1 HEAO-1

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

2.2.1 Organization

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.

2.2.2 Sensitivity

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.

2.3.4 Sensitivity

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 heasarc.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           deg

493790  494056   268.43 -23.89  4/ 2/78  2155   0058 -            2  JUPITER
495106  495421   100.74 -79.31  4/ 3/78  1254   1629 -   3        2  3C390.3
497310  497844    99.18  17.51  4/ 4/78  1358   2004     1        2  CG195+4
501338  501882    84.78 -67.07  4/ 6/78  1148   1800    18        6  LMCX-1
509877  510143   100.83 -79.42  4/10/78  1257   1600 -   3        2  3C390.3
511636  512428   289.75 -60.69  4/11/78  0858   1800     1        7  ESO141-G55
547058  547406   295.21 -11.48  4/28/78  0400   0758     2        6  ngc6814
577908  578170   144.96 -31.75  5/12/78  1900   2159     3        3  MCG5-23-16
586075  586347   244.65  75.73  5/16/78  1555   1902              4  EXG HOLE
592178  592244   308.05  40.19  5/19/78  1322   1407   385   417  3  CYG X-3
598118  598656   116.05 -40.84  5/22/78  0857   1505              5  OSO0752-39
629235  629762     2.69 -40.02  6/ 6/78  0259   0900 -  16        1  NGC 4151
633376  633629   182.93  40.91  6/ 8/78  0206   0459    16    70  3  NGC 4151
637674  638198     2.72 -40.05  6/10/78  0300   0859 -  16        1  NGC 4151
645457  646019   186.63  11.41  6/13/78  1933   0158    23    15  6  M87
652222  652523   182.61  40.59  6/17/78  0032   0358    16    70  3  NGC 4151
663942  664490   188.81  11.95  6/22/78  1352   2007     4    15  4  M87
670184  670726   329.19  40.26  6/25/78  1252   1904     1        4  BL LAC
676337  676873   354.71 -20.70  6/28/78  1053   1701 -   2        2  A1367
677808  678878   179.87  -6.82  6/29/78  0338   1549     2        2  4U1203-06
680487  681002   188.36   1.57  6/30/78  1007   1559     4    36  4  3C273
686754  687294   188.39   1.29  7/ 3/78  0925   1535     4    36  4  3C273
690667  691527   188.49   1.69  7/ 5/78  0557   1544     4    36  4  3C273
696912  697274     1.00  19.44  7/ 8/78  0500   0908     3        2  MKN335
698936  699295   171.74 -36.34  7/ 9/78  0402   0807     3        3  ngc3783
705036  705568   347.80  34.66  7/12/78  0126   0730 -            6  NGC3783
716828  718033   189.70 -40.85  7/17/78  1536   0519 -   2        1  M31
725915  726376   203.01 -43.42  7/21/78  2300   0415    26   181  9  CEN A
733998  734528    22.02  43.79  7/25/78  1857   0100 -  26        5  CEN A
747970  749029    21.41  44.09  8/ 1/78  0956   2159 -  26        8  CEN A
755974  757028    21.10  45.01  8/ 5/78  0500   1700 -  26        7  CEN A
763819  764879    60.91 -16.91  8/ 8/78  2215   1019 -   3        4  A2147
774875  775397   229.71   9.83  8/14/78  0403   1000     4        9  2A1519+097
777463  778332   253.93  34.76  8/15/78  0929   1923   100   171  2  HER X-1
783565  784523   253.87  34.65  8/18/78  0655   1750   100   171  2  HER X-1
786206  786822   227.34 -56.74  8/19/78  1258   1959   850    41  9  CIR X-1
789647  790367   253.84  34.63  8/21/78  0407   1219   100   171  2  HER X-1
791736  792272    60.81  31.32  8/22/78  0352   1000    35    10  6  X PER
793956  794645   193.53 -61.11  8/23/78  0509   1300 -   5        8  GAMMA CAS
796184  796659   253.80  34.51  8/24/78  0630   1155   100   171  2  HER X-1
808801  809520    46.03  40.88  8/30/78  0603   1415    48    46  6  NGC 1275
810376  811231    39.80  40.99  8/30/78  2358   0943              6  NCG 1129
816967  817767    66.81  38.30  9/ 3/78  0258   1205              6  3C111
824619  825670   244.50 -14.28  9/ 6/78  1802   0600   999  4581  2  SCO X-1
826728  827252    65.25   4.72  9/ 7/78  1801   0000     3        6  3C120
828928  829449   253.74  39.69  9/ 8/78  1903   0100     2        2  MKN 501
844268  844639   255.11 -30.98  9/16/78  0135   0549              4  GC HOLE
852776  853294   248.19 -65.47  9/20/78  0224   0818    25   163  5  4U1626-67
857761  858630   257.28  35.76  9/22/78  1107   2100   100   171  3  HER X-1
859913  860783   257.10  35.86  9/23/78  1136   2130   100   171  3  HER X-1
870148  870861    82.24 -51.14  9/28/78  0803   1610 -            2  ZW1727+501
883755  884381    82.44  26.60 10/ 4/78  1852   0200 -  55        6  GX1+4
898186  899057   287.39 -46.38 10/11/78  1503   0059 -   2        2  MK376
902350  903454   279.31   6.51 10/13/78  1426   0300   269    55  3  ser x-1
904359  905123   329.50 -71.05 10/14/78  1318   2200 -   2        2  M82
906395  906749   310.22  -9.78 10/15/78  1227   1630     3     5  3  mkn509
911898  912591   102.43 -32.80 10/18/78  0304   1058 -            2  BETA LY
915004  915846   310.05  -9.65 10/19/78  1425   0000     3     5  3  mkn509
916759  917809    84.72 -70.55 10/20/78  1023   2220 -            3  N EC.POLE
918646  919428    92.98 -64.47 10/21/78  0751   2300 -            3  N EC.POLE
920670  922086   311.50 -13.50 10/22/78  0652   2300              5  mkn509
927362  928239   104.18 -37.40 10/25/78  1101   2100 -   2        3  4U1852+37
933523  934556   309.82  -8.5  10/28/78  0907   2053              6  2A2040-115
939417  940562    14.15 -72.97 10/31/78  0410   1712    69    61  8  SMC X-1
946445  947425   300.43  39.13 11/ 3/78  1208   2318 -   3        3  ZETA PUP
956809  957331    24.60 -68.56 11/ 8/78  1003   1600              2  H0142-69
960947  962094   211.10  69.86 11/10/78  0908   2212     3        7  mkn279
993097  994244   207.97  69.35 11/25/78  1430   2030     3        1  mkn279
1001104 1002523  207.97  69.35 11/29/78  1000   0210     3        7  mkn279
1011755 1012263  333.70  14.00 12/04/78  1100   1700              1  4U2214+13
1051284 1052648  223.10  78.70 12/23/78  0458   2029              9  EXG HOLE
1053371 1054230  281.90  78.90 12/24/78  0443   1430     1        5  3C390.3
1077230 1078885   19.73 -13.32  1/04/79  1211   0701     5        9  EXG HOLE
1080073 1080905  213.91  25.56  1/05/79  2032   0600     2    20  4  NGC 5548
1082236 1083014  213.91  25.56  1/06/79  2101   0559     2    20  4  NGC 5548

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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