HEAO-1 A4 Data and Data Products
Duane Gruber1 & L. Whitlock2
1.1 The Satellite and Mission
HEAO-1, the first of three High Energy Astronomical Observatories, was launched on 1977 August 12, returned its first useful data a week following launch, and returned its last data 1979 January 9. It carried four experiments for non-imaging observations of X-ray sources; these had varying energy ranges extending from < 1 keV to ~ 10 MeV, and varying fields-of-view from half a degree to many degrees. HEAO-1 observations began in scanning mode, but from the middle of the mission onward, the scanning was interrupted with increasing frequency for dwells or "pointings." Roughly 6000 of its 8000 orbits around the Earth were spent in survey, or scanning, mode however. The scanning proceeded along a great circle with the axis always within 0.5° of the sun. The nominal spin period was 33 - 35 minutes. A complete sky scan was obtained each six months.
1.2 The A4 Experiment
The A4 experiment, also known as the UCSD/MIT Hard X-ray/Low-Energy Gamma-Ray Experiment, has been described by Matteson (1978). It consisted of seven inorganic phoswich scintillator detectors surrounded by massive scintillators which served as active anticoincidence against ambient radiations. The two Low Energy Detectors, optimized for the energy range 15 - 200 keV, had area 100 cm2 each and were collimated with slats to a fan beam of 1.7° x 20° FWHM. The slats were inclined at ±30° to the satellite scan direction. The four Medium Energy Detectors, with a nominal energy range of 80 keV to 2 MeV, each had area 45 cm2 and a circular beam of 17° FWHM. The High Energy Detector had a nominal range of 120 keV to 10 MeV, area 100 cm2, and circular beam 37° FWHM.
2 A4 Data and Data Products
2.1 The A4 Catalog
In producing the MIT Sky Maps (see below), the 72 brightest sources were fit for average intensity in four spectral bands during each of two or three passes over the source in the scanning data. These results form the A4 catalog published by Levine et al. (1984). This paper provides a full discussion of the creation of the catalog and of the sky maps, and contains a complete list of the hard X-ray sources visible during the HEAO-1 mission above a threshold of about 30 millicrabs.
2.2 Sky Maps
A four energy band (nominally 13-25 keV, 25-40 keV, 40-80 keV and 80-180 keV) reduction of the scanning data was originally prepared at MIT and later converted to FITS format. With minor qualification, this database can be regarded as an all-sky map; its organization is by scan circle azimuth and by the solar ecliptic longitude of the axis of the scan circle. However, when the longitude repeated after the first year of operation, the later data were not co-added, but kept in sequence. Thus the longitude dimension can also be regarded as a time axis for the duration of the mission. Data were coadded into 900 bins in scan azimuth, making each bin of size 0.4°. In the orthogonal solar ecliptic longitude direction, the bin size was 0.8°. The spin axis direction was nominally stepped 0.5° twice a day in order to follow the sun. Thus the longitude binning can be thought of as equivalent to time steps of 0.8 x 365.25/360 days. For the entire length of the mission, therefore, this dimension is comprised of 643 bins. The set consists of 10 FITS files, each containing a 900 x 643 image. For each detector there are four files containing the co-added counts in each energy band plus a file with live time, which is common for the energy band files and necessary to convert the count sums into rates.
Typically, the A4 detector looked at any part of the whole sky for a few thousand secs every 6 months. However, also on average, when a given point in the sky was observed, it was not looked at "head on", but was at some angle away from the center of the field-of-view such that the effective detector area was 1/4 of the maximum effective detector area. Average intensities with crude spectra for the 72 sources above 30 millicrabs have been placed in the A4 catalog (see above), but sources to 15 millicrabs may be sought with some hope in these maps. Another use of the maps is to investigate the time behavior of known sources with a time resolution of 0.8 days.
2.2.3 Usage of the Sky maps
FTOOLS are expected to be in place by the end of 1995 to transform from map coordinates to equatorial coordinates and time. An aperture response function for the two detectors will also be made available.
2.3 Spectra from Pointed Observations
2.3.1 Organization of FITS Files
For each of the 414 pointed observations in the A4 list, there exists a collection of six FITS files to allow evaluation of the average spectrum of the X-ray source in the range 13-180 keV using XSPEC. The group is made up of three files for each of the two Low Energy Detectors: pha file, background file, and response matrix file. The 64 original detector channels have been reduced to 58 because the instrument setting caused the lowest six channels to carry only zero counts. The energy grid of (coincidentally) 64 bands was determined to oversample the detector resolution of 15-30% by a factor of two to three, depending on energy.
2.3.2 Detector Gain Estimation
Detector gain was variable on short and long time scales, and the response matrix has been created for the average gain determined for the observation. This gain, typically 2.4 - 3 keV, was determined by measuring the centroid of a persistent strong internal background line at 62.7 keV.
2.3.3 Background Estimation
For about one quarter of the pointings, the satellite was regularly slewed (usually every 17.5 minutes) to and from a blank sky position a few degrees from the source. Background was collected here, and experience has shown this off-source background to reliably determine the on-source background to within counting statistics. For the remaining three-quarters of the pointings, it is necessary to estimate the background from an elaborate model which includes time-varying contributions to the detector internal background from prompt and delayed radioactivity in the detector materials in response to dosages of cosmic rays and geomagnetically trapped protons. In a pilot study (Gruber et al. 1989), backgrounds determined for a majority of pointings appear to work well, but a few are clearly in error. A good criterion for most sources is zero flux at higher energies. Background estimates determined this way for "straight" pointings cannot be guaranteed to be accurate to within counting statistics.
Observations ranged in duration from three to 12 hours. With large losses of observing time to earth occultation and transits through the trapped radiation region in the South Atlantic, and in many cases to background determination, time on-source can range from 1000 to 20000 seconds. Background counting rates are typically one count/second in each of the four logarithmic energy bands defined above. The net result is a wide range of sensitivities across the sky. Depending on the observation and energy range, this can vary from a few to a few hundred millicrabs.
3 Current Status
A bibliography of publications about the HEAO-1 satellite and instruments, or results from the data analysis of the four experiments, can be found on legacy.gsfc.nasa.gov in the /heao1/biblio directory.
A set of scan-mode sky maps discussed above was delivered to the HEASARC by John Nousek (Penn State University) as part of an Astrophysics Data program grant. These files are currently on legacy. However, they will eventually be recast in a form more suitable for analysis. Additionally, the existing analysis software will be made more user-friendly.
FITS files for ping-pong pointings, on-source, off-source, and response matrices are currently available. A listing of the pointings is given below. These existing files contain an average spectrum for the entire pointing. So far no decisions have been made concerning making files with finer time resolution. The background model is still not satisfactory, so data from the "straight" pointings are on hold. We have some ideas for improvement of the model and hope to make these data files available in the Spring 1995.
4 Data Access
The HEAO-1 A4 data and products can be found online by FTPing to legacy and then accessing the /heao1 directory. Users can find the bibliography in the /biblio directory and the A4 data and products in the /a4 directory. README files are available to provide additional, current information on available files and filename conventions.
********* HEAO-1 A4 POINTINGS CURRENTLY AVAILABLE ******
start stop R.A. decl. start start stop UFU A4 pp source major frame nr. deg deg date hhmm hhmm Y degReferences
Gruber, D. E., Jung, G. V., and Matteson, J. L. 1989, in "High Energy Radiation Background in Space" eds. Rester and Trombka, (New York:AIP), 232
Levine, A.M., et al. 1984, ApJS, 54, 581
Matteson, James L. 1978, Proc. AIAA, No. 78-35.
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