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The First High Energy Astrophysical Observatory (HEAO-1)

Credit: NASA

The Mission

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 X-ray sky almost three times over 0.2 keV - 10 MeV, provided nearly constant monitoring of X-ray sources near the ecliptic poles, as well as more detailed studies of a number of objects through pointed observations.

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 pointing times 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.

The Instrumentation

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 sensitivity to detect sources as faint as 0.25 micro-Jy at 5 keV for sources with a Crab-like spectrum. Data 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 optical and/or radio counterparts. 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 gamma-ray 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.

HEAO-1 A1 all-sky catalog
Credit: NRL

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 first eclipse seen in a low-mass X-ray binary. The image above depicts the locations of the 842 sources detected.

HEAO-1 A2 map of the galactic center
Credit: NASA

The CXE experiment produced a catalog of AGN and clusters 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 background.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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