ESA steps towards a great black hole census
MOST OF the gamma rays that reach the earth do not come from nearby sources but from celestial objects so far away that they cannot yet be distinguished as individual sources.
This distant gamma-ray emission creates a perpetual glow that bathes the Universe. Most astronomers believe that the unseen objects are supermassive black holes, millions or billions of times heavier than the Sun and each sitting at the centre of a galaxy.
As the black holes swallow matter, the swirling gases release X-rays and gamma rays. Accurately measuring the glow, known as the X-ray and gamma-ray background, is the first step towards calculating how many black holes are contributing to it and how far away in the Universe they are located.
Earth as a giant shield
Astronomers using European Space Agency (ESA)'s orbiting gamma-ray observatory, Integral, have taken an important step towards estimating how many black holes there are in the Universe, according to a European Space Agency press release.
An international team, led by Eugene Churazov and Rashid Sunyaev, Space Research Institute, Moscow used the Earth as a giant shield to watch the number of tell-tale gamma rays from the distant Universe dwindle to zero, as our planet blocked their view.
The new Integral observations were made during January and February 2006 and provide highly accurate data on the gamma-ray background. The key to success was using the Earth as a shield.
Allowing the Earth to enter Integral's field of view goes against the standard set of nominal observations for the satellite, because the optical devices needed to determine the spacecraft's attitude would be blinded by the bright Earth.
Jigsaw puzzle
In the past, different satellites have had to measure the different energies of X-rays and gamma rays, leaving astronomers with the task of having to piece the results together like putting together a jigsaw puzzle.
This operation (Integral's) required remarkable efforts from the ISOC/MOC teams operating the mission, who had to rely on alternative spacecraft control mechanisms.
But the risk was worth it: by measuring the decrease of the gamma-ray flux and by making a model of the Earth's atmospheric emission, the astronomers precisely gauged the gamma-ray background.
Another bonus of the Integral observations is that the observatory's complementary instruments allowed the strength of both X-rays and gamma rays to be measured simultaneously. — Our Bureau
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