The story of QPEs discovery, and the little we know and understand so far.
On December 24th 2018, the ESA satellite XMM-Newton was pointing at a galaxy 240 millions light years away from Earth, located in the direction of the galactic South Pole, named GSN 069. Astronomers were interested in discovering if and how the X-ray and UV emission of this galaxy were correlated, and the Hubble Space Telescope was observing simultaneously with XMM-Newton. A couple of weeks later, when the data had been delivered by the telescopes through radio signals down to Earth, and were ready to be analyzed by the astronomers on ground, the major surprise came from the X-rays.
The X-ray signal, originating in the nucleus of the galaxy GSN 069, that was detected by the XMM-Newton telescope on 24 December 2018.
Astronomers were stunned in observing such sharp, intense variations of X-ray light happening in so short time scales. The two bursts of X-ray light last about one hour, and are separated by nine hours.
I remember Giovanni, appearing at the door of my office with a piece of paper in his hand: “Look at this.” — I looked at that, wide-opened my eyes and asked: “What is this?!” — and Giovanni, looking at me with a half-smile, saying nothing. Me: “Is it GSN 069?!??” — and Giovanni, widening his smile: “Check the other XMM-Newton detectors, please.”
I immediately started my data analysis routines, and a few minutes later could confirm: the extraordinary signal was not an instrumental artifact, as was observed by all the three X-ray cameras onboard XMM-Newton. We were observing a cosmic signal never observed before.