In a research published last week on the journal Astronomy & Astrophysics, scientists from the international collaboration of the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, including the participation of UAB and IFAE researchers and located at the Roque de los Muchachos Observatory, in Garafía (La Palma), announced the discovery of gamma-ray emissions more distant than any other previously detected. The discovery was made possible by the gravitational lensing caused by a massive galaxy between the quasar and Earth, which "repeated" the light produced by the source.



According to Einstein's General Relativity, this phenomenon occurs when light is deflected passing close to a large mass. To a distant observer the mass focuses light like a giant lens. The result is a much brighter, although distorted, image of the source and a chance to see distant objects which might otherwise be far too faint to detect. And just like a lens, light can pass through it with slightly different path lengths. On cosmic scales, this means photons -- parcels of light -- travelling along different lines of vision arrive at slightly different times. If, in addition, the source is variable, these are "imprinted" on the light and when it reaches the Earth millions of years later, the object is visible as it was at that precise moment. According to the theory, this should not depend on the energy of the photons and that is what makes such observations especially important.



QSO B0218+357 is a quasar, a highly compact and energetic object associated with a supermassive black hole in the centre of a galaxy. Over 7 billion years ago a gigantic explosion occurred in this object, which led to the emission of an intense flare of gamma rays, which is the highest-energy form of light currently known. In its long journey toward Earth, these photons passed in the vicinity of a foreground — still distant — galaxy, B0218+357G, over one billion years later. In passing and being deflected, those photons travelling along the shorter path finally arrived at Earth on 14 July 2014 and were observed by the Large Area Telescope on board the orbiting Fermi satellite, which scans the entire sky every 3 hours. The detection of this gamma-ray outburst alerted the astronomical community, and many telescopes worldwide were immediately pointed towards QSO B0218+357 to learn more about this distant cosmic explosion.



Researchers operating the MAGIC telescopes became excited about the possible observation of this object, but unfortunately at that moment there was a full moon. However, they did get a second chance. From the earlier measurements of this object in 2012 by Fermi and by radio telescopes the scientists knew that photons arriving along the longer path should arrive about 11 days later. “In other words, nature could award us with a replay, a second chance to look at the same interesting phenomenon”, says MAGIC Collaboration member Julian Sitarek (researcher at the University of £ódz, Poland and former IFAE member) who led this study, and continues: “When the time came, the MAGIC telescopes were pointed at QSO B0218+357, and, in accordance with the prediction, a flare of very-high-energy gamma rays was observed, making QSO B0218+357 the most distant object detected in the very-high-energy gamma-ray domain to date.” These very-high-energy gamma-rays from any distant source have a high chance to interact with the numerous low-energy photons emitted by galaxies and stars, being lost in the process.



With this observation, MAGIC has doubled the previously known visibility range of the Universe in very-high-energy gamma rays. Observation of the delayed signal from QSO B0218+357 by MAGIC showed for the first time that these very energetic photons are also deflected in agreement with General Relativity and that the signal arriving at the predicted time may rule out some theories about the structure of the vacuum. For the moment, this observation demonstrates a new capability of the very-high-energy gamma-ray observatories and highlights what awaits the next generation of such telescopes, the Cherenkov Telescope Array (CTA) project.



MAGIC telescopes



MAGIC is a ground-based gamma-ray instrument located on the Canary island of La Palma, Spain. The system of two 17m diameter Cherenkov telescopes is currently one of the three major imaging atmospheric Cherenkov instruments in the world. It is designed to detect gamma rays tens of billions to tens of trillions times more energetic than visible light. MAGIC has been built with the joint efforts of a largely European collaboration that includes some 160 researchers from Germany, Spain, Italy, Switzerland, Poland, Finland, Bulgaria, Croatia, India and Japan, and among them scientists from the UAB and from the Institute of High Energy Physics.