Sky & Telescope - August 2019 - 15

DISCOV ERY OF SCO X-1: R. GIACCONI E T A L. / PHYSICAL R E VIE W LE T TERS
19 62; ELECTRO M AG NE TIC SPECTRU M: G REGG DINDIER M A N

Number of counts

450
Turns out, Sco X-1 is the brightest endurEmission from Moon Peak of X-ray emission
Revealing
ing X-ray source outside the solar system.
350
the X-ray Sky
But the rocket launched by Giacconi's team
250
might easily have missed it: A big chunk of
Year
Number of
Counter 2
sky was hidden behind Earth during the
Counter 3
150
X-ray sources
nearly six minutes the rocket was aloft. The
1962
1*
50
discovery was fortunate indeed for Giacconi,
1965
10
120°
180°
240°
300°
360°
0°
60°
as it played a key role in winning him the
1970
60
2002 Nobel Prize in Physics. But it was even
p DISCOVERY Riccardo Giacconi and his team
more fortunate for X-ray astronomy, galvalaunched an Aerobee rocket and, in addition to solar
1974
160
fluorescence off the surface of the Moon, the second
nizing the ﬁeld as well as the imagination.
1980
680
proportional counter onboard detected X-rays
"It was pretty hard to explain [Sco X-1] by
1984
840
streaming from a source identified as Scorpius X-1.
anything except something that you didn't
The third counter, which was pointing in another
1990
8,000
expect to ﬁnd," says former Chandra X-ray
direction, saw a pervasive background of X-ray pho2000
340,000
Center director Harvey Tananbaum (Center
tons. (The first counter failed and isn't shown.)
for Astrophysics, Harvard & Smithsonian).
2010
780,000
Over the next six decades, X-ray astronThe just-graduated Tananbaum joined
2017
1,250,000
omy kept growing, albeit sometimes in ﬁts
the team in 1968, when the ﬁeld of X-ray
*Excluding the Sun.
and starts. As more instruments took to
astronomy was only six years old. He accomData source: NASA's High Energy
Astrophysics Science Archive
space, the data that came back helped conpanied equipment to the launch site off the
Research Center
ﬁrm theorists' wildest imaginings - X-rays
coast of Kenya, an equatorial location where
provided crucial evidence for the existence
Earth's rotation boosts rocket speed. With
and physics of neutron stars, black holes, dark matter, and
the rocket due to launch shortly after midnight, Tananbaum
more. "There's a whole range of activity in the universe that
followed the crew's progress via a computer on the mainland,
would have been completely unknown to us without X-ray
six miles from the rocket platform. He was waiting to perform
astronomy," says Peter Kretschmar (European Space Agency).
a last equipment test about half an hour before launch.
Yet even as 2019 sees the triumphant 20th anniversaries
"They're counting down, and they go into a hold, they're
of the Chandra X-ray Observatory and the XMM-Newton
checking something on the rocket," he recalls. "There's limited
satellite, astronomers are still struggling to maintain their
information ﬂowing, and the Sun comes up and it's starting
space-based window on the X-ray universe.
to get warm. . . . Temperatures inside the payload are going up
and up." High heat and humidity were threatening the mission
before it could make it into space. The platform crew had to
Freedom: Satellites Take to the Sky
decide: scrub or launch.
Not long after the ﬁrst discoveries, scientists began craving
The decision was quick - the count jumped from T minus
more than the few minutes of observing time that a rocket
hours to T minus minutes. "And bingo, it launches," Tanancould grant them. Giacconi led the team at AS&E as they
baum chuckles. That day, December 12, 1970, happened to
began to build a dedicated X-ray satellite - this time with
fall on Kenya's independence day, and Giacconi renamed the
NASA funding.
mission Uhuru (Swahili for freedom) in the country's honor.
The project, ofﬁcially known as Small Astronomical SatelBefore long, computer screens showed so-called "X-ray
lite A, would contain two sets of proportional counters to scan
stars" seething on the sky. Some varied predictably in their
the sky and pinpoint X-ray sources to within a few arcminutes. These proportional counters consisted of beryllium
Energy (electron volts)
casings surrounding interiors that were ﬁlled with argon gas.
0.1
1
10
100
1,000
10,000 100,000 1,000,000
An X-ray photon that penetrated the tube's thin beryllium
windows would collide with an argon atom and initiate an
avalanche of ion-electron pairs proportional to the photon's
energy. Combined with a star tracker that told scientists
Infrared
Visible
UV
X-ray
Gamma ray
which way the satellite was pointing, the proportional counter could record X-rays by their position on the sky.

10,000

1,000

100

0.1

0.01

0.001

Wavelength (nanometers)
u X-RAY VISION Rather than using wavelengths, astronomers typically refer to X-rays in terms of their energy, given in units of electron
volts (eV) or, more often, kiloelectron volts (1,000 eV, or 1 keV). Uhuru, for
example, used proportional counters to detect X-rays in the range from
2 to 20 keV. Nowadays, XMM-Newton sees X-rays between 0.1 and 10
keV. Some instruments, like Integral, go to even higher energies, catching both X-rays and gamma rays (3 to 10,000 keV).

100K
-173°C

10,000K
9,727°C

10,000,000K
~10,000,000°C

Temperature of objects at which this radiation
is the most intense wavelength emitted.
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Sky & Telescope - August 2019

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