Guest Blog: Blazars - A multi-wavelength look at the objects behind the first neutrino traced back to a deep-space source

Posted by Alison Klesman
on Tuesday, July 24, 2018

By Angela Osterman Meyer

Photo Credit: NASA/JPL-Caltech/UCLA

Perhaps the most amazing aspect of studying astronomy is that in solving one mystery of the cosmos you find far more questions than answers; the adventure never ends. The distant and ultra-luminous objects known as blazars perfectly embody this. At right is a blazar (looking bright white) surrounded by nearby stars in an image from NASA’s Wide-field Infrared Survey Explorer. 

Which brings us to the first mystery of blazars: What made astronomers think they weren’t just stars? How are blazars different from stars? The answers come from looking at the same objects in many different kinds of light, a method known as multi-wavelength astronomy.

In radio observations, blazars exhibit brightness that is uncharacteristic of stars. Some blazars even get their names from radio astronomy surveys of the sky, for example PKS 2155-304 in the CSIRO Parkes Radio Telescope’s survey. This unexpected radio brightness also leads to the name “quasar,” short for “quasi-stellar (meaning star-like) radio sources.” The term quasar also describes objects such as Hercules A (below), with radio observations (in pink) revealing jets extending outward from an elliptical host galaxy.

Photo Credit: NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)

From radio and visible light observations of blazars, astronomers observe variability on timescales from hours to years that doesn’t follow a discernible pattern, and distances that place blazars far outside of our own galaxy. The prototypical blazar known as BL Lac gets its name from being designated as a variable star in the constellation Lacerta in 1929 before spectroscopic analysis by Oke and Gunn (1974) revealed a distance of 900 million light-years. That something so distant can be observed fairly easily with a small telescope indicates astounding amounts of energy, far more than a star is capable of emitting.

Photo Credit: NASA/CXC/SAO/H. Marshall et al.

In X-ray and gamma-ray observations, blazars shine bright, and again variably so, in the highest energy kinds of light that scientists are able to observe, indicating that very high-energy processes must be occurring. The X-ray observations at right by the Chandra telescope of the blazar 3C 273 also show a small jet of material emerging, though not the extensive jet seen in Hercules A.

Such observational clues led astronomers to a model explaining blazars: At the very heart, a massive but very compact nucleus containing a super-massive black hole anywhere from a million to a billion times the mass of our Sun. An active accretion disk brings whole stars worth of material to the central black hole. Highly energetic particles make up a luminous and variable jet directed right at us. The direction of the jet relative to us gives us the name “blazar”, or BL Lac-like quasar, as if we’re looking almost right into one of the jets observed in Hercules A instead of off to one side.

But how is there a jet? Where does its incredible energy come from? In gravitational interactions, the strength of the interaction comes from mass. The strong gravitational field of a blazar’s central super-massive black hole pulls nearby matter towards it to form the accretion disk. However, if gravity were the only interaction present, there would be no matter moving away from the black hole, much less matter being organized and accelerated to form jets shooting away at speeds approaching the speed of light. In electromagnetic interactions, the charge of the jet’s particles govern how they interact with their surroundings. The details of the physics governing the jets and how they accelerate particles is an ongoing mystery of blazar astrophysics, but many models include a strong magnetic field that interacts strongly with the charge of the jet’s particles.

Photo Credit: Sophia Dagnello/NRAO/AUI/NSF

If the jet is in fact a charged particle accelerator, how can we hope to observe such charged particles when the only particle that is capable of traveling through deep space undisturbed is the charge-less, nearly massless, and notoriously hard-to-detect particle known as the neutrino?

Sometimes, the universe hands us a bit of a break. As positively charged protons in the blazar jet collide with each other or with photons in the jet, other particles are produced… including neutrinos. But again, more answers present new questions: Where are the neutrinos?

For all the time blazars have been studied, there was no observational answer to that question, until on September 22nd of 2017, the IceCube Neutrino Observatory detected a neutrino from a spot in the sky near Orion’s arm (blazar location labeled in teal crosshairs, below).

Photo Credit: IceCube Collaboration

A few days later, NASA’s Fermi Gamma-ray Space Telescope observed a flare in the same small area of the sky and coinciding with the location of blazar TXS 0506+056. Ground-based observatories MAGIC and VERITAS also detected increases in gamma-ray brightness and energy over the next couple of weeks. But could observations of light and of neutrinos be coming from the same object? If so, this would be the very first such observation other than our Sun and supernova 1987A located in the nearby Large Magellanic Cloud, and from a source billions of light-years away. Astrophysicists carefully examined the distance in the sky between the gamma-ray source and the neutrino source, and the characteristics of the gamma-ray observations before concluding that IceCube had in fact observed neutrinos from a blazar for the first time. Scientists refer to this as a multi-messenger observation because the “messengers” represented two different physical phenomena; light in the form of gamma rays, and neutrinos.

So… what’s next?

Blazar astronomers can use this new evidence to narrow down and refine the best models for the physics of blazar jets. What new predictions will come from these models?

Could any of the other almost 300 neutrinos detected by IceCube, or neutrinos detected from other observatories, be from blazars? Turns out that a dozen were also in fact observed from the location of TXS 0506+056 between September 2014 and March 2015 (IceCube Collaboration 2018). What else will be discovered from the archives?

If blazars are the best embodiment of each answer leading to new questions, then Orion may be the spokes-constellation for reminding us all to never stop looking for new cosmic adventures even in familiar places.

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