1st High Energy Astrophysics Observatory
Encyclopedia
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
three High Energy Astronomy Observatories, HEAO 1, launched August 12, 1977 aboard an Atlas rocket with a Centaur
Centaur (rocket stage)
Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity...
upper stage, operated until 9 January 1979. During that time, it scanned the X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
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 included four large X-ray and gamma-ray astronomy instruments, known as A1, A2, A3, and A4, respectively (before launch, HEAO 1 was known as HEAO A). The orbital inclination was about 22.7 degrees. HEAO 1 re-entered the Earth's atmosphere on 15 March 1979.
A1: Large-Area Sky Survey instrument
The A1, or Large-Area Sky Survey (LASS) instrument, covered the 0.25—25 keV energy range, using seven large proportional counters.It was designed, operated, and managed at the Naval Research Laboratory (NRL) under the direction of Principal Investigator Dr. Herbert D. Friedman, and the prime contractor was TRW
TRW
TRW Inc. was an American corporation involved in a variety of businesses, mainly aerospace, automotive, and credit reporting. It was a pioneer in multiple fields including electronic components, integrated circuits, computers, software and systems engineering. TRW built many spacecraft,...
. The HEAO A-1 X-Ray Source Catalog included 842 discrete X-ray sources.
A2: Cosmic X-ray Experiment
The A2, or Cosmic X-ray Experiment (CXE), from the Goddard Space Flight CenterGoddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
, covered the 2-60 keV energy range with high spatial and spectral resolution. The Principal Investigators were Dr. Elihu A. Boldt and Dr. Gordon P. Garmire.
A3: Modulation Collimator instrument
The A3, or Modulation Collimator (MC) instrument, provided high-precision positions of X-ray sources, accurate enough to permit follow-up observations to identify optical and radio counterparts. It was provided by the Center for Astrophysics (Smithsonian Astrophysical ObservatorySmithsonian Astrophysical Observatory
The Smithsonian Astrophysical Observatory is a research institute of the Smithsonian Institution headquartered in Cambridge, Massachusetts, where it is joined with the Harvard College Observatory to form the Harvard-Smithsonian Center for Astrophysics .-History:The SAO was founded in 1890 by...
and the Harvard College Observatory
Harvard College Observatory
The Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, and was founded in 1839...
, SAO/HCO). Principal Investigators were Dr. Daniel A. Schwartz of SAO and Dr. Hale V. Bradt of MIT.
A4: Hard X-Ray / Low-Energy Gamma-ray experiment
The A4, or Hard X-ray / Low Energy Gamma-ray Experiment, used sodium iodideSodium iodide
Sodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....
(NaI) scintillation counters to cover the energy range from about 20 keV to 10 MeV.
It consisted of seven clustered modules, of three distinct designs, in a roughly hexagonal array.
Each detector was actively shielded by surrounding CsI scintillators, in active-anti-coincidence, so that an extraneous particle or gamma-ray event from the side or rear would be vetoed electronically, and rejected.
(It was discovered in early balloon flight by experimenters in the 1960s that passive collimators or shields, made of materials such as lead, actually increase the undesired background rate, due to the intense showers of secondary particles and photons produced by the extremely high energy (GeV) particles characteristic of the space radiation environment.)
A plastic anti-coincidence scintillation shield, essentially transparent to gamma-ray photons, protected the detectors from high-energy charged particles entering from the front.
For all seven modules, the unwanted background effects of particles or photons entering from the rear was suppressed by a "phoswich" design, in which the active NaI detecting element was optically coupled to a layer of CsI on its rear surface, which was in turn optically coupled to a single photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
tube for each of the seven units.
Because the NaI has a much faster response time (~0.25 μsec) than the CsI (~1 μsec), electronic pulse shape discriminators could distinguish good events××3 in NaI in the NaI from mixed events accompanied by a simultaneous interaction in the CsI.
The largest, or High Energy Detector (HED), occupied the central position and covered the upper range from ~120 keV to 10 MeV, with a field-of-view (FOV) collimated to 37° FWHM
Full width at half maximum
Full width at half maximum is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value....
.
Its NaI detector was 5 in (12.7 cm) in diameter by 3 in (7.62 cm) thick.
The extreme penetrating power of photons in this energy range made it necessary to operate the HED in electronic anti-coincidence with the surrounding CsI and also the six other detectors of the hexagon.
Two Low Energy Detectors (LEDs) were located in positions 180° apart on opposite side of the hexagon.
They had thin ~3 mm thick NaI detectors, also 5 in (12.7 cm) in diameter, covering the energy range from ~10—200 keV.
Their FOV was defined to fan-shaped beams of 1.7° x 20° FWHM by passive, parallel slat-plate collimators.
The slats of the two LEDs were inclined to ±30° to the nominal HEAO scanning direction, crossing each other at 60°.
Thus, working together, they covered a wide field of view, but could localize celestial sources with a precision determined by their 1.7° narrow fields.
The four Medium Energy Detectors (MEDs), with a nominal energy range of 80 keV — 3 MeV, had 3 in (7.62 cm) dia by 1 in (2.54 cm) thick NaI detector crystals, and occupied the four remaining positions in the hexagon of modules.
They had circular FOVs with a 17° FWHM.
The primary data from A4 consisted of "event-by-event" telemetry, listing each good (i.e., un-vetoed) event in the NaI detectors. The experiment had the flexibility to tag each event with its pulse height (proportional to its energy), and a one or two byte time tag, allowing precision timing of objects such as gamma-ray bursts and pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
s.
Results of the experiment included a catalog of the positions and intensities of hard X-ray (10—200 keV) sources, a strong observational basis for extremely strong magnetic fields (of order 1013 G) on the rotating neutron stars associated with Her X-1 and 4U 0115+634, a definitive diffuse
component spectrum between 13 and 200 keV, discovery of the power-law
shape of the Cygnus X-1
Cygnus X-1
Cygnus X-1 is a well-known galactic X-ray source in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3 Wm−2Hz−1...
power density spectrum, and discovery of slow intensity
cycles in the X-Ray sources SMC X-1 and LMC X-4, resulting in approximately 15 Ph.D theses and ~100 scientific publications.
The A4 instrument was provided and managed by the University of California at San Diego, under the direction of Prof. Laurence E. Peterson
Laurence E. Peterson
Laurence E. Peterson is Emeritus Professor of Physics and Director of the Center for Astrophysics and Space Sciences at the University of California, San Diego, California.He received his Ph.D in 1960 from the University of Minnesota....
, in collaboration with the X-ray group at MIT, where the initial A4 data reduction was performed under the direction of Prof. Walter H. G. Lewin.
See also
- HEAO ProgramHEAO ProgramThe High Energy Astronomy Observatory Program was a NASA program of the late 1970s and early 1980s that included a series of three large low-Earth-orbiting spacecraft for X-ray and Gamma-Ray astronomy and Cosmic-Ray investigations. After launch, they were denoted HEAO 1, HEAO 2 , and HEAO 3,...
- Einstein ObservatoryEinstein ObservatoryEinstein Observatory was the first fully imaging X-ray telescope put into space and the second of NASA's three High Energy Astrophysical Observatories...
(HEAO 2) - High Energy Astronomy Observatory 3High Energy Astronomy Observatory 3The last of NASA's three High Energy Astronomy Observatories, HEAO 3 was launched 20 September 1979 on an Atlas-Centaur launch vehicle, into a nearly circular, 43.6 degree inclination low-Earth orbit with an initial perigee of 486.4 km....