Using NASA’s Fermi Space Telescope, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona, and other telescopes, astronomers have detected high-energy gamma-ray emission from an extremely distant galaxy.
Blazars are the most common sources detected by NASA’s Fermi telescope. As matter falls toward a supermassive black hole at the galaxy’s center, some of it is accelerated outward at nearly the speed of light along jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, as illustrated here, the galaxy appears especially bright and is classified as a blazar. Image credit: M. Weiss / CfA.
The gamma rays came from PKS 1441+25, a type of galaxy called a blazar, according to two studies published in the Astrophysical Journal Letters.
This galaxy lies in the constellation Boötes, approximately 7.6 billion light-years away, and has a black hole of about 70 million solar masses at its center.
If placed at the center of our own Solar System, the black hole’s event horizon would extend almost to the orbit of Mars.
High-energy gamma-rays from PKS 1441+25 were detected in April 2015 and observed by a range of telescopes sensitive to different wavelengths.
NASA’s Fermi telescope detected gamma rays up to 33 billion electron volts (GeV). For comparison, visible light has energies between about 2 and 3 eV.
“Detecting these very energetic gamma rays with Fermi, as well as seeing flaring at optical and X-ray energies with NASA’s Swift satellite, made it clear that PKS 1441+25 had become a good target for MAGIC,” said Dr Luigi Pacciani of the Italian National Institute for Astrophysics in Rome, who is a member of the Major Atmospheric Gamma-ray Imaging Cerenkov (MAGIC) experiment.
The MAGIC team detected gamma rays with energies ranging from 40 to 250 GeV. “Because PKS 1441+25 is so far away, we didn’t have a strong expectation of detecting gamma rays with energies this high,” said Dr Josefa Becerra Gonzalez of NASA’s Goddard Space Flight Center, a co-author of the MAGIC study.
More distant blazars show a loss of higher-energy gamma rays thanks to the EBL. From studies of nearby blazars, astronomers know how many gamma rays should be emitted at different energies. If a gamma ray on its way to Earth collides with lower-energy light in the EBL, it converts into a pair of particles and is lost to astronomers. As shown by the graphs at left in this illustration, the more distant the blazar, the fewer high-energy gamma rays we can detect. During the April 2015 outburst of PKS 1441+25, MAGIC and VERITAS saw rare gamma rays exceeding 100 GeV that managed to survive a journey of 7.6 billion light-years. Image credit: NASA’s Goddard Space Flight Center.
The VERITAS system also detected gamma rays with energies approaching 200 GeV.
“With VERITAS, we detected gamma rays from this unusual object at the highest energies observed on Earth,” said Dr Jonathan Biteau of the University of California, Santa Cruz, and the Institut de Physique Nucléaire d’Orsay in France, who is a co-author of the VERITAS study (arXiv.org preprint).
To reach these telescopes, gamma rays from PKS 1441+25 had to avoid a tight net of photons surrounding the vicinity of the black hole, as well as a looser net of photons, the extragalactic background light (EBL), that fills the Universe.
The EBL is a faint glow that pervades the space between galaxies, consisting of photons from all the stars and galaxies that have existed. It is hard to measure because there are so many bright sources of light nearby.
Astronomers have used cosmological models to estimate the EBL, galaxy counts to set lower limits, and gamma rays from blazars to set upper limits. The farther gamma rays have to travel the more likely they are to encounter photons of the EBL and annihilate.
“With PKS 1441+25, we can now place tight constraints on this loose net of photons,” Dr Biteau said.
“Combining the Fermi data with the VERITAS data enabled us to make the constraints on the EBL much tighter,” added Caitlin Johnson of the University of California, Santa Cruz, a co-author of the VERITAS study.
“With observations across the entire electromagnetic spectrum, we have now realized that the location of the gamma-ray emissions for this source has to be at least a tenth of a light year away from the black hole. Otherwise, none of the gamma rays would escape,” Dr Biteau said.
The astronomers estimated that the emission region is probably at a distance of about 5 light-years from PKS 1441+25’s black hole, much further than expected.
According to a leading scenario for the gamma-ray emissions, high energy electrons are accelerated to near the speed of light in the jet, interact with photons, and transfer their energy, boosting the photons to gamma-ray energies.
“These observations constitute a fantastic step forward in our understanding of blazars as cosmic accelerators and as light beacons for gamma ray cosmology,” Dr Biteau concluded.
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Abeysekara A.U. et al. 2015. Gamma rays from the quasar PKS 1441+25: story of an escape. ApJ 815, L22; doi: 10.1088/2041-8205/815/2/L22
Ahnen M.L. et al. 2015. Very high energy γ-rays from the Universe’s middle age: detection of the z = 0.940 blazar PKS 1441+25 with MAGIC. ApJ 815, L23; doi: 10.1088/2041-8205/815/2/L23
