Why are Quasars so Bright?

By Gerrit Verschuur and Joan Schmelz

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 330 MHz (92cm), 1.7 GHz (18cm), 4.7 GHz (6.2cm) and 22 GHz (1.3cm).

“Arecibo’s huge diameter helps compensate for the small size of the RadioAstron dish,” commented Dr. Chris Salter, a 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 microarcseconds 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 coin of the size of US quarter (5 cm across) at 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 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.

Paper: RadioAstron Observations of the Quasar 3C273: A Challenge to the Brightness Temperature Limit
Authors: Y.Y. Kovalev and 19 coauthors, including AO staff members T. Ghosh and C. J. Salter
Reference: ADS Link: http://adsabs.harvard.edu/abs/2016ApJ...820L...9K