MAXI J1659-152

The black hole in the system MAXI J1659-152 appears to have lived up to its reputation. Over the last few billion years, the black hole may have slowly devoured most of a companion star, leaving behind only a tiny remnant that is continuing to pour material into the black hole. This process has pulled the two objects into the tightest orbit yet observed for a black-hole system.

Swift, an orbiting X-ray telescope, discovered MAXI J1659-152 on September 25, 2010, when the system produced a bright flare of high-energy X-rays. The flare-up probably occurred because too much gas piled up in the accretion disk around the black hole, causing an eruption. Later that same day, scientists with MAXI, a Japanese instrument aboard the International Space Station, confirmed the outburst at other X-ray wavelengths.

Several other X-ray space telescopes, as well as telescopes on the ground, followed the system over the following months. Using those observations, astronomers have compiled a preliminary dossier on the system.

It consists of a black hole that is 3 to 20 times the mass of the Sun, with a more likely upper limit closer to 10 times the Sun's mass. The black hole has a dim, red companion star that is just 20 percent as massive as the Sun. The two objects are just 600,000 miles apart (1 million km), and orbit each other once every 2 hours, 25 minutes, which is the fastest black-hole binary orbit yet measured.

The black hole pulls material off the companion star, forming a small accretion disk around the black hole. We view the system almost edge-on, so as the entire system rotates, gas from the companion star blocks some of the hot inner edge of the disk, causing a dip in the system's X-ray light. (The intervals between dips reveals the system's orbital period.)

In 2013, a team led by Erik Kuulkers of the European Space Agency proposed an evolutionary scenario for the system based on observations of MAXI J1659-152 and models of black hole binaries. In this scenario, the system was born about 5 billion to 6 billion years ago. The newborn companion was about 1 to 1.5 times the Sun's mass, and the two bodies orbited each other once every 12 to 21 hours.

The star that formed the black hole was much more massive than the companion, so it quickly consumed the nuclear fuel in its core. The core collapsed to form the black hole, while the star's outer layers exploded as a supernova. The blast gave the system a big "kick," boosting it away from its birthplace at speeds of up to 450,000 miles per hour (200 km/second) and propelling MAXI J1659-152 to its current location thousands of light-years above the plane of the Milky Way galaxy.

The companion was so close to the black hole that it wasn't kicked away by the explosion and high-speed race across the galaxy, so the system remained together.

According to the Kuulkers team scenario, as the companion burned through the hydrogen in its core, its surface expanded and it began to dump gas onto the black hole. Magnetic fields in the system then acted as a brake, slowing the companion's orbital speed and causing it to spiral closer to the black hole, accelerating the transfer of gas. And as the companion and black hole moved closer together, they emitted more gravitational waves, which carried away some of the orbital energy and pulled them even closer together. Eventually, the black hole will ingest the remainder of the companion star -- first gradually, through the continued stream of gas, and eventually, if there's anything left, in one big gulp.

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This document was last modified: April 29, 2013.