Although I had planned to push straight on to look at instrumentation for a true interstellar mission (using Mike Gruntman’s landmark paper on the topic), I want to revise that schedule because of the recently announced antimatter news. We’ll return to the instrumentation issue on Monday, including the tricky question of how a probe designed to reach 400 AU can make effective measurements given its speed (75 km/s in the best case scenario Gruntman looks at). Because that question just gets trickier as speeds ramp up, it’s a major one for planning.
But on to antimatter, a cloud of which has been known to exist around the galactic center since the 1970s, when balloon-based gamma-ray detectors first located it. Gamma rays are significant in terms of antimatter because electrons encountering positrons (their antimatter equivalent) annihilate each other, with their mass converted into high energy gamma rays. So the cloud’s presence is well established. The question since its detection is what could have caused it.
Now a new paper in Nature may offer an answer, noting the asymmetric distribution of the antimatter cloud, which extends further on one side of galactic center than on the other. We’re talking about a cloud some 10,000 light years across, generating the energy of 10,000 Suns. The research team used data from the European Space Agency’s Integral satellite (INTErnational Gamma-Ray Astrophysics Laboratory) to detect the asymmetry. Their paper notes that it matches the distribution of a certain type of binary star systems, the latter thought to contain neutron stars and black holes.
Image: Integral mapped the glow of 511 keV gamma rays from electron-positron annihilation. The map shows the whole sky, with the galactic center in the middle. The emission extends to the right. Credit: ESA/Integral/MPE/G. Weidenspointner.
Are these binary stars the cause of the antimatter cloud? They’re what’s known as ‘hard’ low-mass X-ray binaries. The mechanism at play is that gas from a low-mass star spirals into a black hole or neutron star nearby, with high-energy (hard) X-rays resulting. That and the relative similarity between the distributions of cloud and stars makes the case that the binaries are producing these interesting positrons. In fact, says lead author Georg Weidenspointer (Max Planck Institute for Extraterrestrial Physics), “Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries.”
Of course, what comes immediately to mind at this end is James Bickford’s interesting work on antimatter collection here in the Solar System. As we saw in several earlier posts, Bickford has been advocating collection strategies that would mine the antimatter being formed naturally not only near the Earth but also in abundance further out in the Solar System, especially around Saturn. So the idea of antimatter farming again comes to the front with this renewed reminder that the exotic stuff occurs as a result of astrophysical processes and not just in particle accelerators.
Not that we’re able to tap a cloud like this one, so vast and so much further from Earth. But on a theoretical level, it’s useful to learn more about antimatter production even while we’re discovering the limitations in our existing theories. For the questions the antimatter cloud poses are themselves vast. The low-mass binaries seem associated with the antimatter cloud but we lack knowledge of how they could produce enough positrons to account for it. That probably targets particle jets as the necessary area for investigation, something NASA’s GLAST (Gamma-ray Large Area Space Telescope) may be able to shed further light on. And GLAST is helpfully ready for a 2008 launch.
The paper is Weidenspointner et al., “An asymmetric distribution of positrons in the Galactic disk revealed by big gamma-rays,” Nature 451 (10 January 2008), pp. 159-162 (abstract).