At the centre of many galaxies sits a supermassive black hole swallowing gas. Some of them blast out two narrow jets of particles at nearly the speed of light. Most of the time we see those jets from the side. A blazar is what you see when one of the jets happens to point almost straight at Earth.
Seen down the barrel
Looking into the jet changes everything. Matter moving towards us at up to about 99.5% of the speed of light has its light squeezed and concentrated in the direction of motion. This relativistic beaming makes a blazar appear far brighter than it would from the side. Blazars are among the most luminous objects in the universe, and they shine across the whole spectrum, from radio waves to the most energetic gamma rays.
The same geometry produces an illusion. Blobs of gas in the jets of many blazars appear to move across the sky faster than light. They are not: because they are coming almost towards us, the light they give off later has less distance to travel, which compresses the apparent time and exaggerates their speed.
Flickering giants
Blazars change brightness quickly, often over hours or days. Some flare even faster. Markarian 501 has produced gamma-ray flares lasting about 20 minutes, with the brightness rising in about a minute. Since nothing can vary faster than light can cross it, the region producing the flare must be extremely small for something so powerful.
There are two main kinds: BL Lacertae objects, named after a “star” that turned out to be one, which show few spectral lines, and flat-spectrum radio quasars, which show strong ones. The word blazar, a blend of the two, was coined by Edward Spiegel in 1978.
Messengers
On 22 September 2017 the IceCube detector, a cubic kilometre of ice at the South Pole, caught a neutrino with an energy of about 290 TeV. Telescopes traced it to the blazar TXS 0506+056, which was flaring at the time. It was the first time a high-energy neutrino had been linked to a known source outside our galaxy, suggesting blazars accelerate cosmic rays.
In December 2024 Eduardo Bañados and colleagues announced the most distant blazar known, seen as it was about 750 million years after the Big Bang. Since only a few jets happen to point our way, its discovery implies many similar black holes with jets already existed in the young universe.