In April, NASA’s Fermi Gamma-ray Space Telescope caught a rare cosmic event: an outburst of gamma rays that has been journeying across the universe. The sensational find helped ground-based observatories detect some of the highest-energy light ever witnessed from a distant galaxy.
The gamma rays emerged from PKS 1441+25, an active galaxy known as a blazar and situated so far from the Solar System that it takes 7.6 billion years for its light to reach Earth. This blazar is an extremely bright beacon powered by a massive black hole, with a disk of dust and hot gas surrounding it. Its black hole is estimated to have 70 million times the sun's mass.
The flare from PKS 1441+25 was caught by Luigi Pacciani of Rome’s Italian National Institute for Astrophysics using publicly available Fermi data. He then alerted the scientific community.
What the telescope revealed was gamma rays measuring up to 33 billion electron volts (GeV), going into the portion that shows the highest energy in the telescope's detection range. In contrast, energies in visible light are measured to have between 2 and 3 electron volts only.
In a follow-up to the Fermi discovery, the team behind the Major Atmospheric Gamma-ray Imaging Cherenkov experiment, also known as MAGIC, turned to the said blazar and found gamma rays with 40 to 250 GeV energies.
“Because this galaxy is so far away, we didn't have a strong expectation of detecting gamma rays with energies this high,” reported NASA in a press release.
NASA cited it was unexpected since distance is very crucial for ultra high-energy gamma rays, which turn into particles once they collide with light with a lower energy. For a better understanding of the distance and age of the gamma rays, the universe is 14 billion years old and Earth has been in existence for just 4.5 billion years. This makes the gamma rays half the universe’s age and shows the rays come from a place half a universe away.
In their staggeringly long journey to reach Earth’s telescopes, the gamma rays also had to pass through the extragalactic background light (EBL), a remnant glow made up of a looser net of photons. Once a gamma ray comes across starlight, it converts into two particles, an electron and a positron, making itself invisible to astronomers - which makes its detection and measurement more astonishing.
After this surprising discovery, Arizona’s Very Energetic Radiation Imaging Telescope Array System (VERITAS) also caught gamma rays with energies approximating 200 GeV. PKS 1441+25 is one of two gamma ray sources with energies exceeding 100 GeV that have been witnessed so far.
The marvelous flare offered a look into the EBL’s intensity from a wide range of wavelengths. The measurements, from near-infrared to near-ultraviolet, suggest that surveys of the galaxy have zeroed in on most of the EBL's sources. Since the EBL gets its glow from all the heavenly bodies that have ever existed, probing into it lets scientists track the universe's history.
Associate professor Jonathan Biteau, who headed the data analysis as a postdoctoral researcher in UC Santa Cruz, said this development heralds a new era.
“[W]e can compare source-by-source measurements and start to probe the cosmic evolution of the extragalactic background light,” he said.
Caitlin Johnson, UC Santa Cruz graduate student and Biteau’s co-author, said combining the Fermi data with those from VERITAS enabled them to make constraints on the EBL much tighter. “The window is slowly narrowing,” she said.
The results of the study are to be published in The Astrophysical Journal Letters, which can be viewed online.