First X-ray detection of a colliding-wind binary beyond the Milky Way (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 16 Feb 2007 19:07:59 GMT
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 Yaël Nazé of the Université de Liège 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 Nazé. 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 Nazé. "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 México and Dr Anthony J. Moffat of the Université de Montréal
in Canada.
For more information
Yaël Nazé
Université de Liège, 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 México
Email: gloria @ astroscu.unam.mx
Anthony J. Moffat
Université de Montréal, Canada
Email: moffat @ astro.umontreal.ca
Norbert Schartel
ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMPYIO2UXE_index_1.html ]
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