A “spectacular” outburst from a distant quasar in April of this year gave astronomers the opportunity to measure the density of the optical “fog” that lies between the quasar and Earth—and to deduce the surprising separation of the high-energy emission from the black hole that drives it.
“On April 20, 2015, we got word from the Fermi satellite and the MAGIC telescope observatory that they had spotted a very active source—one that was getting very bright in gamma rays,” says Manel Errando, a research scientist in physics at Washington University in St. Louis. “At the time, I was chair of the committee at VERITAS that decides which sources we’ll observe. We decided to give it a go.”
VERITAS is a telescope array in southern Arizona that is sensitive to very high-energy gamma rays, those with billions of times more energy than the photons to which our eyes are sensitive. Over the next 10 days, it detected high-energy gamma rays from the quasar, known as PKS 1441+25, including some with energies of about 200 GeV.
“We knew that this source emitted low-energy gamma rays, but this time we detected high energy ones that usually don’t make it to Earth,” Errando says.
“This is the first puzzle: how is it possible that gamma rays of such high energies made the trip all the way from this very distant quasar to Earth without getting lost in the fog of visible photons in between? The second puzzle was that the high-energy gamma rays were produced far from the black hole that powers them, not close to it, as you would expect.”
“Everyone who had a telescope was looking at PKS 1441+25, which was active across all wavelengths,” says Errando, corresponding author of a paper in Astrophysical Journal Letters.
“We were surprised to detect high-energy gamma rays,” Errando says, “because they don’t usually make it through. It’s like turning on your high-beams in a fog.”
Very energetic gamma rays are easily knocked off by visible light, the light from galaxies and stars that fills intergalactic space. Lower-energy gamma rays mainly interact with ultraviolet light and X-rays. There are lots of sources of visible photons—stars and galaxies—but not as many X-ray sources that would bother the lower-energy gamma rays.
The arrival of high-energy gamma rays therefore allowed the astronomers to set a limit on the density of the fog of photons between the quasar and Earth, the photons that should make it difficult for those gamma rays to propagate.
Because it takes light time to travel, astronomy can be a time machine; the farther away the source, the farther back in time the light was emitted. The light from PKS 1441+25 traveled at least 7.6 billion years to reach Earth, or more than half of the age of the universe.
Continue to the next page for a great video that details just how unique this observation is…