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Cos-B: 30 Years On (Forwarded)



 
 
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Old August 17th 05, 07:53 PM
Andrew Yee
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Default Cos-B: 30 Years On (Forwarded)

ESA News
http://www.esa.int

17 Aug 2005

Cos-B: 30 Years On

On 9 August 1975 ESA's first scientific satellite, Cos-B, was launched.
Cos-B was ESA's first foray into producing a spacecraft with a single
payload: a high-energy gamma-ray telescope. The mission was a remarkable
success and returned the first detailed observations of gamma-ray emission
from within our galaxy.

Just three days after launch, 12 August the first data was returned and on
17 August 1975 routine science operations commenced. The mission had a
1-year design life, yet, like many missions, survived far in excess of
this and was finally deactivated on 25 April 1982 when the attitude
control gas finally ran out.

The final detection was recorded for posterity and signed by all project
members present in the control room at the time including: project
scientist Kevin Bennett, who now works on ESA's Planck mission, and
Giovanni Bignammi, the current chair of the Space Science Advisory
Committee.

Launch and Orbit

The spacecraft was launched on a Delta 2913 rocket from the Vandenburg Air
Force Western Test Range in California. The orbit of Cos-B was roughly 100
000 km with a period of 37 hours. This eccentric orbit was chosen to
ensure that for most of the time the satellite was outside the Earth's
radiation belts. This led to an efficient viewing programme at the price
of high cosmic-ray background.

The orbital plane was inclined at roughly 90 degrees to the Earth's
equator and the argument of perigee was placed in the fourth quadrant to
ensure that the satellite was in sight of one of the ESTRACK ground
stations.

The Spacecraft

Cos-B was configured as a cylinder 1.40 m in diameter and 1.13 m long,
with the main experiment package occupying the central region. The total
height, including antennas and other structural supports, was 1.71 m. The
total mass at launch was 278 kg, of which the experiment was 118 kg.

The satellite was spin-stabilised at about 10 rpm about its axis of
symmetry, which coincided with the optical axis of the gamma-ray detector.
Sun and Earth sensors were used for attitude measurements from which the
pointing direction could be reconstituted with a precision of about 0.5
deg. The timing accuracy was 0.5 ms or better.

The Cos-B carried a single large experiment, the design and provision of
which was the responsibility of a group of research laboratories known as
the Caravane Collaboration. The gamma-ray detector features a
magnetic-core, wire-matrix spark chamber, SC, triggered by a three-element
(B1, B2, C) scintillation counter telescope. For gamma-ray selection a
plastic scintillator guard counter (A) surrounding these two units is
placed in anti-coincidence to reject triggers due to incident charged
particles. Beneath the telescope is an energy calorimeter (E) consisting
of a caesium iodide scintillator, which absorbs the secondary particles
produced by the incident photons.

Science Summary

The main achievement of Cos-B was the 2CG catalogue of point sources. This
early catalogue was based on the selected data from only 30 observations
made during the first three years of operation. It was searched for the
presence of discrete, or localised, sources of gamma radiation under the
assumption of a flat, or smoothly varying background. Thus 25 candidate
sources were reported. In view of the availability of the total mission
data as well as more realistic background models, the interpretation of
these enhancements has evolved rapidly.

Most of the observations performed with Cos-B were devoted to the study of
the galactic disc. A detailed and complete intensity map of the galaxy
seen in the light of gamma radiation is shown below.

Light curves and spectra were derived for both the Crab and Vela pulsars.
A major discovery resulted from comparing the light curves of the Vela and
Crab pulsars for the different epochs. It is seen that while during
several years the light curve of Vela remains unchanged the Crab's light
curve varies.

Geminga, or 2CG195+4 is one of the brightest sources in the 2CG catalogue.
First discovered by SAS2, the source stands out brightly in a low
background region of the galactic disc. As a consequence of the favourable
signal to noise, an accurate spectrum was determined.

The quasar 3C273 was subjected to repeated observations and the source
appeared in three independent data sets. The combined data yielded an
energy spectrum, which when combined with data from other wavelengths
demonstrated for the first time that the luminosity of 3C273 peaks in the
gamma-ray region.

Legacy

High-energy astrophysics has played a fundamental role in the activities
of the Agency with EXOSAT, XMM-Newton and the INTEGRAL mission all
building on the work of COS-B.

For further information please contact:
SciTech.editorial @ esa.int

IMAGE CAPTIONS

[Image 1:
http://sci.esa.int/science-e/www/obj...bjectid=37775]
A recording, in two dimensions, of the last gamma-ray interaction recorded
in the Cos-B detector before the spaceraft was deactivated.

The plot has been signed by all the members of the Cos-B team present in
the control room when the event came in.

Copyright: ESA

[Image 2:
http://sci.esa.int/science-e/www/obj...bjectid=36278]
Sectional View of the COS-B Experiment

Cos-B carried a single large experiment, the gamma-ray telescope.

The gamma-ray detector features a magnetic-core, wire-matrix spark
chamber, SC, triggered by a three-element (B1, B2, C) scintillation
counter telescope. For gamma-ray selection a plastic scintillator guard
counter (A) surrounding these two units is placed in anti-coincidence to
reject triggers due to incident charged particles. Beneath the telescope
is an energy calorimeter (E) consisting of a caesium iodide scintillator
which absorbs the secondary particles produced by the incident photons.

Copyright: From Bignami et al., Space Sci. Instr. V1 (1975), p. 245-268

[Image 3:
http://sci.esa.int/science-e/www/obj...bjectid=36193]
Milky Way in Gamma Rays


 




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