Seven billion years ago, a black hole at the center of a distant galaxy emitted a spectacular burst of energy. Astronomers missed their chance to study this event properly as a result of interference from the Moon. Eleven days later, however, a different form of interference gave them another chance, and they took full advantage. It marked the first time such a distant event has been studied using the highest-energy telescopes we possess.
On July 14, 2014, the Large Area Telescope aboard the Fermi satellite picked up signs of an outburst coming from a blazar called QSO B0218+357, a black hole aligned in such a way that the bright jets it sometimes emits are visible from Earth. The event was so significant that gamma-ray telescopes dropped their planned observations and turned their attention to this event.
Unfortunately for the MAGIC telescope on Canary Island, this was just two days after the full Moon. Although the Moon doesn’t emit high-energy gamma rays, scattered moonlight does overwhelm its detectors so it can’t pick up anything else.
Fortunately, in almost the exact direction of QSO B0218+357, lies a galaxy with the confusingly similar name B0218+357G. Light from the blazar has to pass close enough to B0218+357G to be affected by its gravity. The light from QSO B0218+357 gets split into two beams. Both eventually reach Earth, but one takes a slightly more roundabout route, and therefore arrives slightly later.
Astronomers at MAGIC calculated the rerun would screen 11 days later, and had the telescope, along with several others that work in the lower energy part of the gamma-ray spectrum, all set to go. Given the length of the lunar cycle, moonlight was never going to be an issue this time. The observations have now been reported in Astronomy and Astrophysics.