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Why are Quasars so Bright? - - Russian RadioAstron Space Telescope and Arecibo Link Together to Find an Answer

01/22/2017 12:50PM

Why are Quasars so Bright? - - Russian RadioAstron Space Telescope and Arecibo Link Together to Find an Answer
Remarkable new observations derived by linking Arecibo Observatory's 305 meter dish with Russia's RadioAstron Space Radio Telescope has provided results that are causing much head scratching in radio astronomical circles. The achievable resolution from this combination of observatories is equivalent to seeing a golf ball on the Moon or a fingernail on the surface of the Earth from a spy satellite in geosynchronous orbit. What used to be a well understood explanation for the mechanism that generates intense radio signals from tiny and very distant quasar nuclei has now been tested in previously impossible ways. The results make it difficult to interpret the data in terms of conventional theories.

The RadioAstron Space Telescope, launched in 2011 by the Russian Federal Space Agency, carries a 10 meter radio dish and is traveling around the Earth in a highly elliptical orbit that takes it out to 350,000 km from Earth -- almost the distance to the Moon. When the signals it receives from a distant quasar are combined with simultaneous data acquired by its Earth-based partners at Arecibo in Puerto Rico, Green Bank in West Virginia, Socorro in New Mexico, and Bonn in Germany, the observations simulate a dish up to 350,000 km in diameter.

"Arecibo's huge diameter helps compensate for the small size of the RadioAstron dish," commented Dr. Chris Salter, Universities Space Research Association (USRA) senior staff astronomer at Arecibo Observatory. "Arecibo's participation is critical to the success of many RadioAstron experiments."

Combining the signals produces what are called fringes, and it was recently reported that quasar 3C273 was detected at a baseline of 170,000 km (106,000 miles). This remarkable achievement showed that 3C273 has structure in its core at least as small as 26 micro-arcseconds across. At the distance of 3C273, this corresponds to a physical diameter of 2.7 light-months. The ability to see such detail is not matched by any other telescope in the world. Optical telescopes, even the Hubble Space Telescope, do not come anywhere near this ability to see detailed structure.

To relate this angular scale to human experience, it is as if you were able to see a golf ball (which is not quite 5 cm across) on the Moon. Or if a spy satellite were in geosynchronous orbit, it would be able to see details as small as a fingernail.

So far RadioAstron and its terrestrial partners have not detected details smaller than the 26 micro-arcseconds in 3C273's core, but already the observations are pushing the theory of radio source emission mechanisms beyond their limit.

Radio astronomers measure the apparent brightness of objects such as quasars in terms of the temperature a solid body subtending the same angular size would have to possess in order to shine with the same intensity. The smaller the angular diameter of the object producing the radio signals, the higher its source temperature must be to produce the observed signal.

The 3C273 data reveal that its brightness temperature must be about 40 trillion degrees K (that is a 4 followed by 13 zeroes). The problem is that the maximum allowed by present theories for radio emission from a quasar is about one trillion degrees.

"Temperatures this high test our understanding of the physics in the vicinity of the supermassive black hole at the heart of 3C273," noted Dr. Tapasi Ghosh, the VLBI staff astronomer at Arecibo Observatory. "We hope that Arecibo/RadioAstron observations of other sources will help shed light on this mystery."

Dr. Yuri Kovalev of the Lebedev Physical Institute in Moscow states "We conclude that it is difficult to interpret the data in terms of conventional incoherent synchrotron radiation." Yet the theory of quasar radio emission that has held sway for nearly 60 years is based on synchrotron radiation.

"Arecibo Observatory may have celebrated its 50th anniversary in 2013, but it continues to make vital observations that challenge our understanding," said USRA's Dr. Joan Schmelz, Director of Arecibo Operations at Arecibo Observatory. "These impressive contributions to the RadioAstron measurements are just one example."

The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Science and the Lavochkin Association of the Russian Space Agency. Scientists at partner institutions in Russia and elsewhere in the world, including Puerto Rico, West Virginia, Massachusetts, New Mexico, Virginia, Germany, the Netherlands, and Australia, collaborate to make RadioAstron the international success that it has turned out to be. Crucial to that success has been the availability of the huge collecting area that is provided by the 305 meter diameter dish at Arecibo, Puerto Rico, without which the effectiveness of the small RadioAstron antenna would be vastly reduced.

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