First X-ray detection of a colliding-wind binary beyond the Milky Way (Forwarded)

ESA News
http://www.esa.int

16 February 2007

First X-ray detection of a colliding-wind binary beyond the Milky Way

Imagine two stars with winds so powerful that they eject an Earth's worth of
material roughly once every month. Next, imagine those two winds colliding
head-on. Such titanic collisions produce multimillion-degree gas, which
radiates brilliantly in X-rays. Astronomers have conclusively identified the
X-rays from about two-dozen of these systems in our Milky Way. But they have
never seen one outside our galaxy -- until now.

Thanks to the European Space Agency's XMM-Newton X-ray observatory, with
help from NASA's Chandra X-ray Observatory, an international team led by Dr
Ya Nazof the Universitde Lie in Belgium has found such a system in a nearby
galaxy. This galaxy, the Small Magellanic Cloud, orbits the Milky Way and is
located about 170 000 light-years from Earth.

The binary system, known as HD 5980, contains two extremely massive stars,
'weighing' about 50 and 30 times the mass of the Sun. Each star radiates
more than a million times as much light as the Sun, meaning they put out
more light in one minute than our host star generates in an entire year.

The sheer photon pressure of this incredible outpouring of light blows off
gas from each star in a supersonic 'wind'. These winds are so powerful that
they carry away roughly an Earth mass each month, a rate 10 thousand million
times greater than the solar wind, and at a speed 5 times faster than the
solar wind itself.

HD 5980's two stars are separated by only about 90 million kilometres,
roughly half Earth's average distance from the Sun. "These stars are so
close to each other that if they were in our solar system they could fit
inside the orbit of Venus," says Naz As a result, the winds smash into each
other with tremendous force, heating the gas and generating enormous numbers
of X-rays.

"The system emits about 10 times more energy in X-rays alone than the Sun
radiates over the entire spectrum," says team member Dr Michael F. Corcoran,
a scientist with the Universities Space Research Association at NASA's
Goddard Space Flight Center in Greenbelt, Maryland.

Using data from Chandra, the same team first reported HD 5980's highly
energetic X-ray emission in 2002. But its origin was uncertain. Data taken
from 2000 to 2005 with XMM-Newton shows that it is indeed produced by a wind
collision.

The stars orbit each other every 20 days in a plane that is edge-on to
Earth's line of sight, so the stars periodically eclipse each other. The
wind collision is thus seen from different angles and through different
amounts of material. XMM-Newton saw the X-ray emission rise and fall in a
repeatable, predictable pattern.

"Similar X-ray variability from massive binaries inside the Milky Way have
been detected, but this is the first indisputable evidence for the
phenomenon outside our galaxy," says Naz "This discovery highlights the
great capabilities of modern X-ray observatories."

XMM-Newton has the largest mirrors of any X-ray observatory ever flown, and
the sheer size of these mirrors allowed astronomers to monitor this distant
system. HD 5980 itself is surrounded by hot interstellar material that
creates a diffuse X-ray glow that makes the object difficult to study. "The
Chandra data allowed us to pinpoint HD 5980 and resolve the system from the
diffuse emission," says Corcoran.

HD 5980 is one of the Small Magellanic Cloud's brightest stars. Situated on
the periphery of the star cluster NGC 346, the two stars are nearing the end
of their lives and will eventually explode as supernovae. The more massive
star, HD 5980A, is passing through a Luminous Blue Variable (LBV) phase -- a
short-lived, erratic stage that only the most massive stars go through. The
most well-known LBV in our galaxy, Eta Carinae, produced a giant outburst
that was recorded by astronomers in the 1840s. HD 5980A experienced a
smaller-scale outburst that was seen in 1993-94. Its companion, HD 5980B, is
an evolved Wolf-Rayet star that has already ejected much of its original
envelope.

"It's interesting to be able to study an extragalactic colliding-wind binary
like HD 5980 as if it were in our own galaxy", says Corcoran. "Colliding
winds provide an important handle on how massive stars shed material. Being
able to study them in external galaxies means we can study the effects of
different compositions and environments on how these massive stars evolve.
From the XMM-Newton data, we can study the delicate balance between the two
winds, and determine the changing strength of the winds."

Note for editors

The results will appear on-line today in The Astrophysical Journal Letters,
in the article titled: "First detection of phase-dependent colliding wind
X-ray emission outside the Milky Way"
(http://arxiv.org/abs/astro-ph/0702403).

Coauthors also include Dr Gloria Koenigsberger of the Universidad Nacional
Autonoma de Mico and Dr Anthony J. Moffat of the Universitde Montrl in
Canada.

For more information

Ya Naz
niversitde Lie, Belgium
Tel: +32 4 366 9720
Email: naze @ astro.ulg.ac.be

Michael F. Corcoran
Universities Space Research Association at NASA/GSFC, Greenbelt, Maryland,
USA
Email: corcoran @ milkyway.gsfc.nasa.gov

Gloria Koenigsberger
Universidad Nacional Autonoma de Mico
Email: gloria @ astroscu.unam.mx

Anthony J. Moffat
Universitde Montrl, Canada
Email: moffat @ astro.umontreal.ca

Norbert Schartel
ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int

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