Remarkable new observations derived by linking Arecibo Observatory's 305-meter dish with the Russian RadioAstron space radio telescope have provided results that are causing much head scratching in radio astronomical circles. What used to be a well-understood explanation of the mechanism that generates intense radio signals from tiny and very distant quasar nuclei has now been tested in previously impossible ways.

The RadioAstron satellite, launched in 2011 by the Russian Federal Space Agency, carries a 10-m 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. This network of telescopes operates at frequencies (wavelengths) of 330 MHz (92 cm), 1.7 GHz (18 cm), 4.7 GHz (6.2 cm) and 22 GHz (1.3 cm).

"Arecibo's huge diameter helps compensate for the small size of the RadioAstron dish," commented Dr. Chris Salter, Universities Space Research Association's (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 3C 273 was detected at a baseline of 170,000 km (106,000 miles). This remarkable achievement showed that 3C 273 has structure in its core at least as small as 26 microarcseconds across.

At the distance of 3C 273, 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 microarcseconds in 3C 273'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 3C 273 data reveal that its brightness temperature must be about 4 x 10^13 K, that is, a 4 followed by 13 zeroes, or 40 trillion degrees. The problem is that the maximum allowed by present theories for radio emission from a quasar is about 10^12 K, which is to say around one trillion degrees Celsius.

"Temperatures this high test our understanding of the physics in the vicinity of supermassive black hole at the heart of 3C 273," 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."

As the 20 authors of the most recent paper, led by Dr. Yuri Kovalev of the Lebedev Physical Institute in Moscow, state, "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, 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-m diameter dish at Arecibo, Puerto Rico, without which the effectiveness of the small RadioAstron antenna would be vastly reduced.

Two of USRA's permanent staff at Arecibo, Drs. C. J. Salter and T. Ghosh, who carry out the on-site observations, are deeply involved in furthering the aims of the project to determine what makes distant source of radio shine as brightly as they do.

Dr. Chris Salter of Universities Space Research Association is presenting these results at a press conference at the American Astronomical Society's meeting in Grapevine, Texas, on January 4, 2017.

Research paper: "RadioAstron Observations of the Quasar 3C 273: A Challenge to the Brightness Temperature Limit," Y. Y. Kovalev et al., 2016 Mar. 20, Astrophysical Journal Letters