You spin me right round

By Tushna Commissariat

It’s not often that we come across a mention of an astronomical event measured in Earth years, let alone months or hours. So suffice to say I was pretty surprised by a recent XMM-Newton finding that talked about a star orbiting a black hole at the furious rate of once every 2.4 hours! Further investigation revealed that this has only broken the previous record by an hour, but these extremely short orbits still have me rather amazed. Certain short orbital period binary stars or pulsars do have even shorter periods of less than an hour, but this star orbits a stellar-mass black hole (it’s about three times more massive than the Sun) that is roughly a million kilometres away from it. The video below, courtesy of the European Space Agency (ESA), is an animation showing one complete orbit of the star.

The dizzy duo was discovered on 25 September 2010 by NASA’s Swift space telescope and was initially thought to be a gamma-ray burst. Later that day, Japan’s MAXI telescope on the International Space Station also spotted a bright X-ray source at the same place. Further observations using other telescopes, including ESA’s XMM-Newton, uncovered the black hole, known as MAXI J1659-152, dining off matter it was accreting from its companion – a star with a mass only about 20% that of our Sun.

According to ESA, XMM-Newton saw several regularly spaced dips in the pair’s emission during an uninterrupted 14.5-hour observation, caused by the uneven rim of the black hole’s accretion disc briefly obscuring the X-rays as the system rotates. The disc is almost edge-on from XMM-Newton’s line of sight. Using the dips, the researchers calculated the orbital period of a mere 2.4 hours, setting the new record for black-hole X-ray binary systems. The previous record holder, Swift J1753.5-0127, has a period of 3.2 hours.

Although both the black hole and the star orbit their common centre of mass, an even more impressive figure is the speed at which the star orbits the black hole – a breakneck two million kilometres per hour – making it the fastest moving star ever seen in an X-ray binary system, according to ESA. The black hole, on the other hand, orbits at a leisurely 150,000 km/h.

“The companion star revolves around the common centre of mass at a dizzying rate, almost 20 times faster than Earth orbits the Sun. You really wouldn’t like to be on such a merry-go-round in this galactic fair!” says lead author of the new research, Erik Kuulkers from the European Space Astronomy Centre in Spain.

The team also saw that the system lies high above the galactic plane, out of the main disc of our spiral galaxy, an unusual characteristic shared by only two other black-hole binary systems, including Swift J1753.5-0127.  “These high galactic latitude locations and short orbital periods are signatures of a potential new class of binary system, objects that may have been kicked out of the galactic plane during the explosive formation of the black hole itself,” says Kuulkers.