The Big One That (Almost) Got Away

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 CoudÈ Feed Telescope, a special auxiliary
system on Kitt Peak National Observatory’s 2.1-meter telescope.

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 Coudè 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 CoudÈ 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 CoudÈ 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.

A colorful artist’s concept of LS 5039 is available on the Internet at:
http://www.noao.edu/outreach/press/pr01/0112images.shtml

An image of the optical tower that directs light into the Coudè Feed is
available at:
http://www.noao.edu/image_gallery/html/im0431.html

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.

NOTE TO REPORTERS: Virginia McSwain can be contacted at 404/651-1369,
mcswain@chara.gsu.edu . Douglas Gies can be contacted at 404/651-1366,
gies@chara.gsu.edu .

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IMAGE CAPTION:

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

For more information:

Douglas Isbell

NOAO Public Information Officer

(520) 318-8214

disbell@noao.edu

Stacie Sutton

Georgia State University

Phone: (404) 651-3576

stacie@gsu.edu