Andrew Yee[_1_]
January 16th 08, 07:27 PM
ESA News
http://www.esa.int
9 January 2008
Integral discovers the galaxy's antimatter cloud is lopsided
The shape of the mysterious cloud of antimatter in the central regions of
the Milky Way has been revealed by ESA's orbiting gamma-ray observatory
Integral. The unexpectedly lopsided shape is a new clue to the origin of the
antimatter.
The observations have significantly decreased the chances that the
antimatter is coming from the annihilation or decay of astronomical dark
matter.
Georg Weidenspointner at the Max Planck Institute for Extraterrestrial
Physics and an international team of astronomers made the discovery using
four-years-worth of data from Integral. The cloud shows up because of the
gamma rays it emits when individual particles of antimatter, in this case
positrons, encounter electrons, their normal matter counterpart, and
annihilate one another.
One signature of positron-electron annihilation is gamma rays carrying 511
thousand electron-volts (keV) of energy. There has been a vigorous debate
about the origin of these positrons ever since the discovery of the 511 keV
emission from the centre of the galaxy by gamma-ray detectors flown on
balloons during the 1970s.
Some astronomers have suggested that exploding stars could produce the
positrons. This is because radioactive nuclear elements are formed in the
giant outbursts of energy, and some of these decay by releasing positrons.
However, it is unclear whether these positrons can escape from the stellar
debris in sufficient quantity to explain the size of the observed cloud.
Other astronomers wondered whether more exotic processes were at work. From
earlier results using much less data, the positron cloud seemed to be
spherical and centred on the centre of the galaxy. Such a shape and position
corresponds to the expected distribution of dark matter in the centre of our
galaxy, so it was suggested that dark matter was annihilating or decaying
into pairs of electrons and positrons, which then annihilated to produce the
gamma rays.
The trouble with this idea, however, was that the dark matter particles
needed to be much less massive than most theories were predicting.
The new results give astronomers a valuable new clue and point away from
dark matter as the origin of the antimatter. Beyond the galactic centre, the
cloud is not entirely spherical. Instead it is lopsided with twice as much
on one side of the galactic centre as the other. Such a distribution is
highly unusual because gas in the inner region of the galaxy is relatively
evenly distributed.
Equally importantly, Integral found evidence that a population of binary
stars is also significantly off-centre, corresponding in extent to the cloud
of antimatter. That powerfully suggests these objects, known as hard
(because they emit at high energies) low mass X-ray binaries, are
responsible for a major amount of antimatter.
A low mass X-ray binary (LMXB) is a celestial system in which a relatively
normal star is being eaten alive by a nearby stellar corpse, either a
neutron star or a black hole. The gravitational field of the stellar corpse
is so strong that it rips gas from the normal star. As this gas spirals down
towards that object, it is heated so much that positron-electron pairs can
be spontaneously generated in the intense radiation field, although the 511
keV emission is probably too weak to be detected from individual LMXBs by
Integral.
"Simple estimates suggest that about half and possibly all of the antimatter
is coming from the X-ray binaries," says Weidenspointner. The other half
could be coming from a similar process around the galaxy's central black
hole and the various exploding stars there. He points out that the lopsided
distribution of hard LMXBs is unexpected, as stars are distributed more or
less evenly around the galaxy. More investigations are needed to determine
whether the observed distribution is real.
Integral is currently the only mission that can see both the 511 keV
radiation and the hard LMXBs. Weidenspointner and colleagues will be
watching keenly to see whether it discovers more LMXBs and, if so, where
they are located.
"The link between LMXBs and the antimatter is not yet proven but it is a
consistent story," says Weidenspointner. It has real astrophysical
importance because it decreases the need for dark matter at the centre of
our galaxy.
Notes for editors:
'An asymmetric distribution of positrons in the galactic disk revealed by
gamma rays' by Georg Weidenspointner et al. is being published on 10
January, in the journal Nature.
For more information:
Georg Weidenspointner, Max Planck Institute for Extraterrestrial Physics
Email: Georg.Weidenspointner @ hll.mpg.de
Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int
Gerald K. Skinner, Astrophysics Science Division, NASA/GSFC
Email: skinner @ milkyway.gsfc.nasa.gov
[NOTE: Images supporting this release are available at
http://www.esa.int/esaSC/SEMKTX2MDAF_index_1.html ]
http://www.esa.int
9 January 2008
Integral discovers the galaxy's antimatter cloud is lopsided
The shape of the mysterious cloud of antimatter in the central regions of
the Milky Way has been revealed by ESA's orbiting gamma-ray observatory
Integral. The unexpectedly lopsided shape is a new clue to the origin of the
antimatter.
The observations have significantly decreased the chances that the
antimatter is coming from the annihilation or decay of astronomical dark
matter.
Georg Weidenspointner at the Max Planck Institute for Extraterrestrial
Physics and an international team of astronomers made the discovery using
four-years-worth of data from Integral. The cloud shows up because of the
gamma rays it emits when individual particles of antimatter, in this case
positrons, encounter electrons, their normal matter counterpart, and
annihilate one another.
One signature of positron-electron annihilation is gamma rays carrying 511
thousand electron-volts (keV) of energy. There has been a vigorous debate
about the origin of these positrons ever since the discovery of the 511 keV
emission from the centre of the galaxy by gamma-ray detectors flown on
balloons during the 1970s.
Some astronomers have suggested that exploding stars could produce the
positrons. This is because radioactive nuclear elements are formed in the
giant outbursts of energy, and some of these decay by releasing positrons.
However, it is unclear whether these positrons can escape from the stellar
debris in sufficient quantity to explain the size of the observed cloud.
Other astronomers wondered whether more exotic processes were at work. From
earlier results using much less data, the positron cloud seemed to be
spherical and centred on the centre of the galaxy. Such a shape and position
corresponds to the expected distribution of dark matter in the centre of our
galaxy, so it was suggested that dark matter was annihilating or decaying
into pairs of electrons and positrons, which then annihilated to produce the
gamma rays.
The trouble with this idea, however, was that the dark matter particles
needed to be much less massive than most theories were predicting.
The new results give astronomers a valuable new clue and point away from
dark matter as the origin of the antimatter. Beyond the galactic centre, the
cloud is not entirely spherical. Instead it is lopsided with twice as much
on one side of the galactic centre as the other. Such a distribution is
highly unusual because gas in the inner region of the galaxy is relatively
evenly distributed.
Equally importantly, Integral found evidence that a population of binary
stars is also significantly off-centre, corresponding in extent to the cloud
of antimatter. That powerfully suggests these objects, known as hard
(because they emit at high energies) low mass X-ray binaries, are
responsible for a major amount of antimatter.
A low mass X-ray binary (LMXB) is a celestial system in which a relatively
normal star is being eaten alive by a nearby stellar corpse, either a
neutron star or a black hole. The gravitational field of the stellar corpse
is so strong that it rips gas from the normal star. As this gas spirals down
towards that object, it is heated so much that positron-electron pairs can
be spontaneously generated in the intense radiation field, although the 511
keV emission is probably too weak to be detected from individual LMXBs by
Integral.
"Simple estimates suggest that about half and possibly all of the antimatter
is coming from the X-ray binaries," says Weidenspointner. The other half
could be coming from a similar process around the galaxy's central black
hole and the various exploding stars there. He points out that the lopsided
distribution of hard LMXBs is unexpected, as stars are distributed more or
less evenly around the galaxy. More investigations are needed to determine
whether the observed distribution is real.
Integral is currently the only mission that can see both the 511 keV
radiation and the hard LMXBs. Weidenspointner and colleagues will be
watching keenly to see whether it discovers more LMXBs and, if so, where
they are located.
"The link between LMXBs and the antimatter is not yet proven but it is a
consistent story," says Weidenspointner. It has real astrophysical
importance because it decreases the need for dark matter at the centre of
our galaxy.
Notes for editors:
'An asymmetric distribution of positrons in the galactic disk revealed by
gamma rays' by Georg Weidenspointner et al. is being published on 10
January, in the journal Nature.
For more information:
Georg Weidenspointner, Max Planck Institute for Extraterrestrial Physics
Email: Georg.Weidenspointner @ hll.mpg.de
Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int
Gerald K. Skinner, Astrophysics Science Division, NASA/GSFC
Email: skinner @ milkyway.gsfc.nasa.gov
[NOTE: Images supporting this release are available at
http://www.esa.int/esaSC/SEMKTX2MDAF_index_1.html ]