Five billion years ago, a blazar abruptly flared, triggered an intense rain of gamma rays. Racing across the universe for millennia, they finally slammed into NASA’s Fermi satellite over several days this June, setting a new record for the most luminous high-energy object we’ve ever seen.

The monster black hole at the center of 3C 279 transforms the heart of the galaxy into a blazar: an active galaxy powered by a supermassive black hole equivalent to a billion suns compacted into our solar system. By a quirk of geometry, the high-energy jets spurting out of the system are oriented straight at us here on Earth, making them blazingly bright. Every so often for reasons we don’t yet understand, it flares up even brighter. This June, it produced a shower of the most luminous shower of gamma rays we’ve ever observed.

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Week-long exposures before and after the gamma ray flare from Blazar 3C 279, including the Vela pulsar (which is normally the brightest gamma ray object). The 10° scalebar covers an angular distance approximately the width of a clenched fist at arm’s length. Image credit: NASA/DOE/Fermi LAT Collaboration

The first gamma ray arrived on June 14, 2015, with the shower’s intensity increasing by an order of magnitude to peak on June 16. The change was beyond abrupt: Italian Space Agency astronomer Sara Cutini explains, “One day 3C 279 was just one of many active galaxies we see, and the next day it was the brightest thing in the gamma-ray sky.” By June 18, the display had faded away.

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The rapid fade of intensity in just a few days is why gamma ray bursts hijack observing schedules: Masaaki Hayashida at the University of Tokyo’s Institute for Cosmic Ray Research explains: “Our priority is to make observations while the object is still bright. Once it’s over, we can start trying to understand the mechanisms powering it.” A month later, our best theory for what happened is something to-be-determined changed with the blazar’s jets, leading to a rapid yet unsustainable outburst of energy.

The Italian Space Agency’s AGILE gamma-ray satellite spotted the flare first, soon followed by NASA’s Fermi Gamma-Ray Space Telescope (formerly GLAST). Next, NASA’s rapid response Swift satellite started follow-up observations, while coincidentally, the European Space Agency’s INTEGRAL spacecraft was already looking in the right direction to add to observations. With the burst event confirmed, more ground-based optical and radio telescopes joined the party.

The visualization put together by NASA Goddard covers a five-degree segment of the sky centered on 3C 279 from data collected by Fermi between June 14 and 17, 2015. Circle size and colour changes with magnitude from small white circles to large magenta ones. The highest-energy ray detected near the end of the shower hits a staggering 52 billion electron volts of energy. Despite being a million times farther away, the 3C 279 balazar burst was four times brighter than the Vela pulsar, the most persistent gamma ray source we’ve discovered and conveniently within the same arc of vision for Fermi’s Long Array Telescope. What exactly caused the abrupt flare is unknown.

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This specific blazar is particularly special in the history of science: a flare from it just after the launch of NASA’s Compton Gamma Ray Observatory (CGRO) set the record for the most distant and luminous gamma ray source observed. In 1991, as now, the flare was not just a shockingly bright gamma ray source but improbably managed to get even brighter in the following days. Robert Hartman, a scientist with the original CGRO team and now with Fermi, recalls, “Its brightness varied substantially, becoming four times brighter within 10 days.”

Mid-June was an abnormally busy time for gamma ray observations: along with observing 3C 279, Fermi also picked up gamma rays from eruptions on the sun (also an unusual occurrence) and a series of outbursts from the black hole binary system V404 Cygni (which erupts every few decades). Overall, this made it the busiest single period in the instrument’s history.

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