Very high-energy gamma rays from the Universe’s middle age: the first detection of the distant galaxy PKS 1441+25

An international team of researchers reports the first detection of very high-energy gamma-ray emission from the distant active galaxy PKS 1441+25. The discovery was made by the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) twin telescopes, located in the Canary island of La Palma (Spain). MAGIC collaboration pointed the telescopes to this object following a flare seen at lower energies by Fermi satellite. The detection was afterwards confirmed by the VERITAS telescopes, located in Arizona (USA). The results in a paper to be published in the Astrophysical Journal Letters


PKS 1441+25 is one of the two most distant active galaxy detected up to such extreme energies. The very high-energy emission is attenuated on its way to Earth due to the interaction with the diffuse light filling the Universe, the extragalactic background light. This diffuse light retains the history of the stars and galaxies evolution and, hence, the story of the evolution of the Universe. PKS 1441+25 was used as a lighthouse to derive information on the evolution of the Universe from half its age up to the present day. Beside its distance, PKS 1441+25 is one of the very few Flat Spectrum Radio Quasars (a family of extremely luminous galaxies carrying hot, high-velocity gas, deep in the gravitational well of their central black hole and shooting with the speed approaching the speed of light plasma jets in our direction) detected at such extreme gamma-ray energies, allowing to study its intrinsic characteristics.

On April 2015, the Large Area Telescope detected an increase of the activity of PKS 1441+25 and MAGIC followed up the alert, discovering VHE gamma rays from the object. Elina Lindfors, PI of the observation project in MAGIC, researcher at Turku University says: "In MAGIC we have always followed closely the behavior of sources in other wavelengths and when they show increased flux in some region of the electromagnetic spectrum, we readily follow them with MAGIC. Discovery of VHE gamma-rays from PKS 1441+25 was yet another success of this observational strategy."

Some of the galaxies are called active, as they produce in their central parts much more light that can be explained by the stellar and dust emission. Active galaxies, hosting at their centers a supermassive black hole with a mass of million up to few billion times the mass of the Sun are amongst the most powerful objects of the Universe and dominate the gamma-ray sky: they are able to accelerate charged particles up to very high energies. Following a classification based on their luminosity and other astrophysical important features, PKS 1441+25 belongs to the Flat Spectrum Radio Quasars family and emits very energetic gamma rays from the vicinity of its central black hole. Flat Spectrum Radio Quasars are enigmatic sources and scientists struggle to explain the emission of VHE gamma rays from them, observations such as those performed by MAGIC give crucial inside into those sources. Gamma rays depict the extreme Universe, but such highly energetic emission suffers absorption in the journey to the Earth due to the interaction with diffuse emission, the extragalactic background light. The extragalactic background light acts as a sort of ‘haze’ which dims the gamma-ray brightness of distant galaxies. This ‘haze’ was generated by stars and dust throughout the history of the Universe. It traces the evolution of the Universe after the appearance of the first stars. In this context, powerful active galaxies, such as PKS 1441+25, can be used as distant lighthouses to infer the characteristics of the extragalactic background light between the Earth and the position of the active galaxy. Miguel Nievas Rosillo, young PhD student at the Universidad Complutense in Madrid says: "The absorption of gamma-rays during their travel from distant galaxies towards us makes their study very challenging. Nevertheless with MAGIC we were able to collect high quality data from a very distant source, confirming the good performance of the instrument at low energies and how powerful the Cherenkov Imaging Technique is.”

The emission detected from the active galaxy PKS 1441+25 has been traveling for half of the age of the Universe. PKS1441+25 and QSO B0218+357, another source recently detected by MAGIC, are the oldest VHE gamma rays ever detected from ground observatories. Marina Manganaro, postdoc at the Institute of Astrophysics of Canary islands (IAC) says: "Analyzing data from such a distant galaxy was very exciting and felt like travelling back in time to learn more about our Universe's history”.

In the paper recently accepted for a publication in the Astrophysical Journal Letters, the current models of the extragalactic background light which constrain the evolution of the Universe from its middle age until today were tested for the first time up to energies never reached before for such distant galaxies.

Razmik Mirzoyan, spokeperson of the MAGIC Collaboration, says: "Hunting very far sources allows gamma-ray astrophysicists finding the limits of the observational universe and probing the low-energy photon fields in space- an important input for cosmology. The universe is only partially transparent for very high-energy gamma rays, measured by MAGIC. While traveling towards the Earth, these can interact with the low energy photons emitted by the galaxies and stars over the evolution of our universe, and vanish by converting into electron-positron pairs. Since 2007, when MAGIC first time broke the record of the farthest source detected in VHE gamma rays (3C279 at the redshift of 0.536), we were hunting for much further sources. Finally we succeeded to "catch" a strong flare of the active galaxy PKS 1441+25 residing in the further half of the universe (redshift = 0.940), practically doubling the reach of the observational universe." 

- MAGIC Paper