How/Why black holes emit X-rays
Many stars are observed to be in binary systems, where two stars are orbiting each other (as the Earth orbits the sun). Another thing to know is that, the more massive a star is the faster it uses up its nuclear fuel (mostly hydrogen); therefore the sooner it “dies”.
If we happen to have a binary star system, and the more massive of the two stars explodes as a supernova and it leaves behind a neutron star or a black hole, then it will result in a binary star system with a normal star and a compact object orbiting each other. All these things working out is rare, but there are over a billion stars in the galaxy, so even rare things happen fairly often.
Now, imagine that the “normal star” then runs out of its fuel. The first thing it will do is expand as it enters its “red giant phase”, as our Sun will about 4,000,000,000 years from now. Then, some of the star’s outer atmosphere will spill over onto the black hole. It will eventually fall in, and in the process become very hot. We can observe this hot gas with X-ray telescopes, so we call this an X-ray binary.
As far as the significance of the X-ray emission, it is to let us observe the effects of the black hole, and therefore learn something about it. Black holes do not emit light, in fact they are so dense that they trap it. Therefore, the best way to learn about them is by observing the material they affect. Observing X-rays from an X-ray binary is one effective way of doing this.
How white dwarfs mimic black holes
(white dwarfs : When a star like our sun gets to be very old, after another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of the its mass remaining, it will shrink to a fraction of its radius and become a white dwarf star. read more about white dwarfs here)
The research by Professor Phil Charles, Professor Malcolm Coe and postgraduate student Liz Bartlett has appeared in The Astrophysical Journal that is devoted to recent developments, discoveries, and theories in astronomy and astrophysics.
The Southampton Physics and Astronomy team are part of a global collaboration - with colleagues in Taiwan, South Africa, Poland, Australia and Italy - that has revealed that bright X-ray flares in nearby galaxies, once assumed to indicate the presence of black holes, can in fact be produced by white dwarfs.
They made the discovery by detecting a dramatic, short-lived X-ray flare that was picked up by an X-ray telescope on the International Space Station. Using optical telescopes in South Africa and Chile, the Southampton astronomers showed that the flare, called XRF111111 as it happened on 11 November, 2011, was located in the Small Magellanic Cloud. These Magellanic Clouds are between 160,000 and 200,000 light years away and are the nearest satellite galaxies to the Milky Way.They are visible to the naked eye from the Southern Hemisphere.
The flare from XRF111111 was so luminous that astronomers initially thought it was likely to be a black hole producing X-rays but further research by Phil and his team revealed that its X-ray temperature was so low that it had to be a white dwarf instead.
“We think that this incredible X-ray flash was not due to accretion onto a black hole but was instead due to a nova explosion on a white dwarf that took place close to a hot massive star. This was something that we, as astronomers, have never seen before. “This surprising result shows that, in the right circumstances, white dwarfs are capable of mimicking black holes, the most luminous objects we know of.”
picture : binary white dwarf stars credits to : Tod Strohmayer (GSFC), CXC, NASA, Illustration: Dana Berry (CXC)