Big one that almost got away
Posted: August 29, 2001

Astronomers have discovered a double star system, known as a massive X-ray binary, with an orbit so elongated that the twosome appears to have barely survived the disruptive effects of the supernova that transformed them into a fountain of X-ray energy.

Georgia State University doctoral student Virginia McSwain and collaborators observed a mysterious X-ray source called LS 5039 from 1998 to 2000 using the National Science Foundation's Coude Feed Telescope, a special auxiliary system on Kitt Peak National Observatory's 2.1-meter telescope.

A painting of the LS 5039 system from a vantage point near the the neutron star or black hole. Gases from the massive, blue star are pulled by gravity into a hot accretion disk swirling around the neutron star or black hole. Some of this material is then ejected in fast moving jets originating from the center of the disk. Credit: Bill Pounds
LS 5039 was among a number of possible new massive X-ray binary systems found with the ROSAT X-ray satellite by astronomers from France, Germany, and Brazil. X-ray binary systems consist of a star that is losing mass to its partner, which is the remnant of a supernova, either a neutron star or a black hole.

A subsequent study by Spanish and German radio astronomers showed that LS 5039 was shooting out jets of gas at speeds comparable to the speed of light. Such high-speed jets had been found before in only a few massive X-ray binary systems.

McSwain's observations confirm that LS 5039 is a close X-ray binary system: one extremely bright, massive star and a companion that is nearly invisible to human eyes, which marks the remains of the stellar explosion.

She found that the two objects orbit each other once every 4.1 days in a surprisingly eccentric orbit, the most highly distorted orbit of any massive X-ray binary with such a short period.

"Their orbital period must have been close to two days prior to the explosion, so that when the supernova occurred, the stars were so close that they were almost touching," said McSwain. "The supernova event would have been an awesome sight for anyone unlucky enough to have witnessed the event so nearby."

Astronomers expect that the orbit of a binary system becomes elongated when a supernova occurs within it. The amount of stretching is related directly to the amount of mass shed by the exploding star.

The extremely distorted orbit found by McSwain indicates that the dead star in LS 5039 must have lost more than 15 times the mass of the Sun in the cataclysm that wracked the pair. The precise implications for the history of LS 5039 are stunning, according to astronomer Douglas Gies, McSwain's supervisor at GSU.

"This binary survived a supernova explosion, but just by the barest of margins," said Gies. "If the mass loss in the supernova had been slightly greater, the pair probably would have broken apart."

Most massive stars form in pairs, and astronomers have long puzzled what happens when a supernova occurs in a close binary system.

Astronomers agree generally that the non-exploding component will survive the blast, but the fate of the exploding star is uncertain. Most of the star's mass is flung into space, but a small portion of its core is forced into a tiny ball of nuclear matter called a neutron star. In some cases, the core shrinks even further, and its huge gravitational pull hinders even light from escaping. Such an object disappears from view and forms a black hole.

If it loses too much mass, the remnant of the explosion probably escapes from the binary altogether, and the two stars part ways. If the system remains bound, then the neutron star or black hole companion may eventually begin to pull gas away from the survivor star, as the latter grows in size with age.

The stolen gases heat up to millions of degrees as they swirl around the neutron star or black hole, and form a bright source of X-rays in the sky. Astronomers have found more than 60 so-called massive X-ray binary systems in the Milky Way so far.

"Our measurements suggest that the LS 5039 companion is a neutron star, but a black hole remains a possibility," Gies added.

McSwain's observations with the Coude Feed spectrograph recorded the telltale shifts in color that accompany the motion of the visible star in LS 5039 as it swings towards Earth and then away during its orbit.

The Coude Feed enables spectroscopic work to be performed independently of the main 2.1-meter telescope on Kitt Peak, effectively making it a dual observing facility. A separate system feeds light to the Coude Spectrograph, a large instrument built into a building underneath the main mirror shed.

LS 5039 appears in the constellation Scutum as seen from Earth. A detailed article on this research will be published in the September 1 issue of Astrophysical Journal Letters.

Kitt Peak National Observatory is a division of the National Optical Astronomy Observatory (NOAO), Tucson, AZ. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation.