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astrophysics

Explosion of energy

Combination of techniques makes it possible to identify weaker sources of gamma rays

A team of Brazilian astrophysicists and American astronomers has identified a very rare kind of galaxy, known as a blazar, located 10 billion light-years from the Earth (each light-year corresponds to 9.5 trillion kilometers) – one of the most distant cosmic objects so far encountered. Among the 93 galaxies of this kind now known, this is the weakest source for the emission of gamma rays and radio waves. Even so, it gives off a thousand times more than the whole of the Milky Way, and 100 quadrillion times more than the Sun.

Called 3EG J2006-2321, it was only possible to discover this blazar by uniting refined research techniques with others that could be called archaic. It has been ages since all you had to do was to point the telescope towards the sky and find new galaxies. Nowadays, the search for heavenly bodies, above all the more distant ones, calls for the integration of techniques that detect the energy emitted in the different forms of electromagnetic radiation from one extreme of the spectrum to the other – from gamma rays to radio waves, passing through X-rays and visible light. Each kind of radiation works like a piece of a jigsaw puzzle that fits, and slowly gives an idea of the image that is going to be formed.

The discovery came from information from the Energetic Gamma Ray Experiment Telescope (Egret) of the Compton Observatory, the satellite of the American space agency NASA that swept the skies from 1991 to 2000 in search of sources of gamma rays and catalogued a few possible new galaxies. But it was a telescope from the University of São Paulo (USP)’s Institute for Astronomy, Geophysics and Atmospheric Sciences (IAG), equipped with a digital camera and calcite crystals – a substance used by the Vikings a thousand years ago to navigate, as it makes it possible to locate the position of the Sun on the basis of the polarization of the light -, that confirmed the identification of the blazar. This cooperation perfected the method for detecting these objects, using the association of the analysis of the emission of gamma rays, radio waves and visible light.

“It is clearer all the time that information has to be gathered on various wavelengths to understand an object better”, explains astrophysicist Antônio Mário Magalhães, from the IAG, who coordinated the Brazilian stage of this study, published in the April issue of the Astrophysical Journal magazine. Putting the techniques together makes it possible to understand better the structure and the formation of the blazars already known, and to identify other weak sources of gamma rays, which is radiation made up of light of a wavelength in the order of a thousandth trillionth part of a meter (the size of the nucleus of an atom), and with millions of times more energy than visible light.

Blazars belong to a class of galaxies with active nuclei and that show a characteristic that distinguishes them from calmer galaxies, like the Milky Way: the central region of the active galaxies, concentrated in a region smaller than the Solar System, emits more energy than the rest of the galaxy, justifying the name of active galactic nuclei, or AGNs. The recently identified blazar, even with a relatively low emission of gamma radiation among those now known, releases 1040 watts (the number 1 followed by 40 zeros).

The researchers believe that the energy from active galactic nuclei is the result of a black hole with a very high mass, between 100 million and 1 billion Suns, located in the center of the galaxy, which may remain active for 100 million years. Each year, this black hole eats up the equivalent of one Sun and lets off jets of gas in opposite directions at a speed close to the speed of light, with up to 100,000 light-years in length. The electrons from these jets emit radiation with wavelengths in the ranges of X-rays, radio waves and visible light. It is presumed that it is the radio photons that are scattered by the energy-full electrons, are converted into gamma rays.

Just with the traditional method, the astronomers were making any progress. After analyzing 271 sources of radiation from this same range of the electromagnetic spectrum cataloged by the Egret telescope, Paul Wallace, from Berry College, selected one object that showed a greater possibility for being a blazar among those still not identified. This probable galaxy, although it gives off little energy, had one of the marks of active nuclei: the flow of energy would vary with time. Unlike objects observed with an optic telescope, though, the gamma ray sources come from a direction in the skies that is not known with precision.

Sources of radio
Following a complementary path, Dave Thompson, from NASA’s Goddard Space Flight Center, analyzed sources of radio, which have the least energy of those in the electromagnetic spectrum, which in the skies appeared close to the source of gamma rays. Thompson found six objects that could correspond to the gamma source identified by Wallace. Four were ruled out straight away. The rest were peculiar: they generated a flow of energy in radio waves between four and six times weaker than normal for a blazar.

Jules Halpern, from Colombia University, took some optical images and noted that one of the sources was a normal galaxy and the other, a dot. Being a dot, it could be a blazar or any other heavenly body inside or outside our galaxy. But the spectrum of the radiation emitted by this object showed the so-called redshift, indicating that the source was very distant from the Milky Way.

Magalhães had the last word. With the polarimeter that he himself had designed, the team from the IAG studied the two sources of radio, but in visible light. Coupled to the IAG’s telescope at the National Astrophysics Laboratory, in Brasópolis, Minas Gerais, the polarimeter measures which fraction of the light captured is polarized, that is, vibrates on a single plane. In six hours of observation, on two nights in 2000 and 2001, the astronomers from São Paulo concluded their diagnosis, associating 3EG J2006-2321 with the distant source of visible light that also emitted radio: the dot showed a variation in the polarization of light, another characteristic of the blazars.

The next step: to assess the polarization of the light of another 15 possible blazars from Egret with a polarimeter connected to a telescope at Cerro Tololo, in Chile. “The identification of sources not yet studied will help to get to know the origins of the most energy-full photons that permeate the Universe”, Magalhães concludes.

The Project
Maintenance of the Activities on Astronomic Polarimetry (nº 97/11299-2); Modality Regular assistance line to research; Coordinator Antonio Mario Magalhães IAG/USP; Investment R$ 73,106.21

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