From VERITAS scientist Wystan Benbow: "Here is the VERITAS sky catalog in Galactic Coordinates (i.e. those with the Galactic Plane at the equator). The sky catalog is all objects which VERITAS has seen (& released publicly). The different colors of dots represent different kinds of astrophysical objects. The blue regions are those best visible to VERITAS. Almost all of the marked objects have also been detected by Fermi LAT." Courtesy of VERITAS.
Two spots in the night sky look a bit brighter this week, thanks to collaborative efforts between the Large Area Telescope, which is the primary instrument aboard the Fermi Gamma Ray Space Telescope, and VERITAS, or the Very Energetic Radiation Imaging Telescope Array System, an array of ground-based telescopes in Amado, Arizona. The two collaborations announced last week the discovery of one new blazar and one likely blazar; if the second is confirmed these will be the third and fourth blazars that the observatories have collaborated to find.
Blazars are a type of active galactic nucleus (AGN), the very violent systems that lie at the center of some distant galaxies. The systems each include a massive black hole, some of which are thought to be billions of times more massive than the sun. Blazars are distinct from other AGN because they emit two jets of energetic gas out their poles, which point toward Earth, making them distinguishable by observatories like VERITAS. These violent and energetic systems radiate radio waves or X-rays, but they are identified by their emission of very-high-energy gamma rays.
The first new discovery appeared as a bright spot in the publicly available Fermi LAT data, and VERITAS scientists decided to investigate. They confirmed that it radiated in the TeV range, and that it aligns with an AGN, and hope to soon confirm if it is indeed a blazar. VERITAS made an announcement on October 25. The second high-energy source was jointly spotted in the sky by VERITAS and LAT, who communicated with each other to identify it as a blazar. The results were announced in a joint astrophysics telegram on October 29.
VERITAS is a targeted telescope array that looks for light in the TeV range--that means light with energy even higher than the gamma rays being searched for by Fermi (Fermi searches between 10 and 300 GeV, where 300 GeV is equal to 0.3 TeV). These extremely high-energy photons commonly come from very energetic, violent sources in the universe, such as blazars. VERITAS hopes to glean information about these objects, which could also provide answers to larger questions about our universe.
The trouble for VERITAS is knowing where in the sky to look for these TeV gamma ray sources (the trade off is a tremendous increase in sensitivity, making the two observatories complimentary). VERITAS can only look at a section of the sky about as wide as seven full moons. Without being able to scan the entire sky very quickly, VERITAS looks to observations made by radio and X-ray telescopes, and makes a list of objects that they think might radiate in the TeV range. Upon investigation, there are hits and misses: some of the objects on the list do emanate the high-energy gamma rays, and some don't.
LAT offers VERITAS a tremendous new source list of potential high-energy gamma ray sources. Because the energy range of Fermi is much closer to that of VERITAS than radio or X-ray telescopes, LAT should reveal more potential TeV sources than radio or X-ray telescopes alone, and save VERITAS time and effort by reducing the number of sources they investigate that turn out to be misses.
"The degree to which LAT can help us can't be overstated," said University of Delaware Professor of Physics and Astronomy and VERITAS researcher Jamie Holder, after his talk at the Fermi symposium. In his talk, one of his slides read, "The LAT is changing the way we do TeV astronomy!"
And VERITAS can help Fermi, too.
In the first three months of Fermi data collection, VERITAS identified the blazar RGB J0710+591. They alerted LAT, which searched its data and found that the blazar radiated lower energy gammas as well.
Shortly after, and still within the LAT's first three months of data collection, LAT saw a bright spot in the sky and alerted VERITAS, which confirmed that the spot radiated in the TeV gamma ray range. This blazar, PKS-1424+24, was the first Very High Energy Gamma Ray Source that was identified as a result of the LAT data.
As Fermi continues to collect data, and its resolution increases, the likelihood that VERITAS will identify a bright source first will decline. At the same time, there is a rare type of blazar that, at this point in its data collection, LAT does not see. Over time, the rare blazars could appear in the LAT data, and together the two observatories hope to learn more about these powerful objects.
