High-powered jets beaming at us from distant galaxies may hint at the existence of an exotic particle, an Italian team suggested recently on arXiv.org. The study is highly speculative — a fact the team is quick to point out — but the proposed particle’s characteristics match those put forward by other researchers to explain another astrophysical problem.
So why is it so speculative? Because the particle requires an extension to standard physics. But before you shake your head and click away from this page, consider that pushing the boundaries of run-of-the-mill physics is the bread and butter of scientific advancement. And in this case, there may soon be a straightforward way to test whether the exotic particle actually exists.
Jets of superhot, ionized gas shoot out from some galaxies’ cores, funneled out along magnetic field lines by the galaxies’ central supermassive black holes. These active galaxies can have a variety of orientations from Earth’s point of view, and when the jet points straight at us the object is called a blazar. Blazars are incredibly luminous, especially in gamma rays, the most energetic form of photons. But because these galaxies are so far away many of the higher-energy gamma rays won’t survive the journey to us: en route they’ll collide with lower-energy photons between us and them, thereby making the blazars look fainter and less energetic than they really are.
What astronomers have found is that, even at the minimal level of intergalactic interference, we’re still seeing blazars’ most energetic gamma rays too well. This problem led a trio of Italian physicists to suggest in 2007 that photons from a blazar’s jet are fickle: they oscillate between being photons and an as-yet-undetected particle, called an axion-like particle.
Axions (and axion-like particles, or ALPs) are hypothetical, low-mass particles that interact rarely with matter. Part of their definition is that they can turn into photons (and vice versa) when in a strong magnetic field. ALPs are attractive because they may help explain a perplexing discrepancy between our understandings of two fundamental physical forces, the strong and the weak.
ALPs are also attractive because, unlike gamma rays, they can pass pretty much unmolested through a sea of lower-energy photons. If photons could oscillate between being photons and ALPs as they encounter magnetic fields along their journey between blazar and Earth, the observed blazar power could increase by a factor of 10.
So far, there’s no experimental evidence for ALPs. Blazars could provide that long-sought evidence. Unlike the 2007 scenario (elaborated last year in Physical Review D), the new study looks at what happens to photons behind the curtain of the blazar’s innermost sanctum. The jet’s base should lie inside the so-called broad-line region (BLR), an area of hot, rapidly moving clouds emitting lots of optical and ultraviolet radiation. These photons suffuse the region inside the BLR just as a room with lamps along the walls is filled with light, explains study coauthor Fabrizio Tavecchio (Astronomical Observatory of Brera, Italy). Impeded by these photons, gamma rays have trouble escaping. But there are plenty of high-energy gamma rays seen, more than expected if this interference is such a problem.
To add insult to injury, observations of the blazar PKS 1222+216 with the Cherenkov MAGIC telescopes in the Canary Islands caught the blazar’s gamma-ray brightness doubling in a mere 10 minutes. To be so quickly variable, the emitting region must be compact. Tavecchio and his collaborators suggested in 2011 that the gamma rays might actually originate outside the BLR, avoiding the whole interference conundrum, but because the source is small that would put it a couple of light-years from the black hole, and there’s no evidence to support that.
On the other hand, if the gamma rays could oscillate between being photons and ALPs, it could explain everything. Interacting with the magnetic field inside the blazar could switch the photon to an ALP, allowing it to traverse the BLR unimpeded; when the particle hit another magnetic field later on — perhaps that of the Milky Way — it could flip back, allowing us to observe it.
I admit, it sounds farfetched. Yet ALPs in various forms are actually a common prediction of expansions to the standard model of particle physics. What’s more, the particular particle characteristics supported by Tavecchio and his colleagues to explain how gamma rays make it from one side of the BLR to the other are similar to those put forth to explain the larger blazar-to-Earth problem.
Naturally, researchers are responding with caution. “It’s possible that these observations point to the existence of an axion-like particle,” says Clare Burrage (University of Nottingham, England). “But they may also just be telling us that blazar physics is more complicated than we thought.”
Still, says Alessandro de Angelis (University of Udine and the National Institute of Nuclear Physics, Italy), who coauthored the 2007 paper and its successor, the fact that the particle’s properties match those suggested by his team makes the new study “fascinating.” Various non-exotic explanations exist for the extra high-energy gamma rays, but they’re all rather unsatisfactory, he explains. And fickle photons aren’t a new idea: axion-like particles are just the current flavor in favor. These things could be out there, he says.
The neat part of the ALP suggestion is that researchers may be able to verify the particle’s existence in a few years, perhaps with the hoped-for Cherenkov Telescope Array or planned upgrades to the Deutsches Elektronen-Synchrotron ALPS experiment in Germany. This experiment shoots laser photons through a strong magnetic field at a wall to see if some photons pop up on the other side — where they have no right to be, according to standard physics.
“I always like it when people present models that are straightforwardly testable, as this one is,” says Katherine Mack (University of Cambridge, England). “If you're really looking for evidence of new physics these days, astronomical observations are where you’re most likely to find it, unless you have your own personal LHC.”
It’s worth noting that this particular brand of ALP could not explain dark matter, Mack says. It just wouldn’t contribute enough to the universe’s mass to be a large fraction of the missing matter.