A star like our own Sun in a nearby galaxy is gradually being eaten away by a small but ravenous black hole, losing the equivalent mass of three Earths every time it passes close.
The discovery by University of Leicester astronomers is reported today (7 September) in Nature Astronomy and provides a ‘missing link’ in our knowledge of black holes disrupting orbiting stars. It suggests a whole menagerie of stars in the process of being consumed that still lie undiscovered.
The team was supported by the UK Space Agency and the UK Science and technology Facilities Council (STFC).
The astronomers were alerted to the star by a bright X-ray flash that seemed to come from the centre of the nearby galaxy 2MASX J02301709+2836050, around 500 million light-years away from the Milky Way. Named Swift J0230, it was spotted the moment it happened for the first time using a new tool developed by the scientists for the Neil Gehrels Swift Observatory. They rapidly scheduled further Swift observations of it, finding that instead of decaying away as expected, it would shine brightly for 7-10 days and then abruptly switch off, repeating this process roughly every 25 days.
Similar behaviour has been observed in what are termed quasi-periodic eruptions and periodic nuclear transients, where a star has material ripped away by a black hole as its orbit takes it close by, but they differ in how often they erupt, and in whether it is in X-rays or optical light that the explosion is predominant. The regularity of Swift J0230’s emissions fell between the two, suggesting that it forms the ‘missing link’ between the two types of outburst.
Using the models proposed for these two classes of event as a guide, the scientists concluded that the Swift J0230 outburst represents a star of a similar size to our own sun in an elliptical orbit around a low-mass black hole at the centre of its galaxy. As the star’s orbit takes it close to the intense gravitational pull of the black hole, material…