NASA’s premier gamma-ray space telescope may be changing tack in the coming months from an equal-opportunity scan of the whole sky to a pattern that prioritizes the center of our Milky Way galaxy. The new strategy could help the Fermi telescope find more spinning stars called pulsars, observe a cloud on a collision course with the galaxy’s supermassive black hole and, just maybe, find evidence of dark matter.
In March 2013, after five years of business as usual, the Fermi team put out a call for alternative observing strategies. Five proposals came in, and in August a review panel convened to discuss the ideas. Ultimately, the panel recommended a new strategy based on a proposal to prioritize observing the galactic center.
Until now, Fermi has covered the full sky every three hours. Under the new plan, the telescope would still scan every spot on the sky at least once a day, but would favor the center of the galaxy, building up better statistics on its measurements there. The strategy is a compromise between a pure focus on the galactic center and the many projects that rely on observations elsewhere in the sky. “When people understood that this mode was possible and we looked at it more quantitatively, the concerns about how you’re throwing away some science to favor other science really became less sharp,” says Fermi team member Seth Digel at the SLAC National Accelerator Laboratory in Menlo Park, Calif., who served on the review panel. A final decision on the new strategy will come after a public commenting period ends and the team ensures the change won’t harm the spacecraft or put undue stress on its reaction wheels or other components.
One of the most tantalizing, but perhaps long-shot, goals of observing the galactic center is to clarify whether a possible signal of gamma-ray light in the energy range of 130 giga-electron-volts (GeV) truly exists. First called out in April 2012 by Christoph Weniger, then at the Max Planck Institute for Physics in Germany and now of the University of Amsterdam in the Netherlands, the signal has not reached the level of statistical significance necessary to claim a discovery. Researchers have multiple reasons to doubt whether this 130-GeV line really exists—but if it does, it’s a “smoking-gun signal of dark matter,” says Meng Su of the Massachusetts Institute of Technology.
No particles currently in the stable of the Standard Model could explain this line, Su says. If it exists and is not the result of an instrument error, it is likely caused by the annihilation of dark matter particles packed in tight in the dense center of the Milky Way. One of the leading candidates for dark matter is a class of particles called WIMPs (weakly interacting massive particles) that would only very rarely interact with regular matter, but would exert a gravitational pull. WIMPs are theorized to be their own antimatter partners, meaning if two WIMPs got close enough, they would destroy each other, as matter and antimatter do on contact. The explosions could create photons of light at 130 GeV. (Although 130 GeV sounds conspicuously close to the 125-GeV mass of the recently discovered Higgs boson particle, the similarity is probably a coincidence, researchers say.) “If the alternative observing strategy is finally adopted, it may turn out to be a necessary step in our quest to understand dark matter,” says Nestor Mirabal of the Complutense University of Madrid in Spain, who co-authored the white paper proposing the galactic center observing strategy with Weniger, Su and others.
Many scientists have doubts that the 130-GeV line is dark matter. Since it was first noticed, the signal strength has not been improved by the addition of more data over the past year. Furthermore, the feature falls in an even narrower energy range than the telescope’s energy resolution (the smallest energy spread that can be distinguished by the instrument), raising doubts that a real signal would be that sharp. And there may also be a 130-GeV signal in Fermi observations of Earth’s limb, which is where cosmic rays hitting the planet’s atmosphere create a gamma-ray glow. This feature would not be expected if the 130-GeV photons were coming from dark matter. Either way, however, the signal is mysterious, and getting more data should help sort out its origin. “We have a feeling we can smell whether the signal is real or not after one year” of the new observing strategy, Su says.
But dark matter isn’t the only motivation for pointing Fermi at the galactic center. “By itself, it’s probably not a compelling enough reason to look,” says Eric Charles, also at SLAC, who also served on the panel that reviewed alternative observation strategies. “There is not one single reason we necessarily want to do this—there’s a lot of reasons.”
Spending more time looking toward the Milky Way’s heart should also allow Fermi to discover more pulsars, which are thought to be common in the inner galaxy. These dense stars are made of matter packed so tightly that their atoms have condensed into neutrons and, as they spin rapidly, they emit a beam of light that sweeps in a circle like a lighthouse signal. Probing the workings of these exotic objects is a prime goal of Fermi’s.
Another motivation is a rare event expected to take place soon at the galaxy’s center. A cloud of gas called G2, discovered in 2011, is due to be devoured by the giant black hole there, and may release gamma-rays in the process. “It’s a once-in-a-lifetime event to watch material being accreted by a supermassive black hole,” Su says. The Fermi team determined that the new strategy should begin no later than December, if it is adopted, in part to take advantage of this opportunity.