The ASM instrument, built by the Lab for High Energy Astrophysics at NASA- Goddard Space Flight Center, provided continuous coverage of the entire sky, except for a 20 degree band straddling the satellite's equator. The ASM was intended to act as an early detection system for transients, and to monitor the variability of bright ( > 0.2 Crab) galactic sources. The instrument consisted of a pair of X-ray pinhole cameras, each covering opposite halves of the sky, with gas-filled imaging proportional counters. Position determination of sources was accomplished through position-sensitive anode wires and satellite rotation. Each camera had a 1-cm^2 aperture. Overall telemetry constraints limited the duty cycle for any given source to 1 percent. With the low telemetry rate provided for this instrument (1 bit/s), temporal and spectral information were sacrificed for the sake of all-sky coverage. Hence, spectral information was limited to a single 3 - 6 keV bandpass, and temporal resolution was limited to the satellite orbital period, ~100 minutes.
"All-Sky Monitor Databases Update", by L. Whitlock and P. Tyler, Legacy,
Kaluzienski, L. J. 1977, Ph.D. thesis, Univ. Maryland.
Holt, S. S. 1976, Ap. Space Sci., 42, 123.
Background contributions were from the diffuse X-ray sky, and the internal detector background. The contribution from the sky background is determined by the detector geometry, and is approximately 2 cts / element. The internal background arising from high energy charged particles was minimized by detector design features, including active anti-coincidence rejection of events at the anode ends and radiation monitors. The background measured in flight was found to be 7 cts/element near the satellite pole and 1-2 cts/element just outside the equatorial exclusion zone.
Overall, the detector efficiency and gain were stable throughout the 5.5 years of operation. However, long-term gain variation, combination of data from the "north" and "south" counters, and occasional low count rates due to spatial offsets may affect the observed source intensities. A closer examination of constant sources like the Crab illustrate the gain difference. Near MJD 2442718, a gain change of 10 percent occurred in the "south" counter, causing a decrease in the average Crab count rate from 1.28 cts s-1 cm-2 (computed with the A0535+26 outburst removed) to 1.11 cts s-1 cm-2. In August 1976, the gain in the south counter was adjusted. See Legacy #2 ASM artticle for more details.
Lightcurves were obtained by fitting multiple satellite orbit accumulations (up to ~ 1/2 day) with the locations and intensities of known sources. Large differences (> 3 sigma) between observed and expected count rates were flagged and investigated as new sources or outbursts of persistent sources. The sky catalog used was the Third Uhuru (3U) Catalog, which has some differences in source positions from their true positions.
The lightcurves in the ARIEL5 database were obtained from Dr. Steve Holt's final production tape, which covers all the data for all sources in the 3U catalog. The brightest sources and known transients were chosen to be included here. The algorithm for extracting the lightcurves from the tape used two flags: an occultation correction flag and a data quality flag. The occultation flag came about because Holt noticed, when analyzing the Cyg X-1 data, a yearly variation attributable to solar X-rays scattering or fluorescing off the Earth's atmosphere. This served to give counts for sources occulted by the Earth. Holt determined the average correction empirically from a few such sources. Both occultation-corrected and uncorrected lightcurves are included in the ARIEL5 database. The second flag is a data quality flag. Lightcurves included here corresponded to the *Best* combination of selection criteria for accepting a given datum. These selection criteria include omitting data taken near the pole, the dead zone, or the Sun, and requiring that the source be detected only in the element where it is expected to appear.
The Ariel 5 binary extension has the following columns:
TIME time in MJD RATE occultation-corrected flux in photons/cm^2/s ERROR error on occult.-corrected flux in phot/cm^2/s UNCOR-RATE observed flux in photons/cm^2/s ERR-UNCOR-RATE error on observed flux in photons/cm^2/s DQF integer data quality flag
HEASARC User Hotline.