A Black Hole Feeding on an Ageing Star Powers a Mysterious Microquasar

A new paper by Qin Han and Xiang-Dong Li of Nanjing University, due to be published in the Astrophysical Journal, explores what’s at the heart of an object known as SS433 – the first known microquasar, which was discovered in 1979. SS433 is at the centre of a nebula called W50 (pictured above), and despite studying the two for over 40 years, astronomers don’t know exactly how they formed. W50 is a supernova remnant and this paper concludes that if the supernova left behind a black hole, it must be eight times the mass of the Sun and it must be feeding on a donor star which is 24 times the mass of Sun.

SS433 was originally a binary star system. One star collapsed in a supernova to form the black hole, and very shortly afterwards the other star entered old age. Stars expand as they reach the end of their lives (when they begin to run out of fuel), and this one is expanding so much that it’s unable to hold on to its outer layers against the black hole’s overwhelming gravitational pull. The black hole lifts away material which rapidly spirals down towards the event horizon, getting compressed and heating up as it falls. The excess energy this releases drives powerful jets flowing outwards, away from the black hole. Those jets give W50 its elongated shape, and you can see them in radio waves in the below animation:

Jets of material from SS 433, seen in radio wavelengths. [Image credit: Mioduszewski et al., NRAO/AUI/NSF]

SS433’s jets are travelling at 26% the speed of light. By regular standards that’s really fast – it would get you from New York to London in about 70 milliseconds – but for these kinds of jets it’s actually unusually slow. Normally you’d expect the jets to be travelling at over 99% the speed of light and that’s part of what makes SS433 so interesting. The speed of the jets depends on how quickly material is falling into the black hole, and that in turn depends on the ratio between the black hole’s mass and the donor star’s mass. This new paper helps explain things a little, by narrowing down what makes sense for SS433 – it needs just the right types of stars, of just the right masses, both getting to the end of their lives at nearly the same time.

Featured image credit: NRAO/AUI/NSF, K. Golap, M. Goss; NASA’s Wide Field Survey Explorer (WISE)

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