The first astrophysical radio jet has been detected from an outbursting white dwarf nova, an observation that wouldn’t have been possible without the help of amateur astronomers, and one which suggests that all compact accreting binary systems are capable of creating radio jets.
White dwarfs are the remnants of stars that have exhausted their nuclear fuel and expelled their outer layers, leaving only a hot, dense, compact core behind. Traditionally, there has been little evidence for radio jets emanating from dwarf novae, which is a phenomenon more usually associated with accretion discs, black holes, neutron stars, X-ray binaries and young stellar objects in the process of forming. However, models of jet formation say they should occur in nearly all accreting sources with discs and outbursts, a claim that provided the impetus for a team of determined radio astronomers set on catching a dwarf nova in the act.
The new observations concern the variable star SS Cygni, which consists of a pair of stars in orbit around one another, one a white dwarf and the other an ordinary dwarf star. They are orbiting so close that the gravitational pull of the white dwarf pulls matter off the other star, forming an accretion disc. Occasionally, this disc flares up, becoming hot and bright for a few weeks before fading again.
Artist concept of a dwarf nova system, where the gravitational field of a white dwarf pulls matter off the surface of a companion star into a disc surrounding the white dwarf. The disc occasionally goes into outburst, making the dwarf system hundreds of times brighter. It is now believed that radio jets are formed during the first few hours of an outburst. Image: A. Beardmore, University of Leicester.
SS Cygni was just one of a number of potential radio burst suspects highlighted by the American Association of Variable Star Observers (AAVSO) to amateur observers, but was the only one seen to put on a radio jet display. It was first spotted by variable star observer Stanislaw Swierczynski from Poland on April 24, 2007, and 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. In total, amateur variable star astronomers provided nearly 1500 observations of SS Cygni over the following three weeks, 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.