This artist’s concept shows an active supermassive black hole with a jet streaming outward at nearly the speed of light. Such black holes are often found at the hearts of elliptical galaxies. Not all black holes have jets, but when they do, the jets can be pointed in any direction. If a jet happens to shine at Earth, the object is called a blazar. Image credit: NASA / JPL-Caltech.
In 2011 Francesco Massaro, currently a researcher at the University of Turin in Italy, and Raffaele D’Abrusco, a postdoc in the astrophysics group at the University Federico II of Naples, Italy, using all-sky data collected by NASA’s Wide-field Infrared Survey Explorer (WISE), discovered that blazars — the rarest and most extreme class of active galactic nuclei, typically detected in the high-energy gamma-rays — display infrared emission with unique properties that set them apart from all other extragalactic sources.
Using this discovery, Dr. Massaro and Dr. D’Abrusco developed a procedure to find new infrared candidate blazars. However, a piece of the puzzle was still missing: the conclusive identification of these astronomical powerhouses can only be obtained through observations of their visible light in tiny wavelengths intervals, what astronomers call optical spectrum.
For this reason, at the end of 2011, they assembled an international team of experts and started a large observational campaign to validate the nature of the largest possible sample of candidate blazars selected with their methods through spectroscopy.
The last paper in a series of six that present the results of this project was published Jan. 27 in the Astrophysical Journal.
“Blazars, one of the largest known classes of gamma-ray sources, are characterized a highly energetic emission that arises from a jet pointing towards the Earth originating from rapidly spinning supermassive black holes in the center of giant elliptical galaxies,” the researchers said.
“The radiation from these jets is generated by particles accelerated to velocities very close to the speed of light, that emit over the whole electromagnetic spectrum, from radio frequencies up to the highest energies ever detected, comparable to those released by thousands of supernovae exploding simultaneously. This powerful emission makes blazars the masters of the extragalactic gamma-ray sky.”
“Discovering the astronomical sources observed at lower energies that can be associated to gamma-ray sources has recently attracted feverish interest. It is because scientists hope that the understanding of the connection between well-known classes of objects and the elusive high-energy wonders will reveal something completely unexpected about the Universe and its evolution, including, possibly, the true nature of the dark matter.”
The Fermi Gamma-ray Space Telescope, an international collaboration between Italy, France, Japan, Sweden and the United States, since its launch in 2008 has helped astronomers to put into focus and map the high energy sky with unprecedented resolution and depth.
Although Fermi has already significantly improved our knowledge on the number and characteristics of the most energetic sources in the local Universe, about one third of the gamma-ray sources detected by Fermi are still of unknown origin.
The team of scientists coordinated by Dr. Massaro and Dr. D’Abrusco has tried to change this.
“Our observations represent a major improvement of our knowledge on the demographics of the unidentified gamma-ray sources and gamma-ray blazars,” said Dr. Massaro, who, after leaving the Harvard-Smithsonian Center for Astrophysics where he met Dr. D’Abrusco, has worked at the Stanford and Yale Universities in the United States before coming back to Italy at the University of Turin through the Young Researcher Program ‘R. L. Montalcini.’
“In the near future, the outcome of our long-term project will allow us to put new more stringent constraints on the nature, abundance and behavior of the dark matter that permeates and drives the evolution of the Universe.”
“When we discovered that the infrared emission of Fermi blazars follows a specific pattern, our first idea was to apply our findings to the search of new infrared sources that could be responsible of the emission associated to the unidentified gamma-ray sources,” Dr. D’Abrusco added.
“While our method turned out to be very effective at selecting potential candidates, we could only confirm their nature by obtaining their optical spectra. Thus, we started a large project to verify the nature of the largest available sample of candidate blazars selected with our method.”
“We collected spectra in observatories located all around the planet,” Dr. D’Abrusco said.
“After five years and tens of observing nights spent working with telescopes in Arizona (Kitt Peak National Observatory and Multiple Mirror Telescope), California (Palomar Observatory), Chile (Southern Astrophysical Research Telescope and Magellan), Canary Islands (Telescopio Nazionale Galileo and William Herschel Telescope) and Mexico (National Astronomical Observatory in Sa Pedro Martir and Guillermo Haro Observatory), we recently reached the milestone of 200 confirmed gamma-ray blazars among completely unknown blazars and sources whose nature was uncertain before our observational campaign.”
The project has also produced other unexpected results. Two members of the team coordinated by Dr. Massaro and Dr. D’Abrusco have discovered a handful of transitional blazar whose spectral continuum changes significantly over the time, and two blazars lacking a radio counterpart, which is a rarity even among the rarest class of extragalactic sources.
These efforts were led by two Ph.D. students, Nuria Alvarez-Crespo at the University of Turin and Federica Ricci at the University of Roma Tre, that on behalf of the collaboration acquired, reduced and analyzed the spectra of several candidate blazars, having the opportunity to live for the first time the unique experience of working the night shifts at remote telescopes.
The collaboration also includes: A. Paggi, D. Milisavljevic and Howard Smith of Harvard-Smithsonian Center for Astrophysics (United States); M. Landoni at Brera Observatory (Italy); N. Masetti of the Institute of Cosmic Physics of Bologna (Italy); V. Chavushyan and V. Patino-Alvarez of the Instituto Nacional de Astrofisica, Optica y Electronica (Mexico); E. Jimenez-Bailon of the Instituto de Astronomia, Universidad Nacional Autonoma de Mexico; D. Stern of NASA’s Jet Propulsion Laboratory (United States); J. Strader and L. Chomiuk from Michigan State University (United States); C. C. Cheung of the Naval Research Laboratory (United States); S. Digel from Stanford Linear Accelerator Center; G. Tosti of the University of Perugia (Italy); F. La Franca at the University of Roma Tre (Italy); M. Kagaya and H. Katagiri from Ibakari University (Japan).
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N. Álvarez Crespo et al. 2016. Optical spectroscopic observations of gamma-ray blazar candidates. V. TNG, KPNO, and OAN observations of blazar candidates of uncertain type in the Northern Hemisphere. Astronomical Journal 151, 32; doi: 10.3847/0004-6256/151/2/32
