A gravitational lens observed at very high energies
Research
A collaboration including ETH physicists at the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) observatory reports the discovery of the most distant gamma-ray source ever observed at very high energies — thanks to the 'replay' of an enormous flare by a galactic gravitational lens, as predicted according to Einstein’s theory of General Relativity.
MAGIC is a ground-based gamma-ray instrument located on the Canary island of La Palma, Spain. It is designed to detect gamma rays tens of billions to tens of trillions times more energetic than visible light. MAGIC has been built in a joint effort of a largely European collaboration that includes about 160 researchers from Germany, Spain, Italy, Switzerland, Poland, Finland, Bulgaria, Croatia, India and Japan. Actively involved in the study now published in the journal Astronomy & Astrophysics was the ETH group of Adrian Biland at the Institute for Particle Physics (IPP).
Bending light
According to Einstein's theory of General Relativity, light is deflected when passing by 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 that might otherwise be far too faint to detect. And just like in the case of a conventional lens, light can pass through the gravitational lens with slightly different path lengths. On cosmic scales, this means photons — packages of light — traveling along different lines of sight arrive at slightly different times. If, in addition, the source is variable, this is 'imprinted' on the light with a time delay relative to a fixed first arrival. And this delay should not depend on the energy of the photons, according to theory. Veryfying this in observations is especially important.
Missed opportunity ...
QSO B0218+357 harbours a supermassive black hole in a galaxy located halfway across the Universe from Earth. 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. On their long journey towards Earth, these photons passed in the vicinity of a foreground — but still distant — galaxy, B0218+357G, over one billion years later. In passing and being deflected, those photons traveling 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 burst alerted the astronomical community, and many telescopes worldwide were immediately pointed at QSO B0218+357 to learn more from this distant cosmic explosion. Researchers operating the MAGIC telescopes became excited about the possible observation of this object in the window of very-high-energy gamma rays. These could provide the most extreme perspective of this burst. But, unfortunately, at that time there was full moon in La Palma, which prevented the operation of the MAGIC telescopes.
... but a second chance
From the earlier measurements of this object in 2012 using the Fermi satellite and radio telescopes, the MAGIC scientists knew that photons arriving along the longer path should hit Earth 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 Julian Sitarek (University of Łódz, Poland), who led this study for the MAGIC collaboration, 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.” With this observation, MAGIC has doubled the previously known visibility range of the Universe in the window of very-high-energy gamma rays. The 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 the theory of General Relativity, a result that is both striking and potentially profound, as the signal arriving at the predicted time may rule out some theories of the structure of the vacuum. That aspect awaits further analysis. For the moment, this observation demonstrates a new capability of the very-high-energy gamma-ray observatories and highlights what awaits us when the next generation of such telescopes becomes available.
(Text adapated from external pageMAGIC press release)