USRA | Arecibo Observatory Radio Data Crucial to Understanding why Quasars are so Bright
http://www.usra.edu/news/pr/2017/quasars/
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).
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.
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."
