Sky & Telescope - August 2019 - 16

Cygnus X-1 proved more of a puzzle; there
emissions, such as Centaurus X-3: This source
was no predicting its tumbles up and down
brightened and faded every 4.8 seconds, while
in brightness. The biggest clue appeared when
its average brightness went up and down over
astronomers combined Uhuru data with
a roughly two-day period. Others, like Cygnus
visible-light observations, which showed a blue
X-1, varied randomly but hugely on time scales
supergiant star orbiting the X-ray source every
as short as a tenth of a second, implying that
5.6 days. The short period implied an enclosed
whatever was radiating was extremely small.
mass at least 10 times the Sun's - too masAt ﬁrst, scientists had no idea what could be
sive for a white dwarf or neutron star. Soon,
powering these strong X-ray sources. "While
Cygnus X-1 became the poster child for the
you're trying to ﬁgure it out, discovery at that
point is challenging," Tananbaum says. "It's not p Riccardo Giacconi hangs out existence of black holes.
with the Uhuru flight spare.
All in all, Uhuru cataloged several hundred
just exhilarating, I mean; it's very frustrating."
X-ray sources, tracking some of them for years.
Kretschmar thinks part of the struggle was
It was a remarkable achievement considering the onboard
philosophical. "The astronomy community was maybe not
data recorder failed after only six weeks: For most of its time
fully believing that black holes and neutron stars would really
in space, the mission could only transmit data in real time,
exist," he says. Some scientists especially opposed the idea of
as it was ﬂying over ground stations. Then a key component
black holes. "A lot of this was marginal, at the fringe of sciof the star tracker, which determined where the satellite was
ence," Kretschmar adds.
looking at any given time, burned out after inadvertently
But understanding dawned quickly. Cen X-3's regular,
crossing the Sun. Christine Jones (then a Harvard graduate
seconds-long pulsations are a signature feature of pulsars,
student conducting research at AS&E) found a workaround,
spinning neutron stars detected in X-rays rather than radio.
using the spacecraft's magnetometers to localize sources.
The source's longer, two-day variation comes about because
Nevertheless, after two years the mission was on its last legs.
the pulsar is in an eclipsing binary system.
Fortunately, other missions were on their way. Ariel V, a
q UHURU Before the spacecraft could be shipped to Kenya for launch
British-American satellite launched in 1974 from Kenya, was
to space, it underwent preflight tests at the NASA Goddard Space Flight
equipped with higher sensitivity, a better ability to pinpoint
Center (left, shown with Project Manager Marjorie Townsend and a colsources, and, ultimately, a lifetime twice as long as Uhuru's.
league). An artist's concept (right) shows the spacecraft in flight.
That same year, Martin Elvis (now at the Center for Astrophysics, Harvard & Smithsonian) joined the Ariel V team at
Leicester University, UK. He started his graduate studies just
as many of the astronomers left for Kenya's warmer and sunnier weather. "But it was incredibly lucky for me," he says. He
recalls spending his student days monitoring the basement
printer as data began coming back. "I remember watching the
plots come out. We'd have these pen plotters that would do
this 'djj djj djj,' drawing little boxes on the sky on very long
rolls of paper. . . . You'd see these one-dimensional scans of
the sky, a little histogram going around 360°, and you'd see
sources popping up."
"We found not just new sources but new types of sources,"
Elvis adds. "We'd publish one after another - oh, we found a
distant cluster of galaxies; oh, we found an active galaxy!"

GIACCONI: CENTER FOR ASTROPHYSICS, HARVARD & SMITHSONIAN; UHURU PREFLIGHT: NASA; UHURU ARTIST CONCEPT: SMITHSONIAN ASTROPHYSICAL OBSERVATORY; TIMELINE INFOG R A PHIC: TERRI DUBÉ, SOURCE: JON ATH A N MCDOWELL

X-ray Explorers

THE EVOLUTION OF X-RAYS Over the decades, astronomical X-ray explorations have advanced from
relatively short-lived projects to decades-long missions. With the exception of the galvanizing 1962 rocket
ﬂight, this graphic shows only orbital X-ray missions.

Aerobee
rocket

1960

1961

1962

AUGU S T 2 019 * SK Y & TELESCOPE

Cosmos 215

1963

1964

1965

1966

1967

1968

Orbiting
Solar
Observatory
(OSO) 6

1969

Sky & Telescope - August 2019

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