- Email: [email protected]
Searching for a black hole
image celestial X-ray sources, and measure luminosity and spectral energy distribution. The instruments selected are: Very advanced X-ray cameras for the prime focus positions of the three telescopes; Two reflection grating spectrometers to measure the spectra of X-ray sources; A telescope operating at visual wavelengths to enable both X-ray and optical emissions to be studied at the same time. These instruments will be provided by groups led by the respective principal investigators: Dr G. F. Bignami (Milan), Dr A. C. Brinkman (Utrecht) and Dr K. 0. Mason (London), and will involve some 70 scientists from 29 institutes in seven European countries supported by five groups from the USA. A list of scientists throughout Europe involved in XMM is available from ESA Headquarters and other establishments. The development of the X-ray telescope optics, which has already been started by ESA, will be supported by the selected ‘Telescope Scientist’, Dr B. Aschenbach (Munich). XMM is the second major cornerstone mission of E&A’s long-term scientific programme ‘Horizon 2000’. It will be ESA’s largest scientific satellite when launched in 1998. XMM has a planned lifetime of over ten years and should ensure that Europe’s astronomers will maintain their position in the forefront of astrophysical research into the next century.
4.4. SEARCHING FOR A BLACK HOLE’”
The quest for black holes, the cosmic vacuum cleaners predicted by the general theory of relativity, has triggered an unusual experiment. At an ESAsponsored conference held in Segovia, Spain, in October 1987, 92 astronomers from 15 countries agreed to collaborate in the use of an 11-year old astronomical satellite, the International Ultraviolet Explorer (IUE), to search for the enigmatic black hole in the nucleus of the galaxy NGC5548. The NGC5548 experiment, which started in December 1988, was scheduled to last for about 8 months. It calls for one observation of the galaxy every four days with the IUE satellite. The gas surrounding the candidate black hole is emitted in response to radiation given off by matter falling into it. By monitoring simultaneously both the radiation originating near the object and the swirling gas around it, the distance of the gas clouds can be determined. Five months into the observing programme, the first results have been obtained by Dr J. Clavel of the ESA IUE observatory. Dr Clavel commented: “The campaign is going well beyond what we hoped for and, best of all, the galaxy c4)From ESA News Release No. 13,23 May 1989
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in question has behaved according to the initial expectations”. When combined with an estimate of the gas velocity as obtained from Doppler measurements, the mass of the object can be inferred. Preliminary results from the NGC5548 experiment show that the gas is only seven light-days away from the black hole candidate and yields at least 20 million solar masses for its mass in that galaxy. Black holes are thought to be formed when stars, with mass larger than three times that of the Sun, collapse under their own weight in a supernova explosion at the end of their life. What is left behind is so dense and exerts such a tremendous gravitational pull on its environment that even light cannot escape. This is why black holes are ‘black and therefore difficult to detect. One probable site for black holes are the nuclei of the so-called active galaxies and quasars. These objects typically emit as much energy as 100 billion Suns from within a volume hardly larger than that of the solar system. Nuclear fusion, the power source of stars, is not nearly efficient enough to release that much energy in such a tiny volume. Only accretion (nuclear fusion converts about 1% of the total energy available, whereas accretion converts lo%, i.e. 10 times more bang out of accretion) of stars and interstellar gas on to a 10 to 100 million solar masses black hole can account for the phenomenon. Whole stars can be broken into pieces by such massive holes. The gas thus freed falls on to the hole at near the velocity of light and, in doing so, releases a tremendous amount of energy. As much as 40% of the remaining mass swallowed by the hole can be converted into light by this process. Until now astronomers have strong evidence for only one black hole candidate in the Cygnus constellation; the mass of this candidate is ten times larger than that of the Sun. Black holes can only be studied indirectly through the impact they have on their environment. Such studies, which involve complex techniques and require a large amount of observing time on expensive satellites, are beyond the means of a single astronomer, and require the setting up of large consortia. The IUE mission is a joint project of ESA, NASA and the UK Science and Engineering Research Council (SERC), which provides an observing facility for uv spectrophotometry (1150 A-3200 A) of celestial objects for astronomers from all countries. ESA’s contribution to the satellite involved the construction of the solar panels and, for the operational phase, the building and operation of the Vilspa ground station at Villafranca, Madrid, where the observatory supplies integral support for European guest observers. The IUE satellite was launched by a NASA Delta vehicle into a geosynchronous orbit on 26 January 1978. Its use is open to the scientists via an annual call for proposals. IUE recently celebrated its 11th year of successful operation in orbit.
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4.4. SEARCHING FOR A BLACK HOLE’”
The quest for black holes, the cosmic vacuum cleaners predicted by the general theory of relativity, has triggered an unusual experiment. At an ESAsponsored conference held in Segovia, Spain, in October 1987, 92 astronomers from 15 countries agreed to collaborate in the use of an 11-year old astronomical satellite, the International Ultraviolet Explorer (IUE), to search for the enigmatic black hole in the nucleus of the galaxy NGC5548. The NGC5548 experiment, which started in December 1988, was scheduled to last for about 8 months. It calls for one observation of the galaxy every four days with the IUE satellite. The gas surrounding the candidate black hole is emitted in response to radiation given off by matter falling into it. By monitoring simultaneously both the radiation originating near the object and the swirling gas around it, the distance of the gas clouds can be determined. Five months into the observing programme, the first results have been obtained by Dr J. Clavel of the ESA IUE observatory. Dr Clavel commented: “The campaign is going well beyond what we hoped for and, best of all, the galaxy c4)From ESA News Release No. 13,23 May 1989
32
in question has behaved according to the initial expectations”. When combined with an estimate of the gas velocity as obtained from Doppler measurements, the mass of the object can be inferred. Preliminary results from the NGC5548 experiment show that the gas is only seven light-days away from the black hole candidate and yields at least 20 million solar masses for its mass in that galaxy. Black holes are thought to be formed when stars, with mass larger than three times that of the Sun, collapse under their own weight in a supernova explosion at the end of their life. What is left behind is so dense and exerts such a tremendous gravitational pull on its environment that even light cannot escape. This is why black holes are ‘black and therefore difficult to detect. One probable site for black holes are the nuclei of the so-called active galaxies and quasars. These objects typically emit as much energy as 100 billion Suns from within a volume hardly larger than that of the solar system. Nuclear fusion, the power source of stars, is not nearly efficient enough to release that much energy in such a tiny volume. Only accretion (nuclear fusion converts about 1% of the total energy available, whereas accretion converts lo%, i.e. 10 times more bang out of accretion) of stars and interstellar gas on to a 10 to 100 million solar masses black hole can account for the phenomenon. Whole stars can be broken into pieces by such massive holes. The gas thus freed falls on to the hole at near the velocity of light and, in doing so, releases a tremendous amount of energy. As much as 40% of the remaining mass swallowed by the hole can be converted into light by this process. Until now astronomers have strong evidence for only one black hole candidate in the Cygnus constellation; the mass of this candidate is ten times larger than that of the Sun. Black holes can only be studied indirectly through the impact they have on their environment. Such studies, which involve complex techniques and require a large amount of observing time on expensive satellites, are beyond the means of a single astronomer, and require the setting up of large consortia. The IUE mission is a joint project of ESA, NASA and the UK Science and Engineering Research Council (SERC), which provides an observing facility for uv spectrophotometry (1150 A-3200 A) of celestial objects for astronomers from all countries. ESA’s contribution to the satellite involved the construction of the solar panels and, for the operational phase, the building and operation of the Vilspa ground station at Villafranca, Madrid, where the observatory supplies integral support for European guest observers. The IUE satellite was launched by a NASA Delta vehicle into a geosynchronous orbit on 26 January 1978. Its use is open to the scientists via an annual call for proposals. IUE recently celebrated its 11th year of successful operation in orbit.
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