The strange case of binary X

Behind the X-ray source called 4U 1630-472 there is only a black hole that pulls matter from its companion star. A regular periodic emission of high-energy radiation there would be a third celestial body, another star gravitationally bound to the binary system. The discovery was obtained from a study led by Fiamma Capitanio (INAF).
Investigations on the X-ray source called 4U1630-472 begin its first observations in 1969. Since then and thanks to the use of instruments that are more precise, the scientists were able to reconstruct its ‘identikit’: a pair of celestial objects orbiting around each other consists of a black hole with a mass of about ten times that of the Sun and a star of the small size, maybe smaller than the Sun.


Credit: ESA/NASA

Both components of this binary system are very old and so close together that the black hole is literally tearing his partner all the matter that composes. Around the black hole, it is then formed a disk of matter accreting which plummets towards it, heats up and emits X-rays is thanks to this flow of radiation that we can detect and study systems of stars as 4U1630-472, astronomers call binary 4U 1630-472 X. It is an old favorite for astronomers: in fact, among the binary systems of its kind, is one of the most active issues that very often in the band X. Since 1969 were observed more than 20 rapid X-ray emissions, which are called outburst. In addition, the outburst of 4U 1630-472 has a unique feature in the world of binary X: repeat it at regular intervals, with a period of about 600 days. But perhaps behind this methodical behavior there is a third, dark companion.
And what emerges from the results of a new analysis on the source led by Fiamma Capitanio (IAPS-INAF in Rome) and attended by Richard Bell and Giovanni De Cesare (both INAF-IASF Bologna) with Carlo Ferrigno (ISDC of Geneva, Switzerland), just published in the journal Monthly Notices of the Royal Astronomical Society. In the careful study of the issue of binary X, recorded by various space observatories over 5 years, from 2006 to 2010, the researchers found something unexpected: a particular behavior that outburst, along with their regular, can be explained by the presence of a third celestial body, probably another star, orbiting 4U 1630-472 and form with it a triple system of stars. “We Astronomers call these systems” hierarchical “,” Flame said Capitanio. “When the third body approaches the periastron probably induces a gravitational instability in the accretion disk around the black hole that this causes a sudden blast of X-rays.”
Normally, in binary star systems X, outbursts have caused sudden increase in the amount of matter that is ‘poured’ from the companion star toward the black hole. In the case of 4U 1630-472 there would be an additional mechanism of initiation that every 600 days, however, the lights in the X-ray source, regardless of the amount of material transferred from the companion star: the passage of a third star. This is the first case in which a growing binary system, where the more solid component is a black hole, it discovers the presence of a further body.

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