Do Black Holes Twinkle?

Dr Rajan Chhetri is a guide at Sydney Observatory. He researches Active Galactic Nuclei (AGNs) and below discusses the interesting phenomenon that blacks holes might twinkle.

We’ve all sung or heard the nursery rhyme “twinkle twinkle little star”. Celestial objects of a certain small size twinkle (for example stars twinkle, but the Moon does not). Twinkling of the stars is caused when light from distant stars arrives to us through different paths when traveling through Earth’s atmosphere, due to the atmospheric turbulence. Can we then expect other heavenly bodies to twinkle as well? To understand this, we need to know that the light that we see with is only a small part of different “lights” that are emitted by heavenly bodies, known as the electromagnetic spectrum. Physicists have named different parts of the electromagnetic spectrum as defined by the wavelength of light (in other words, the amount of energy they carry). In descending order of their energy (and increasing order of their wavelengths), different types of light are Gamma ray, X-ray, ultraviolet, visible, infrared, microwave and radio waves. While all other electromagnetic waves get absorbed by the atmosphere, only the visible and radio waves make their way all the way to the earth’s surface in significant amounts. So, it is natural to ask if celestial objects twinkle in radio wavelengths as well.

Centaurus A radio jets above the Australia Telescope Compact Array (ATCA). Image Credit: Ilana Feain, Tim Cornwell & Ron Ekers (CSIRO/ATNF); ATCA northern middle lobe pointing courtesy R. Morganti (ASTRON); Parkes data courtesy N. Junkes (MPIfR); ATCA & Moon photo: Shaun Amy, CSIRO.
Centaurus A radio jets above the Australia Telescope Compact Array (ATCA).
Image Credit: Ilana Feain, Tim Cornwell & Ron Ekers (CSIRO/ATNF); ATCA northern middle lobe pointing courtesy R. Morganti (ASTRON); Parkes data courtesy N. Junkes (MPIfR); ATCA & Moon photo: Shaun Amy, CSIRO.

Indeed, in the 1960s the phenomenon of twinkling of sources producing radio wavelengths was studied in detail. In radio wavelengths, the objects that we are used to see in the sky, namely the stars and planets, are less bright compared other distant objects. Instead of stars, a whole range of new objects appear in the sky when you look away from the plane of the Milky Way galaxy. These are the radio galaxies, possibly with active supermassive black holes at their centres, also known as the active galactic nuclei (AGNs). Such supermassive black holes are extremely massive objects – with their masses a few million times the mass of our Sun confined to a very small space- and outshine the whole galaxy by up to a thousand times. Radio galaxies, on the other hand, are extremely large objects extending a great distance compared to the galaxy that we see in visible wavelengths and can subtend large angles in the sky. For example, Centaurus A (in the image above), one of the closest and strongest radio galaxies covers an area of 200 full Moons in the sky! While the radio galaxies are extremely large (extending up to a few million light years across), their progenitor AGNs are confined to a comparatively very small volume – similar to the size scale of the solar system. Since these objects are at extremely large distances, the cores of these galaxies only subtend a very small angle in the sky. This makes them twinkle in radio wavelengths. This twinkling of radio sources is a result of the fluctuations in the solar wind rather than the Earth’s atmosphere and is currently being used to identify these AGNs in data collected by the new telescopes such as the Murchison Widefield Array (MWA) in Western Australia. The MWA is a precursor to the Square Kilometre Array (SKA) – one of the most ambitious projects of human kind. This data from the MWA will be used to better understand one of the most curious objects in the sky – the AGNs. So, next time you look at the stars and see them twinkle, perhaps think of the nursery rhyme in a slightly different way:

Twinkle twinkle radio star,
How I wonder what you are.
Up above our galaxy so high,
Hopefully understandable through MWA eyes!

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