Chapter 1
INTRODUCTION

1.1  Ariel-V mission overview

Ariel-V (formerly known as UK5) was the fifth British scientific satellite in the UK/US collaborative space reasearch programme. The mission was conceived in the late 1960s, only a few years after the first discovery of non-solar X-rays in 1962. At that time rocket flights had provided a total of less than 2 hours of on-source observations, and revealed about 30 X-ray sources along with an unresolved background. Of the discrete sources, few had positively identified. These factors lead to a rather general scientific aim for the Ariel-V mission:

The identification and examination of X-ray sources other than the Sun ...

Ariel-V was launched by a NASA/LTV Scout-B rocket from the Italien San Marco Kenya launch platform 5 km off the coast of Kenya on 1974 Oct 14 10:47 (local) into a near circular orbit (apogee 556 km, perigee 512 km) with an inclination of 2.9^ƒ and orbital period ~ 97 min. The spacecraft was successfully de-spun, and following a week of experiment switch-on and checkout, the observing programme started on 1974 Oct 22 with an observation of Cyg X-2. A more detailed description of the Ariel-V prgramme from its inception in 1967 to post-launch operations in given in Smith & Courtier (1976).

1.2  Payload Description

1.2.1  Overview

The Ariel-V s/c was a spin-stabilized satellite with a total mass of 134 kg. Physically it resembled a spinning octagonal barrel of dimensions 86.6 x 95 cm. The scientific payload consisted of 6 instruments (Figure 1.1), designed to complement each other, with each putting emphasis on a separate function as its prime aim:

• Expt A (viewing along the spin axis) - a rotation modulation collimator (utilising the spacecraft spin) behind which are two detector:
  • a CMA, with slectable filters covering the 0.3-6.0 keV band

  • a group of proportional counters covering the 1.9-20 keV band.

  The fov is a cone of semi-angle 10^ƒ to 20^ƒ depending on the which of the detectors one considers. Expt A is usually referred to simply as the Rotation Modulation Collimator (RMC).
  (from MSSL/Univ. Birmingham, UK)

• Expt B (viewing perpendicular to the spin axis) - 4 Argon/Xenon proportional counters (with total area ~ 600  cm²) sensitive in the 0.9-18 keV band. Expt B is usually referred to as the Sky Survey Instrument (SSI).
  (from Univ. Leicester, UK)

• Expt C (viewing along the spin axis) - Proportional counter with honeycombed collimator CHECK sensitive in the 1-30 keV band.
  (from MSSL, UK)

• Expt D (viewing along the spin axis) - Bragg crystal spectrometer & proportional counter sensitive in the 1.5-8 keV band using two large plane crystals:
  • one of PET

  • one of lithium fluoride, with a honeycomb collimator

  (from Univ. Leicester, UK)

• Expt F (viewing along the spin axis) - collimated CsI(Na) scintillation counter sensitive in the 26-1200 keV band. Expt F is usually referred to simply as the Scintillation Telescope (ST).
  (from Imperial College, London, UK)

• Expt G (viewing perpendicular to the spin axis) - 2 pin-hole cameras & gas imaging proportional counters sensitive in the 3-6 keV band. Expt G is usually referred to simply as the All-Sky Monitor (ASM).
  (from NASA/GSFC)

The rest of the payload includes:

• The Attitude Measurement and Control System (AMCS)

• Power Supply Systems

• Telemetry, OBC & Data Handling sub-systems

• Background Monitor

1.2.2  The AMCS

Ariel-V was a spin-stabilized s/c (equal to ~ 10 ±2  r.p.m.) with initial de-spin and spin maintenance provided by a cold gas (propane) jet system. The s/c attitude was measured using a combination of 2 pairs of tilted sun slit sensors (1^ƒ×180^ƒ & 1^ƒ×160^ƒ fov), and 2 Earth albedo sensors (4^ƒ circular fov). With the aid of in-orbit calibration using the startracker provided as part of Expt A, and the positions of (then) known bright X-ray sources, attitude reconstruction was generally possible with an accuracy of ~ 0.1^ƒ. Spin-axis drift was reduced (to less than ~ 0.1^ƒ per orbit) by a dipole magnetorquer which minimized the magnetic dipole moment on the s/c.

1.2.3  Power Supply

Body-mounted solar cells on seven of the eight sides of the s/c provided > 41 W at the maximum Sun-array angle of ±45^ƒ. A single nickel cadmium (3.0  A  hr) battery provided power storage capacity.

1.2.4  Telemetry, OBC & Data Handling

The VHF (137.68 MHz) system radiated and recieved from 4 antennae mounted from the base of the s/c. The telemetry bit rate was 2048  bit  s^-1. Two cores stores was used for data compression and storage, each with a capacity of 4098 8-bit words and servicing three of the scientific experiments at a combined rate of up to 1.5×10⁴  events  s^-1. Ground stations were provided at Quito, Equator and Ascension Island, and the operational control centre at the SRC Appleton Lab, UK.

1.2.5  Background Monitor & Rejection System

This subsystem was provided as a measure against the contamination of the scientific payload by spurious background counts during passage of the s/c through areas of high intensity radiation (especially the radiation belts, SAA). Two detectors were used - the guard counter in Expt C, and the anti- coincidence scintillator shield of Expt D.

References

Kaluzienski, L.J., 1977. PhD Thesis, Univ. Maryland.

Smith, J.F. & Courtier, G.M., 1976. Proc. R. Soc. Lond. A., 350, 421.

SRC, 1973. The Scientific Payload for the UK5 Satellite, SRC Report 1973 Apr.

Whitlock, L, Lochner, J. & Rhode, K., 1992. Legacy, 2, 25.

Figures

Chapter 2
Ariel-V ASM DETECTOR CHARACTERISTICS

2.1  Overview

As described in Section 1.2.1, the All Sky Monitor (ASM; also known as Expt G) was one of six X-ray instruments on the Ariel-V satellite. The ASM instrument was built by the LHEA at NASA/GSFC (see Holt 1976; Holt et al. 1979; Holt et al. 1979a) and was the first true X-ray imaging experiment to be flown on a satellite, with the imaging accomplished via two pinhole cameras mounted @ 90^ƒ to the s/c spin axis (Figure 1.1), and provided continuous coverage of the entire sky, except for a 16^ƒ band straddling the satellite's equator (Figure 1.2). The ASM operated from 1974 October 18 to 1980 March 10, and was primarily intended to act as an early detection system for transients, and to monitor the variability of bright ( > 0.2 Crab) Galactic sources.

Overall telemetry constraints limited the duty cycle for any given source to 1%. With the low telemetry rate provided for this instrument (1  bit  s^-1), 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 (equal to ~ 100 minutes).

2.2  The Pinhole Cameras

The pair of X-ray pinhole cameras (known as the 'North' & 'South' detectors), each covering opposite halves of the sky, with gas-filled imaging proportional counters. As the available spave onboard the spacecraft did not allow a stationary arrary of multi-anode proportional counters, X-ray 'imaging' was accomplished through the fan-beam response of the individual collimators (Figure 2.1), position-sensitive anode wires and satellite rotation.

The final experiment configuration had apertures of size 1  cm², aperture-anode distance 15 cm, and an active anode length of 30 cm for a one-dimensional 90^ƒ fov (Figure ). The box enclosing the detector (including a dividing wall between the two cameras) was constructed of ~ 0.1 mm titanium backed with aluminum (to absorb the titanium K-shell flurorescent photons) in a honeycombed structure.

2.2.1  The Detector Coordinate Frame

To monitor the sky, X-ray events were assigned locations in the spacecraft coordinate system. These locations were binned into 16 latitude elements and 32 longitude elements, with each element ~ 10^ƒ ×10^ƒ. The origin of these s/c coordinates was the Sun, and hence the coordinate system moved 1^ƒ per day. Dead bands existed at the s/c equator, as mentioned above, and at the s/c poles, due to poor spatial resolution. With typical source sizes of 3^ƒ ×5^ƒ, flux from a given source may be divided between as many as four elements. (The ASM also operated in an Octant mode, in which the 512 elements were devoted to only 1/16 of the sky. The greater sensitivity was used for monitoring a particular source in outburst, or more crowded regions of the sky. However, at the present time ther are no plans to include the Octant mode data in the HEASARC database).

2.3  In-Orbit Performance

2.3.1  Gain Variations

Overall, the ASM 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 such as the Crab illustrate the gain difference. Figure 2.2 shows the half-day averages for the Crab obtained by the process described below. The outburst of A0535+26 is prominent at MJD 2442516-2442553. Near MJD 2442718, a gain change of 10% occurred in the ßouth" counter (Figure 2.3, based on Kaluzienski 1977), causing a decrease in the average Crab count rate from 1.28  ct  s^-1  cm^-2 (computed with the A0535+26 outburst removed) to 1.11  ct  s^-1  cm^-2. In 1976 August, the gain in the ßouth "counter was adjusted. ... to compensate, and whats the MJD ?

2.3.2  Background

Background contributions were from the diffuse X-ray sky, and the internal detector background. The contribution from the sky is determined by geometry of the detector, and amounts to ~ 2  ct per 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 range from 7  ct per element near the satellite pole and 1-2 ct per element just outside the equatorial exclusion zone.

References

Kaluzienski, L.J., 1977. PhD Dissertation, Univ. Maryland.

Holt, S.S., 1976. Ap. Space Sci., 42, 123.

Holt, S.S. et al., 1979. Astrophys. J., 233, 344.

Holt, S.S. et al., 1979a. Astrophys. J., 227, 563.

Whitlock, L, Lochner, J. & Rhode, K., 1992. Legacy, 2, 25.

Figures

Chapter 3
KNOWN PROBLEMS AND BUGS

3.1  Problems associated with Data Analysis

3.2  Software Bugs

None known

References

Chapter 4
STANDARD ASM DATA PRODUCTS WITHIN THE OGIP ARCHIVE

4.1  Overview

Due to the nature of the instrumentation, the only data products available for each source are:

• ...

Light curves were obtained by fitting multiple satellite orbit accumulations (up to ~ 1/2 day) with the locations and intensities of known sources. Large differences ( > 3 s) 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 light curves in the HEASARC 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 light curves 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 light curves are included in the HEASARC database. The second flag is a data quality flag. Light curves 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.

References

Whitlock, L, Lochner, J. & Rhode, K., 1992. Legacy, 2, 25.

USEFUL LINKS TO OTHER HTML PAGES

The following useful links are available (in the HTML version of this document only):

• Ariel-V Calibration files in the OGIP CALDB (OGIP Calibration Memo CAL/AR5/95-007)
• The OGIP Calibration Database Home Page
• The HEASARC Home Page