NOTICE:

This Legacy journal article was published in Volume 5, November 1994, and has not been updated since publication. Please use the search facility above to find regularly-updated information about this topic elsewhere on the HEASARC site.

HEAO-1 A4 Data and Data Products

Duane Gruber1 & L. Whitlock2

1UCSD

2HEASARC/USRA


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 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           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

References

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.


Next Proceed to the next article Previous Return to the previous article

Contents Select another article




HEASARC Home | Observatories | Archive | Calibration | Software | Tools | Students/Teachers/Public

Last modified: Monday, 19-Jun-2006 11:40:52 EDT