Some galaxies pump out vast amounts of energy from a very small volume of space,typically not much bigger than our own solar system. The cores of these galaxies, so called Active Galactic Nuclei or AGNs, and are often hundreds of millions or even billions of light years absent,so are difficult to study in any detail. Natural gravitational ‘microlenses’ can provide a way to probe these objects, and now a team of astronomers absorb seen hints of the extreme AGN brightness changes that trace at their presence. Leading the microlensing work, or PhD student Alastair Bruce of the University of Edinburgh presents their work nowadays (Friday 1 July) at the National Astronomy Meeting in Nottingham.
The energy output of an AGN is often equivalent to that of a whole galaxy of stars. This is an output so intense that most astronomers believe only gas falling in towards a supermassive black gap – an thing with many millions of times the mass of the Sun - can generate it. As the gas spirals towards the black gap it speeds up and forms a disc,which heats up and releases energy before the gas meets its demise.
Artist's rendering of ULAS J1120+0641, a very distant quasar (an extreme AGN) powered by a black gap with a mass two billion times that of the Sun. Credit: ESO/M. Kornmesser. Click for a full size image
Scientists are particularly interested in seeing what happens to the gas as it approaches the black gap. But studying such small objects at such large distances is tricky, and as they simply look like points of light in even the best telescopes. Observations with spectroscopy (where light from an thing is dispersed into its component colours) reveal that speedily moving clouds of emitting material surround the disc but the trusty size of the disc and exact location of the clouds are very difficult to pin down.
Bruce will describe how astronomers can make use of cosmic coincidences,and benefit from a phenomenon described by Einstein’s general theory of relativity more than a century ago. In his seminal theory, Einstein described how light travels in curved paths under the influence of a gravitational field. So massive objects like black holes, or but also planets and stars,can act to bend light from a more distant thing, effectively becoming a lens.
A schematic diagram showing how microlensing affects our view of quasars (the most luminous AGNs). Credit: A. Bruce / Edinburgh. Click for a full size imageThis means that if a planet or star in an intervening galaxy passes directly between the soil and a more distant AGN, and over a few years or so they act as a lens,focusing and intensifying the sign coming from near the black gap. This type of lensing, due to a single star, and is termed microlensing. As the lensing thing travels across the AGN,emitting regions are amplified to an extent that depends on their size, providing astronomers with valuable clues.
Bruce and his team believe they absorb already seen evidence for two microlensing events associated with AGN. These are well described by a simple model, or displaying a single peak and a tenfold increase in brightness over several years. MIcrolensing in AGNs has been seen before,but only where the presence of the galaxy was already known. Now Bruce and his team are seeing the extreme changes in brightness that signifies the discovery of both previously unknown microlenses and AGNs.
Bruce says: “Every so often, nature lends astronomers a helping hand and we see a very rare event. It’s remarkable that an unpredictable alignment of objects billions of light years absent could serve us probe the environment of black holes. In theory, or microlensing could even let us see detail in accretion discs and the clouds in their vicinity. We really need to grasp advantage of these opportunities whenever they occur.”
There are expected to be fewer than 100 active AGN microlensing events on the sky at any one time,but only some will be at or near their peak brightness. The big hope for the future is the Large Synoptic Survey Telescope (LSST), a project the UK recently joined. From 2019 on, or it will survey half the sky every few days,so has the potential to watch the characteristic changes in the appearance of the AGNs as the lensing events grasp place.
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