Image by Dr. Rob Hynes represents what the quiescent (inactive) black hole binary looked like when the observations were done.

Professor Phil Charles of the University of Southampton is part of an international team which has discovered a stellar mass black hole -- the first ever found in our galactic halo. This region of space lies above and below the main spiral arms of our galaxy, thousands of light years above the Milky Way galactic plane.

The discovery was made from observations using the newly refurbished telescope at the MMT (Multiple Mirror Telescope) Observatory in Arizona, and will be reported shortly in an issue of Astrophysical Journal Letters.

"It soon became clear from our observations that this object, which was first detected a year ago by the Rossi X-ray Timing Explorer, had to be a black hole at least six times as massive as our sun," said Professor Charles, head of astronomy at Southampton's Department of Physics & Astronomy.

Together with collaborators Dr Mark Wagner of the University of Arizona, Craig B. Foltz, director of the MMT Observatory (MMTO), Sumner Starrfield of Arizona State University, and Jorge Casares and Tariq Shahbaz of the Instituto de Astrofisica de Canarias in Tenerife, Professor Charles has assembled the finest 'quiescent' observations (those taken during a period of inactivity) yet made of this phenomenon. These quiescent observations have also made accurate mass measurements possible.

The black hole, which is approximately 6,000 light years away, betrays its presence by the effect it has on the motion of an orbiting star, smaller than our sun, which is slowly transferring matter to the black hole. The period of orbit is short -- only 4.1 hours -- which means that, astronomically speaking, this binary star is nearing the end of its 'active' life.

"We are intrigued to find such an object in the galactic halo," said Professor Charles. "Because this is so far above the galactic plane, there is almost no interstellar medium between us and the object, so we can study it in detail like no other object in its class."

It was Professor Charles' colleague at Southampton, Dr Robert Hynes who, intrigued by the object's unusual location within our galaxy, initiated the first extreme ultra-violet observations of such an object, made possible because of the unobscured line of sight.

The giant MMT, formerly a six-mirrored instrument, equal to a 4.5-m telescope, is a joint facility of the University of Arizona and the Smithsonian Institution. It now features a 6.5-meter, stiff, lightweight borosilicate 'honeycomb' mirror spin-cast and polished at the UA Mirror Lab. The new telescope was dedicated in May 2000 and scientific observations with it began in earnest only last autumn.

Regular brightness variations in the star were monitored with Spanish telescopes at the Observatorio del Teide in Tenerife, and contributed significantly to the accuracy of the black hole mass.

The system was X-ray bright a year ago because X-rays are produced by gas falling from the normal star towards the black hole. The gas near the black hole is heated by friction and gravity to millions of degrees, and the heat energy is radiated as X-rays. But once the X-rays fade away and the system becomes quiescent (inactive), this allows the normal star to become visible, hence providing the opportunity to measure the mass of the black hole.

Because the system is now faint -- more than 160,000 times fainter than can be detected with the unaided human eye -- it took a precision giant telescope to get optical spectra (separated colours of light) in only a few hours time.

Dr Robert Hynes' work on the ultra-violet observations was undertaken as part of a collaboration, partially funded by the Leverhulme Trust.