Link Found Between Bright Galaxies and Unidentified Gamma-Ray Sources
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 5, 2001
Greenbelt, Md. -- Finnish astronomers, compelled to uncover the nature of at least some of the 170 gamma-ray sources that have resisted classification all these years, have detected strong and variable high-frequency radio emission from active galactic nuclei that may likely be the source of the enigmatic gamma rays themselves.
The report is being presented today to the Gamma 2001 meeting in Baltimore, Maryland by Drs. Merja Tornikoski and Anne Lähteenmäki from the Metsähovi Radio Observatory, Finland, and Drs. Esko Valtaoja and Markku Lainela from the Tuorla Observatory, Finland.
"The study gives new information about the high radio-frequency properties of the relatively sparsely studied Southern Hemisphere sources and helps in understanding the gamma-ray emission mechanism in active galactic nuclei", said Dr. Merja Tornikoski, the lead author.
Gamma rays represent the most energetic form of radiation. These high-energy photons are produced in cataclysmic events, such as the flow of extremely hot gas towards a black hole or the collision of atomic particles moving at nearly the speed of light.
Of the 271 sources of gamma rays detected by the EGRET instrument aboard the Compton Gamma Ray Observatory, 66 are "high-confidence" active galactic nuclei (AGNs) or quasars, essentially galaxies with extremely bright central regions likely fueled by hot gas accreting onto a supermassive black hole. Twenty-seven sources are "low-confidence" AGNs; and 170 sources cannot be firmly identified with any known object.
The Finnish team confirms the AGN-nature of nine of the low-confidence identifications of Southern sources, and furthermore suggests that nine of the so far unidentified EGRET sources in the Southern Hemisphere may also be AGNs. Identifying gamma-ray sources is difficult because EGRET could not establish precise locations, and several astronomical objects possibly responsible for the gamma-ray emission may fall within the EGRET field of view.
Astronomers have associated some of the unidentified sources with sources of galactic origin, i.e. located in the Milky Way galaxy. But because many EGRET sources are indeed AGNs, an obvious hypothesis is that some of the unidentified ones are AGN-type extragalactic sources. Tornikoski said her group detected strong high-frequency radio emission (in the millimeter-wavelengths) from AGNs close to several unidentified EGRET sources.
Earlier work by this Finnish group suggested the high-frequency radio waves are connected to high-energy gamma-ray emission in AGNs. Previously, astronomers used mainly low-frequency radio data in their quest to identify gamma-ray sources, assuming that the higher-frequency radio waves would only be fainter and thus less useful.
"This does not have to be the case," Dr. Tornikoski said. "Even though the 'generic' behavior of an active galactic nucleus is such that a source faint at the low frequencies would be too faint to detect at the high frequencies, we have several counterexamples of sources that are unusually bright at the high radio-frequencies. These seem to be the ones that are active also at gamma-ray energies. The high radio-frequency activity and gamma-ray activity in these AGNs occur synchronously, making millimeter-wave studies an important tool in understanding these objects."
The Finnish group used the 15-meter (49') Swedish-ESO Submillimeter Telescope (SEST), located at the European Southern Observatory site of the La Silla mountain in Chile. The telescope is ideal for making observations at relatively high radio frequencies; the group used the frequencies 90 GHz and 230 GHz, which correspond to the wavelengths of 3 mm (0.12 inches) and 1.3 mm (0.05 inches), respectively.
"We know that all EGRET-detected AGNs are strong and variable sources also at higher radio frequencies," Dr. Tornikoski said. "What we do not know yet is whether all sources bright and variable in the high radio-frequency domain are also potential gamma-ray sources, if we manage to observe them at the peak of their activity."
Next generation gamma-ray space instruments -- particularly the Gamma Ray Large Area Space Telescope, or GLAST, scheduled for a 2006 launch -- will probe further into the gamma-ray emssion of AGNs. This work was supported by the Academy of Finland. Earlier work on the identification of EGRET-detected sources with active galactic nuclei has been published by Dr. John Mattox et al. in 1997, Dr. Steven Bloom et al. in 1997, Dr. Alma Zook et al. in 1997, and Dr. Jorge Combi et al. in 1999.
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