
than the sun. Furthermore, certain
physical processes were taking
place within the source and
these had never been seen in the
laboratory. After weeks of analysis,
the team concluded that this must
be a new class of stellar object.
Search for the source
There was no candidate in the solar
system to account for the intense
radiation. The most likely source
was named Scorpius X-1 (Sco X-1
for short) after the constellation
within which it was located. Herb
Friedman at the NRL confirmed
the result using a detector with
a larger area and better resolution
than the AS&E instrument. Sco X-1
is now known to be a double star
system and is the brightest, most
persistent X-ray source in the skies.
Further launches revealed a sky
dotted with X-ray sources, both
galactic and extra-galactic. In a
short space of time, the team had
detected a disparate set of celestial
oddities emitting X-rays. These
included supernova remnants,
binary stars, and black holes.
Today, more than 100,000 X-ray
sources are known.Toward Chandra
By the mid-1960s, instruments
were becoming ever more sensitive.
Detectors were able to record X-rays
one thousand times weaker thanCOSMIC RADIATION
Sco X-1 just five years after Giacconi’s
discovery. Initially proposed by
Giacconi in 1963, Uhuru, the first
satellite dedicated solely to X-ray
astronomy, was launched in 1970. It
spent three years mapping X-rays.
This all-sky survey located 300
sources, including a bizarre object
in the center of the Andromeda
galaxy, and it earmarked Cyg X-1
as a potential black hole. Uhuru
also found that the gaps in galaxy
clusters are strong sources of
X-rays. These apparently empty
regions are in fact filled by a low-
density gas at millions of degrees
Kelvin. Although thinly spread,
this “intercluster medium” contains
more mass than that of all of the
cluster’s galaxies combined.
In 1977, NASA launched its High
Energy Astronomy Observatory
(HEAO) program. HEAO-2,
renamed the Einstein Observatory,
was equipped with highly sensitive
detectors and revolutionized X-ray
astronomy. With its fused quartz
mirrors, the telescope was a million
times more sensitive than that of
Giacconi’s 1961 discovery rocket.
Einstein observed X-rays emanating
from stars and galaxies, and even
from planetary aurorae on Jupiter.The Chandra X-ray Observatory
was launched by NASA in 1999. It
was initially planned to operate for
five years, but was still in use in 2016.
Active regions of the sun are revealed
by combining observations from many
telescopes. High-energy X-rays are
shown in blue; low-energy X-rays green.