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Nature13 September 2001
 
Nature © Macmillan Publishers Ltd.

Wandering star

Locating black holes is a difficult business. They emit nothing but feeble Hawking radiation, remaining effectively invisible in the sky, so astronomers must infer their position from indirect effects such as the glow from material sucked in from neighbouring bodies. Around a dozen likely black holes of a similar mass to our sun have been discovered using this method, and new observations reported in Nature this week by Mirabel et al. reveal surprising behaviour of one of them.

The 'X-ray nova binary' XTE J1118+480 consists of a pair of objects orbiting one-other. One, a main-series star, is faintly visible in the visual range. But the existence of its black-hole twin was only betrayed when researchers saw a powerful X-ray burst coming from the system last year, consistent with the super-heating of gases as they were ejected from the star and accreted onto the black hole. Analysis revealed it had a mass of around six times that of the Sun.

What is unusual about XTE J1118+480 though is its position in the sky, well outside the plane of the Galaxy and at a greater angle than the other known black holes. It also has a high velocity across the sky, indicating an eccentric orbit around the galactic centre. What could have pushed the system so far from its original orbit, assumed to be in the galactic plane?

The only event powerful enough to kick XTE J1118+480 so far out of line would its own supernova. Supernovae explosions are sometimes asymmetrical, theoretically giving the star a powerful thrust as it collapses. This is the favoured explanation for other off-plane black holes, but calculations by Mirabel's team indicate that even this titanic event would give an insufficient kick without also blasting its partner out of orbit.

The astronomers argue that the black hole in XTE J1118+480 may not have had its origins in the galactic plane at all, but is instead the remnant of a far older star formed in the early galactic halo. This suggests that we may have failed to spot many other black holes, misinterpreting them as other objects or simply looking in the wrong direction.

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