skyandtelescope.com • AUGUST 2019 17Among its discoveries, Ariel V provided a crucial measure-
ment of the curious X-ray blobs fi rst discovered in Uhuru
data. The bright blobs had already been identifi ed with galaxy
clusters, but the new observations revealed the emission’s
source: huge clouds of million-degree gas trapped by the clus-
ters’ gravity. Ultimately, these X-rays would play a key role in
exploring the nature of dark matter and dark energy.From Counting to Seeing
Both Uhuru and Ariel V viewed the X-ray universe with
proportional counters, but such devices have limited sensi-
tivity and ability to pinpoint source locations. What X-ray
astronomy needed was a telescope — an optic that could focus
incoming X-rays.
The trouble is, high-energy photons don’t refl ect off most
materials; they penetrate them. At best they can refl ect at
grazing incidence angles, skipping off a material like a smooth
pebble across a pond. That makes focusing enough X-rays to
make a decent image a diffi cult prospect.
A 1952 design by German physicist Hans Wolter showed
how to focus X-rays by allowing them to graze off of not just
one but two specially shaped mirrors. Wolter didn’t pursue the
design once he realized it was impractical for X-ray micros-
copy, but Giacconi and Bruno Rossi (then at MIT) saw its
greater value for telescopes. Even as Giacconi continued to
build conventional proportional counters over much of the
next two decades, he worked behind the scenes on plans forthe very fi rst X-ray telescope: the Einstein Observatory.
Launched on November 13, 1978, Einstein carried four
Wolter-type mirror pairs nestled within each other to focus
a greater number of photons. For the fi rst time, astronomers
could view details in the X-ray universe on the level of a few
arcseconds, nearly equivalent to the focus of large, ground-
based optical telescopes. Einstein
resolved turbulent structure in
supernova remnants and hot blobs
of gas in galaxy clusters. Moreover,
the space telescope was 100 times
more sensitive to X-rays than Ariel
V, opening a new window on fainter
(and often farther) objects. The
number of sources jumped from
hundreds to thousands.
Within months the Einstein tele-
scope had solved a mystery remaining
from the 1962 rocket fl ight: Unlike
the dark night sky, the X-ray sky is
bright no matter where you look.
Where do all of these X-rays come from? Some scientists
suggested that hot gas fi lled the space between galaxies and
clusters; others argued that couldn’t be, or the universe would
collapse under its own weight. But as Einstein discovered
more and more sources, it found increasing numbers of active
galaxies at fainter luminosities and greater distances. It soonpHEARTBEAT & CHAOS Uhuru detected regular pulsations from Centaurus X-3 (left), which fl ashed every 4.8 seconds. (The overall intensity also
changes in this plot, but it is an artifi cial effect: The spacecraft was rotating, so its detector’s response increased and then decreased as it rolled over the
source.) Cygnus X-1 (right), on the other hand, exhibited large, random variations on incredibly short time scales.pCYGNUS X-1 A giant
blue companion star
orbits the X-ray-emitting
source Cyg X-1 — the
poster child for the exis-
tence of black holes.70 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980
OSO 7
Uhuru (SAS A)Salyut 4 space stationAryabhata
Ariel VSkylab S
Experiment
Astronomische Nederlandse SatellietOSO 8High Energy
Astrophysical
Observatory
(HEAO) 1
Einstein (HEAO 2)Hakucho (CORSA B)Time (bins of 0.096 second)Countsp
er0.^0
96
s
econd1020304050 Centaurus X-Time (bins of 0.096 second)0100200300400500C
ounts^
per^0.^0
96
s
econd Cygnus X-CENTAURUS^ X-^3
AND^ CYGNUS^ X-^1
LIG
HTCURVES
:^ H.^ T
AN
ANBAUM/^ I
NTERNATIONAL^ AST
RONOMI
CAL^ UNION^ SYMPOSIUM^
NO.^5
5 ,^
19
73
(^2 )
;^ CYGNUS^ X-^1
ARTIST^ CONCEPT
:^ NASA^ /CXC^ /M.^ W
EISSModel of pulsations Data collected by Uhuru