Observatories may identify the sources of our new discoveries.
Over the last few years, we've witnessed the start of two radically new ways of doing astronomy. For all of human history, everything we've learned about the cosmos has come from observing photons, from high-energy gamma rays down to the cosmic microwave background. But since the opening of the IceCube observatory at the South Pole, we've been able to track ultra-high-energy cosmic neutrinos. And earlier this year, the updated LIGO detector spotted gravitational waves, ushering in the ability to observe ripples in space itself.
While neutrinos and gravitational waves are a very different means of looking at the cosmos, the data that they generate has to be integrated with everything we've already learned about the Universe. In other words, when we spot the neutrinos or gravitational waves, it would be helpful to observe photons associated with whatever event is producing them. That will help us integrate the new information with things we already know about and come to grips with anything we don't know about.
This week, possible successes were announced, as people identified a likely source of cosmic neutrinos, as well as a possible detection of a gravitational-wave-generating event using more traditional astronomy.
Do the wave
The merger of black holes, in and of itself, shouldn't be expected to produce any photons—they're black, after all. But black holes are often surrounded by material that their gravity has drawn in. That material could react to the merger in a way that generates photons. Although we don't have a detailed understanding of how this would work with a pair of stellar-mass black holes, the possibility that it would occur was taken seriously. As the Advanced LIGO gravitational wave detector was being tested, the team behind the Fermi Gamma Ray Telescope added software triggers to the data analysis pipeline that could identify photons produced by a gravitational-wave-producing event.
LIGO spotted the gravitational waves. So what happened with Fermi? A draft of a paper that is currently under review tells that story.
For starters, none of the software triggers fired off when LIGO detected the gravitational waves. That could be because the event that produced them was in the wrong location. The authors of the manuscript calculate that 25 percent of the area the LIGO team has identified as containing the source was blocked by the Earth from Fermi's perspective.
Nevertheless, they scanned 30 seconds' worth of Fermi data at the time of the LIGO event, looking for anything unusual. Two weak events occurred during that time. One was 11 seconds after the LIGO detection and in the wrong location. The second, however, was only 0.45 seconds after and in roughly the right spot. The authors calculate the false detection rate at 0.0022, which indicates they're pretty confident it's real.
The authors describe it as "a weak but significant hard X-ray source," and say the energy of the photons reached Mega-electronVolt energies. The energies involved (it was producing about 1026 Megatons a second at these wavelengths) mean it's too energetic to be one of the transient sources normally detected by Fermi, such as neutron stars that are drawing in matter.
Although the location of the source from Fermi's perspective makes it difficult to determine precisely where it is, it's possible to combine that imprecise positioning data with the imprecise data from LIGO. The result is that the 90 percent confidence area for the source shrinks to one-third its original size. If the two events are truly one and the same, this will help astronomers searching for the aftermath in other wavelengths.
BigBird a blazar?
The IceCube facility relies on an array of photodetectors suspended in the ice below the South Pole. The hardware is able to track incoming particles as they bump into the atoms of the ice, producing light in the process. Among these particles are neutrinos from a variety of sources, but of particular interest are the extremely high-energy ones.
What's extreme in this context? The Large Hadron Collider, our highest energy hardware, can accelerate protons to energies of a handful of Tera-electronVolts. Some of the neutrinos IceCube spotted were in the neighborhood of two Peta-electronVolts, three orders of magnitude higher. It's fair to wonder what could possibly give these lightweight, uncharged particles those sorts of energies.
One model for their production involves the jets produced by active supermassive black holes. Within these jets, protons and other charged particles get accelerated to high energies and are able to interact with high-energy photons in the environment. These interactions can produce pions, an unstable particle that decays in a process that produces a neutrino. The neutrino then inherits some of the energy of its parent pion and continues traveling in the same direction.
For the neutrino to reach Earth, the jets from the black hole must be pointing at us. We've already identified galaxies where this is the case; they're called blazars, due to the incredible amount of energies the jets send in our direction. There's a large catalog of blazars that we've identified through various astronomical surveys.
Naturally, when IceCube identified its first Peta-electronVolt neutrinos (nicknamed Ernie, Bert, and BigBird), researchers started searching the blazar catalog for potential sources in the direction that the neutrino came from. Unfortunately, these searches came up blank.
But now, a large international team has found a possible candidate as the source of BigBird, a blazar known as PKS B1424–418. This blazar was included in the original analysis of PeV neutrinos but wasn't considered a good candidate, as it was relatively quiet at the time. But black hole jets are one of the rare astronomical phenomena that can change suddenly, within the span of a few years. And the team found that PKS B1424–418 had ramped up its activity at about the time BigBird was detected—an event they call a "blazar outburst."
The identity of the sources for Ernie and Bert are a mystery. But, as IceCube continues to gather data, the chances of identifying the sources should go up. And the same is true for the ongoing work with LIGO, where another four potential gravitational wave detections are still being analyzed.
The arXiv. Abstract number: 1602.03920 (About the arXiv). Under review at The Astrophysical Journal.
Nature Physics, 2015. DOI: 10.1038/NPHYS3715 (About DOIs).