X-ray Explorers
16 AUGUST 2019 • SKY & TELESCOPE
emissions, such as Centaurus X-3: This source
brightened and faded every 4.8 seconds, while
its average brightness went up and down over
a roughly two-day period. Others, like Cygnus
X-1, varied randomly but hugely on time scales
as short as a tenth of a second, implying that
whatever was radiating was extremely small.
At fi rst, scientists had no idea what could be
powering these strong X-ray sources. “While
you’re trying to fi gure it out, discovery at that
point is challenging,” Tananbaum says. “It’s not
just exhilarating, I mean; it’s very frustrating.”
Kretschmar thinks part of the struggle was
philosophical. “The astronomy community was maybe not
fully believing that black holes and neutron stars would really
exist,” he says. Some scientists especially opposed the idea of
black holes. “A lot of this was marginal, at the fringe of sci-
ence,” Kretschmar adds.
But understanding dawned quickly. Cen X-3’s regular,
seconds-long pulsations are a signature feature of pulsars,
spinning neutron stars detected in X-rays rather than radio.
The source’s longer, two-day variation comes about because
the pulsar is in an eclipsing binary system.Cygnus X-1 proved more of a puzzle; there
was no predicting its tumbles up and down
in brightness. The biggest clue appeared when
astronomers combined Uhuru data with
visible-light observations, which showed a blue
supergiant star orbiting the X-ray source every
5.6 days. The short period implied an enclosed
mass at least 10 times the Sun’s — too mas-
sive for a white dwarf or neutron star. Soon,
Cygnus X-1 became the poster child for the
existence of black holes.
All in all, Uhuru cataloged several hundred
X-ray sources, tracking some of them for years.
It was a remarkable achievement considering the onboard
data recorder failed after only six weeks: For most of its time
in space, the mission could only transmit data in real time,
as it was fl ying over ground stations. Then a key component
of the star tracker, which determined where the satellite was
looking at any given time, burned out after inadvertently
crossing the Sun. Christine Jones (then a Harvard graduate
student conducting research at AS&E) found a workaround,
using the spacecraft’s magnetometers to localize sources.
Nevertheless, after two years the mission was on its last legs.
Fortunately, other missions were on their way. Ariel V, a
British-American satellite launched in 1974 from Kenya, was
equipped with higher sensitivity, a better ability to pinpoint
sources, and, ultimately, a lifetime twice as long as Uhuru’s.
That same year, Martin Elvis (now at the Center for Astro-
physics, 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 sun-
nier 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!”pRiccardo Giacconi hangs out
with the Uhuru fl ight spare.qUHURU Before the spacecraft could be shipped to Kenya for launch
to space, it underwent prefl ight tests at the NASA Goddard Space Flight
Center (left, shown with Project Manager Marjorie Townsend and a col-
league). An artist’s concept (right) shows the spacecraft in fl ight.1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 19Orbiting
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