The HEAO-1 A2-Cosmic X-ray Experiment (CXE)

artist concept of HEAO-1 The A2 experiments consisted in 3 main detectors LED, MED and HED. The archive for the LED data includes only maps of the sky at different energy range. No other data are available for the LED.
* Data Products

  • A2 Maps Two sets of sky maps are available. One set, generated at HEASARC, includes the MED and HED data and are available in several standard coordinate systems and projections (see description in "HEAO-1 and the A2 Experiment" Allen, Jahoda, and Whitlock 1994, Legacy, 5, 27. These can can be viewed using Skyview interface.
    A second set of maps were generated for the LED and MED data at Penn State University. These are available for different energies in Aitoff projection together with an exposure map.

  • A2 Lightcurves : Pointing and Scanning
    Pointing Lightcurves were extracted from the XRate database for a subset of positions included in the HEAO-1 A2 pointing catalog (available as the A2POINT database at HEASARC). The A2POINT database contains the pointing positions of the A2 experiment when the primary instrument was either A2 or A1, located opposite to A2. The lightcurves were extracted only when the A2 was the pointing primary instrument. Note for 89 pointing, with the A2 as primary instrument, no XRate data are available and therefore no lightcurves are available. For few known sources, data were extracted using the best known position for that source rather than the A2POINT position. For each good pointing there are associated 4 lightcurves (in FITS format) extracted from the MED (Medium Energy Detectors) and HED3 (High Energy Detectors) small and large fields of view respectively. A single GIF file containing plots of the 4 lightcurves is also associated to each pointing. The lightcurves are background subtracted and collimator corrected, with an integration time of 5.12 seconds. Data were included only if the source position was within 15 % of the collimator response. The background applied was derived from the standard background values listed in the HEAO-1 A2 data analysis system (LHEA June 1988) user guide. Because of the large A2 FOV more than one source could be in the detector collimator. The database A2LCPOINT, from which the lightcurves can be retrieved, contains a flag "CONFUSED" to indicate that other sources may contribute to the total count rate and/or variability. The confusion flag was derived by cross correlating the A2 pointing position with source positions in the following catalogs: the HEAO-1 A3 MC LASS Catalog, HEAO-1 A2 Piccinotti Catalog, and the X-ray binaries catalog.

    Scanning The lightcurves are also available for the scanning phase. These can be accessed from the database A2LCSCAN . They were obtained using data from the A2 MED (small and large field of view) and HEDs (3 units, each with small and large field of view) detectors during the scanning phase.

  • A2 Spectra The A2SPECTRA database accesses the spectra (and associated files) obtained from the A2 MED (small and large field of view) and HEDs (1 and 3 small and large filed of view) detectors during the pointed phase. The spectra were generated for all HEAO-1 A2 pointed observations except for those in more unusual HEAO-1 modes (RAM-11 and "radical RAMs"). Thus the set of spectra created are only a subset (though a fairly complete one) of the entire catalog of HEAO-1 A2 pointed observations.

    Two sets of spectra are associated to each database entry. In the first set, the source is on the collimator axis. In the second set instead the source is significantly off-axis or not even within the collimator aperture due to the satellite wobbling (here after off-source spectra). A background spectrum for each set was derived using blank sky observations and associated, using several criteria, to the first set (spectrum with the source on axis) and to the second set (off-source spectrum). Since not always the best background file was chosen additional background files are available from the A2SPECBACK database. For each set the following spectral files are available:

     * 2 MED spectra one for the small and other for the large field of view and
       2 associated background files (all in FITS).
     * 2 HED3 spectra one for the small and other for large field of view and
       2 associated background files (all in FITS).
     * 2 HED1 spectra one for the small and other for the large field of view and
       2 associated background file (all in FITS).
    Not all the spectra for each set are present. The first set of files, source on-axis, should be used for a given pointing for spectral analysis (filename suffix _p). The second set instead most likely contains background data or data from near-by sources, and/or data from the target at a low collimator angle (filename suffix _po).This set in general is not useful to derive source spectral behaviours, but in a few cases can be used as background spectrum (if no contaminating sources are in the FOV) for the first set of files.
    For the MED there is only one response file for all the spectra and this is valid also for the HD1 detector. For the HD3 instead 3 responses are available valid for different days through the mission. The database record for each entry the appropriate response for all the detectors.
    A plot viewing the MED-S and HD3-S background subtracted spectra is also available in GIF format.

    Note: The area in the file refer to the nominal on-axis area of the detector and the spectra have not been corrected for the reduction in the collimator efficiency during the period when the source was observed off-axis. Therefore the count rate and the normalization in the spectral fitting maybe be incorrect.

    * The Raw Data : the rates All data from the HEAO-1 A2 instrument (MED and HED) were placed in the so called MAX database regardless of quality or format. To facilitate and speed the data analysis, the MAX data were separated in four subsets : GETSRC, PHA, XRATES and DSDISK. The GETSRC consisted of MAX data only for selected sources. The PHA and XRATES data contained spectra and rates (for several energy bands) respectively along with several other parameters for the entire mission. The DSDISK dataset instead contained rate data integrated every 0.25 degrees in the sky. HEASARC has reformatted in FITS only the XRATE and the DSDISK dataset.

  • XRATES : A2 Raw Rates Data The raw rate FITS files were created from the XRATE database. These contains rates in eight energy bands, known as discovery scalars, for the MED (small and large field of view, FOV) and HEDs (1,2 and 3 small and large field of view) data obtained during the HEAO-1 scanning and pointed phase. Data from the HED detectors 1, 2, and 3 large and small FOV are all included in a file. A separate file contains the MED (large and small FOV) data. Two data files are available for each instrument (MED and HED) with different integration time. One contains only the 5.12 seconds data the other only the 1.28 seconds data. Each FITS file includes up to twelve hours of data and represents a single tape file from the original XRATES database.
    The FITS file consists of one BINTABLE extension. It contains the rate data along with the digital status flags, data flags, source in field-of-view, pointing mode, electron contamination, earth occultation, and the position of the moon. Each row in the file corresponds to 40.96 seconds major frame. The rate data are stored in columns as an array of 32 or 8 elements respectively in the file with 1.28 and 5.12 seconds integration. There are eight separate columns of rates in one FITS file, each containing data for one discovery scalar. The energy boundary of the first four discovery scalars were fixed, instead the definition of the
    discovery scalar 5-8 changed throughout the course of the mission.

  • DSDISK The DSDISK data set consists in two files each containing rates from the MED and the HED3 experiments, taken during the mission lifetime (about one and half year). One file has the small FOV (3° x 3°) data for the MED and HEDs together with the discovery scalars 5 and 7, the other has large FOV (3° x 1.5°) data together with discovery scalars 6 and 8. There are no data for the HED2 or HED1. The data are organized in one FITS BINTABLE extension with the same structure of the original file. Each row contains rates from about 0.5 days stored as an array of 1440 elements, corresponding to one full scan. Each array element represents a collection of 1.28 seconds exposures within 0.25 degrees of the sky. Each files contains 4 columns of rates (for different discovery scalars) for the HED3 and 4 for the MED.

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