There’s a list out there somewhere of the most extreme things in the Universe. Blazars must certainly be on that list. Astronomers used to think that blazars were variable stars, but strangely, they didn’t change in brightness in any predictable way. But then in the 1970s astronomers realized that these objects were actually millions of light-years away. They were outside our galaxy, and yet they were so bright they outshone all the rest of the stars in their galaxy.
So what is a blazar? Simply put, it’s the core of an active galaxy, where the galaxy is oriented face on, so a relativistic jet blasting out of the galaxy is oriented directly towards the Earth.
All large galaxies seem to contain supermassive black holes. There are times when these black holes are actively feeding on infalling material. In fact, so much material tries to get into the black hole that it backs up into an accretion disk around the center of the galaxy. The gravitational pressure is so extreme that the material heats up to millions of degrees and becomes like a star, emitting a tremendous amount of radiation. The rapidly spinning black hole generates a powerful magnetic field that whips up the material into jets that blast above and below the black hole. Material caught in these jets is accelerated nearly to the speed of light and fired out for hundreds of thousands of light-years.
When we see a blazar, we’re looking at an actively feeding galaxy face on. Furthermore, one of the relativistic jets is oriented so that it’s pointed directly towards us, and we can see the radiation emitted by both the black hole and the jet.
Even though these blazars can be as far as 9 billion light-years away, they’re still detectable by Earth-based instruments. Now that’s bright.
We have written many articles about galaxies for Universe Today. Here’s about a blazar observation.
ASTRONOMERS OBSERVE BIZARRE BLAZAR WITH BATTERY OF TELESCOPES
Quasars visible at gamma-ray energies are called “blazars.” Blazars are among the most energetic objects in the universe and are fueled by supermassive black holes at the core of certain giant elliptical galaxies. An international team of astrophysicists using a combination of ground- and space-based telescopes have uncovered surprising changes in radiation emitted by an active galaxy. The picture that emerges from these first-ever simultaneous observations with optical, X-ray and new-generation gamma-ray telescopes is much more complex than scientists expected and challenges current theories of how blazars generate the radiation they emit.
The galaxy, called PKS 2155-304, emits oppositely directed jets of particles traveling near the speed of light as matter falls into a central supermassive black hole; this process is not well understood. In the case of blazars, the galaxy is oriented such that we’re looking right down the jet.
PKS 2155-304 is located 1.5 billion light-years away in the southern constellation of Piscis Austrinus and is usually a detectable but faint gamma-ray source. But when its jet undergoes a major outburst, as it did in 2006, the galaxy can become the brightest source in the sky at the highest gamma-ray energies scientists can detect — up to 50 trillion times the energy of visible light. Even from strong sources, only about one gamma ray this energetic strikes a square yard at the top of Earth’s atmosphere each month
Atmospheric absorption of one of these gamma rays creates a short-lived shower of subatomic particles. As these fast-moving particles rush through the atmosphere, they produce a faint flash of blue light. The High Energy Stereoscopic System (H.E.S.S), an array of telescopes located in Namibia, captured these flashes from PKS 2155-304.
One of the H.E.S.S. telescopes in Namabia. Credit: H.E.S.S.
Gamma rays at lower energies were detected directly by the Large Area Telescope (LAT) aboard NASA’s orbiting Fermi Gamma-ray Space Telescope. “The launch of Fermi gives us the opportunity to measure this powerful galaxy across as many wavelengths as possible for the first time,” says Werner Hofmann, spokesperson for the H.E.S.S. team at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany.
With the gamma-ray regime fully covered, the team turned to NASA’s Swift and Rossi X-ray Timing Explorer (RXTE) satellites to provide data on the galaxy’s X-ray emissions. Rounding out the wavelength coverage was the H.E.S.S. Automatic Telescope for Optical Monitoring, which recorded the galaxy’s activity in visible light.
Between August 25 and September 6, 2008, the telescopes monitored PKS 2155-304 in its quiet, non-flaring state. The results of the 12-day campaign are surprising. During flaring episodes of this and other blazars, the X- and gamma-ray emission rise and fall together. But it doesn’t happen this way when PKS 2155-304 is in its quiet state — and no one knows why.
What’s even stranger is that the galaxy’s visible light rises and falls with its gamma-ray emission. “It’s like watching a blowtorch where the highest temperatures and the lowest temperatures change in step, but the middle temperatures do not,” says Berrie Giebels, an astrophysicist at France’s École Polytechnique who works with both the H.E.S.S. and Fermi LAT teams.
“Astronomers are learning that the various constituents of the jets in blazars interact in fairly complicated ways to produce the radiation that we observe,” says Fermi team member Jim Chiang at Stanford University, Calif. “These observations may contain the first clues to help us untangle what’s really going on deep in the heart of a blazar.”
Ghostly particle caught in polar ice ushers in new way to look at the universe
If astronomers are right, a ghostly particle that lit up an instrumented swath of ice beneath the South Pole on 22 September last year was a messenger from a distant galaxy. The particle was a neutrino, electrically neutral and almost massless, which means its path could be traced back to the extragalactic event that created it. Cued by IceCube, the Antarctic detector, the orbiting Fermi Gammaray Space Telescope found that the neutrino likely came from a far off blazar, a hugely bright source of radiation powered by a supermassive black hole.
Astronomers have long been tantalized by the prospect of using neutrinos, which move at almost the speed of light and rarely interact with other matter, to learn about violent cosmic events. The new finding, reported today in Science, could mark the founding event of neutrino astronomy. The detection also triggered a powerful example of another new trend, multimessenger astronomy, in which telescopes and other instruments studied the flaring blazar in all parts of the electromagnetic spectrum, from gamma rays to radio waves.
A neutrino-producing blazar could also help solve a decades-old mystery in astronomy: Where do the extremely high energy protons and other nuclei that occasionally bombard Earth come from? Known as ultrahigh-energy cosmic rays, these particles have a million times more energy than has ever been produced in an earthbound particle accelerator, but what boosts them to such colossal energies is unknown. Suspects have included neutron stars, gamma ray bursts, hypernovae, and the radiation-spewing black holes at the center of some galaxies, but whatever the source, high energy neutrinos are a likely byproduct. If the IceCube team is right, blazars could be the first confirmed source of these cosmic rays.
A year and half ago, physicists working with the massive IceCube particle detector—a 3D array of 5160 light sensors buried kilometers deep in ice at the South Pole—spotted ghostly subatomic particles called neutrinos from beyond our galaxy. The discovery is Nobel-caliber stuff, some physicists say, as, except for a burp from a nearby supernova explosion in 1987, neutrinos from the far reaches of the cosmos had eluded capture. However, IceCube saw only about a dozen cosmic neutrinos per year, a rate at which the $279 million detector might never see enough of them to work as advertised: as a neutrino telescope with which to view the heavens in a whole new way. But as the data continue to come in, researchers are optimistic that a big enough detector should be able study the sky through neutrinos. IceCube researchers are pushing to expand the array, and other researchers have developed approaches that they say could be cheaper and more efficient. More important, a convergence of observations suggests that cosmic neutrinos spring from the same astrophysical sources as other particles from space: highly energetic photons called gamma rays, and mysterious ultra-high energy cosmic rays—protons and heavier atomic nuclei that reach energies a million times higher than humans have achieved with particle accelerators. If so, physicists have only one mystery to solve.