The rapid relationship between a black hole and a red dwarf has been revealed by ESA’s XMM-Newton Space Telescope. The pair, dubbed MAXI J1659-152 , were found to be tangoing in a fast-paced dance of one orbit per 2.4 hours and are separated by around one million kilometres.
“Most likely the two stars were born together,” says lead researcher Erik Kuulkers, who is currently situated at ESA’s European Space Astronomy Center in Spain. “One was a heavy star (say, greater than 20 solar masses), the other one was a 1 to 1.5 solar mass star. The heavy star evolved faster and ended up as a black hole after having undergone a supernova.”
Kuulkers states that because of the supernova the binary system was kicked out of the plane of our Milky Way and, during the life of the binary, the red dwarf was feeding the ravenous black hole. “They probably started orbiting each other with a period of about 0.5 to 0.8 days and, in time, evolved to the current value,” Kuulkers tells All About Space. “It’s because the two stars are orbiting each other that it takes a long time for the orbit to shrink. They don’t just fall towards each other.”
This black hole weighs in at a hefty three times the mass of the Sun compared to its somewhat lighter red dwarf companion, which is around 20% the mass of the Sun. Orbiting a common centre of mass at a speed of around 150,000 kilometres per hour, the black hole’s velocity pales in significance to its rapid companion which hurtles through space at around two million kilometres per hour – almost 20 times faster than Earth orbits the Sun. As the black hole’s gravity strips gas from the companion star, which loses mass to become a small red dwarf, it forms a a disc of gas around it which glows hot and interactions between the companions and the disc lead to powerful outbursts of X-rays and gamma-rays, initially noticed by NASA’s Swift satellite.
“Enhanced emission of X-ray or gamma rays was noted by Swift from a location not previously known to exhibit an astronomical object,” says Kuulkers. This meant that, back in September 2010, MAXI J1659-152 was originally designated as a gamma ray burst. However, with an explosion of emission that lasted longer than your standard gamma ray burst along with the proximity to our Galaxy’s plane, meant the astronomers had to rethink their initial conclusion. Could it have been an X-ray transient binary?
“It was also detected independently that same day by the Japanese MAXI instrument onboard the International Space Station, and they gave the source the name it has now,” says Kuulkers. “Soon thereafter it was followed up at many wavelengths (mainly because it was first thought to be a gamma ray burst), and there’s now a wealth of information floating around.”
Using the rapid response of XMM-Newton, Kuulkers and his team were able to trace out several regularly spaced dips in the emission in a 14.5 hour observation of the system from an edge-on vantage point. The dips, caused by the rim of the black hole’s accretion disc which temporarily obscures the X-rays as the system rotates, allowed the orbital period of the pairing to be determined. The new result, which sets a new record for the fastest black hole X-ray binary system, beats previous winner Swift J1753.5-0127 by almost an hour.
Kuulkers believes that, over time, the orbit of the pair will shrink further, shortening the period of the binary. “As the orbit shrinks, the donor star will lose mass,” he explains. “This does not go on forever (the two stars will not spiral in, or the black hole will not eat the companion in the end), but at some point the donor loses so much mass that it starts evolving and its radius increases. This has the effect of increasing the orbit again. I guess this can take another few billion years.”
Finding X-ray binaries with short orbital periods high above the galactic plane has hinted towards a potential new class of binary system. “If certain stars have similar characteristics, astronomers eagerly call it a new class,” says Kuulkers. “There are currently no other short period black hole systems known in the Galactic plane.”
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Images courtesy of D. Ducros/ESA (top) and NASA (bottom)