The First High Energy Astrophysical Observatory
The first of NASA's three High Energy Astrophysical Observatories, HEAO-1
was launched aboard an Atlas Centaur rocket on 12 August 1977 and operated
until 9 January 1979. During that time, it scanned the entire
sky almost three times over 0.2 keV - 10 MeV, provided nearly constant
monitoring of X-ray sources near the
as well as more detailed studies of a number of objects through pointed
HEAO-1 was primarily a survey mission, dedicated to systematically mapping
the X-ray sky every 6 months. HEAO-1 had a 93 minute orbital period, and,
while in scanning mode, spun with a nominal period of 33 minutes. Each spin
traced out a great circle of constant ecliptic longitude. Every twelve hours,
the spin axis was moved approximately 0.5 degrees in order to keep it pointed
at the Sun; thus, after 6 months, the entire sky had been observed. After the
first ~ 100 days of the mission, scanning was interrupted from time to time to
point the detectors at particular objects of interest. These
became more frequent until 9 January 1979, when the gas used to control the
spacecraft attitude ran out. The systems were shutdown, and HEAO-1 drifted in
a decaying orbit
until March 1979, when it burned up on re-entry into the atmosphere.
HEAO-1 carried four instruments all used primarily in a scanning mode, with
a small number of pointed observations.
A1 - Large Area Sky Survey experiment (LASS)
The experiment had sufficient
detect sources as faint as 0.25 micro-Jy at 5 keV for sources with a Crab-like
was collected in either a 5 or a 320 millisecond timing resolution mode: Full
sky coverage for both time resolutions was achieved before the mission's end.
A1 was a product of the Naval Research Laboratory.
A2 - Cosmic X-ray Experiment (CXE)
The A2 experiment was designed to primarily study the large
scale structure of the
Galaxy and the
Universe, yielding high quality spatial and spectral data over the energy
range 2-60 keV. A2 was a collaborative experiment between NASA-GSFC and
Cal Tech/Penn State University.
A3 - Modulation Collimator (MC)
The scanning Modulation Collimator (A3) instrument was designed to
measure the positions of X-ray sources with sufficient precision to identify
A3 was a collaborative effort between the Massachusetts Institute of
Technology and the Smithsonian Astrophysical Observatory.
A4 - Hard X-Ray / Low Energy Gamma-Ray Experiment
The A4 experiment was designed to extend the energy range of the HEAO-1
mission into the low-energy
region. It was sensitive to photons with energies from 15 keV to 10 MeV,
allowing an investigation of higher-energy sources. A4 was a collaborative
effort between the University of California at San Diego and the Massachusetts
Institute of Technology.
The Science Results
All of the instruments aboard HEAO-1 produced a significant number of
discoveries of X-ray sources, transients, bursters,
pulsars, and more.
Below are listed a few specific items.
The LASS experiment produced a catalog containing 842 sources from a
systematic sky survey sensitive down to 0.001 Crab. It also discovered
aperiodic variability in Cyg X-1 on timescales on a few milliseconds and the
seen in a low-mass X-ray
image above depicts the locations of the 842 sources detected.
The CXE experiment produced a catalog of
of galaxies, a definitive broad-band spectrum of the diffuse X-ray
background, and pulsations in the cataclysmic variables SS Cygni and U Gem.
The image above shows how individual X-ray sources could be see in the
galactic center region.
The MC experiment data led to the identification of several hundred optical
companions and source classifications. This work produced a 600+ source
catalog of LASS/MC sources. The identifications included many previously
unknown active coronal type
stars, CVs, AGN, and
clusters of galaxies.
The results from the A4 experiment include a catalog of 40+ high-energy
sources. The data also allowed investigation into the origin of the diffuse