Major improvements to methods used to process observations from NASA's Fermi Gamma-ray Space Telescope have yielded an expanded, higher-quality set of data that allows astronomers to produce the most detailed census of the sky yet made at extreme energies. A new sky map reveals hundreds of these sources, including 12 that produce gamma rays with energies exceeding a trillion times the energy of visible light. The survey also discovered four dozen new sources that remain undetected at any other wavelength.
This image, constructed from more than six years of
observations by NASA's Fermi Gamma-ray Space Telescope, is the first to
show how the entire sky appears at energies between 50 billion (GeV)
and 2 trillion electron volts (TeV). For comparison, the energy of
visible light falls between about 2 and 3 electron volts. A diffuse glow
fills the sky and is brightest in the middle of the map, along the
central plane of our galaxy. The famous Fermi Bubbles, first detected in
2010, appear as red extensions north and south of the galactic center
and are much more pronounced at these energies. Discrete gamma-ray
sources include pulsar wind nebulae and supernova remnants within our
galaxy, as well as distant galaxies called blazars powered by
supermassive black holes. Labels show the highest-energy sources, all
located within our galaxy and emitting gamma rays exceeding 1 TeV. Credits: NASA/DOE/Fermi LAT Collaboration. unlabeled image , labeled image
Watch Fermi scientists explain why they're so
excited about Pass 8, a complete reprocessing of all data collected by
the mission's Large Area Telescope. This analysis increased the LAT's
sensitivity, widened its energy range, and effectively sharpened its
view through improved backtracking of incoming gamma rays. Credits: NASA's Goddard Space Flight Center. Download the video in ultra-HD at NASA's Scientific Visualization Studio
Using 61,000 Pass 8 gamma rays collected over 80 months, Ajello and
his colleagues constructed a map of the entire sky at energies ranging
from 50 billion (GeV) to 2 trillion electron volts (TeV). For
comparison, the energy of visible light ranges from about 2 to 3
electron volts.
"Of the 360 sources we cataloged, about 75 percent are blazars, which are distant galaxies sporting jets powered by supermassive black holes," said co-investigator Alberto DomÃnguez at the Complutense University in Madrid. "The highest-energy sources, all located in our galaxy, are mostly remnants of supernova explosions and pulsar wind nebulae, places where rapidly rotating neutron stars accelerate particles to near the speed of light." One famous example, the Crab Nebula, tops the list of the highest-energy Fermi sources, producing a steady drizzle of gamma rays exceeding 1 TeV.
Astronomers think these very high-energy gamma rays are produced when lower-energy light collides with accelerated particles. This results in a small energy loss for the particle and a big gain for the light, transforming it into a gamma ray.
For the first time, Fermi data now extend to energies previously seen
only by ground-based detectors. Because ground-based telescopes have
much smaller fields of view than the LAT, which scans the whole sky
every three hours, they have detected only about a quarter of the
objects in the catalog. This study provides ground facilities with more
than 280 new targets for follow-up observations.
"An exciting aspect of this catalog is that we find many new sources that emit gamma rays over a comparatively large patch of the sky," explained Jamie Cohen, a University of Maryland graduate student working with the Fermi team at NASA's Goddard Space Flight Center in Greenbelt. "Finding more of these objects enables us to probe their structures as well as better understand mechanisms that accelerate the subatomic particles that ultimately produce gamma-ray emission." The new catalog identifies 25 of these extended objects, including three new pulsar wind nebulae and two new supernova remnants.
Ajello presented the findings Thursday at the 227th meeting of the American Astronomical Society in Kissimmee, Florida. A paper describing the catalog has been accepted for publication in The Astrophysical Journal Supplement.
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and
particle physics partnership, developed in collaboration with the U.S.
Department of Energy and with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
For more information about NASA's Fermi, visit: www.nasa.gov/fermi
