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Radio astronomers catch a glimpse of an astrophysical jet

For thie first time, astronomers have detected a radio jet from a dwarf nova.
Provided by AAVSO, Cambridge, Massachusetts
An artist's concept of a dwarf nova system
An artist's concept of a dwarf nova system. The gravitational field of a white dwarf pulls matter off the surface of a companion star into a disk surrounding the white dwarf. The disk occasionally goes into outburst, making the dwarf nova system about 100 times brighter than in quiescence. Radio jets are now believed to be formed during the first few hours of an outburst, during the rapid brightening stage.
A. Beardmore, University of Leicester
June 10, 2008
A paper by Dr. Elmar Koerding (U. Southampton, UK) and collaborators was published in the journal Science June 6 describing what is believed to be the first astrophysical radio jet detected from an outbursting dwarf nova. The observations were performed with assistance from the American Association of Variable Star Observers (AAVSO) and amateur variable star observers around the world who helped Koerding and his team catch the very beginning of the April 2007 outburst of the star SS Cygni. This is the first time a radio jet from a dwarf nova has been clearly detected, and suggests that the creation of radio jets is possible by all compact accreting binary systems.

The variable star SS Cygni consists of a pair of stars in orbit around one another, one a white dwarf and the other an ordinary dwarf star. White dwarfs are the remnants of stars that have exhausted their nuclear fuel and lost their outer layers, leaving only a hot, dense, compact object behind. The white dwarf and its orbiting companion are so close together that the gravitational pull of the white dwarf pulls matter off the other star, forming an accretion disk. Occasionally, this disk flares up, becoming hot and bright for a few weeks before fading again — a dwarf nova outburst. Radio jets are well known phenomena that can occur in many types of star that involve accretion disks, including black hole and neutron star X-ray binaries, young stellar objects in the process of forming, and even some types of white dwarf binaries. But there has been little evidence for radio jets in the dwarf novae before now. The absence of radio jets in dwarf novae outbursts posed a problem for models of jet formation which say they should occur in nearly all accreting sources with disks and outbursts.

Elmar Koerding and collaborators Michael Rupen (NRAO), Christian Knigge (U. Southampton), Rob Fender (U. Southampton), and Vivek Dhawan (NRAO) sought to catch a dwarf nova in the act. They called upon the global community of amateur variable star observers to assist them in detecting a dwarf nova outburst during its earliest stages. In an international campaign organized by the AAVSO, amateur astronomers kept watch on a number of dwarf novae in hopes of catching one within just a few hours of the start of an outburst. Observers from all over the world worked together to provide the needed information to the radio astronomers; Stanislaw Swierczynski, a variable star observer in Poland, was the first to detect a possible outburst of SS Cygni April 24, 2007.

Within a few hours observers in Canada, the United States, Norway, and Finland provided the confirmation that an outburst was indeed underway. Their observations set in motion radio telescopes in the US and the UK that finally obtained evidence for a transient radio jet that appeared right at the start of the outburst — just as models predicted. Amateur variable star astronomers provided nearly 1500 observations of SS Cygni over the following three weeks, clearly showing the evolving outburst and providing important physical information used in the interpretation of the radio data.

"This project is a perfect example of the kind of things the amateur variable star community is capable of," said AAVSO Staff Astronomer Dr. Matthew Templeton. "It's difficult to monitor variable stars over long periods of time from professional observatories, because observing time has to be shared by many different programs and there just aren't enough telescopes to go around. But amateur observers in the variable star community are monitoring thousands of stars every night, and they can provide exactly the kind of notification needed to make this kind of observation." This is nothing new to the AAVSO and its observers; in the past year, amateur astronomers have provided observations enabling observations with the Hubble Space Telescope, Spitzer Space Telescope, the XMM-Newton X-ray satellite, and the Chandra X-ray Observatory, as well as providing important data for many other ground-based research programs.
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