The University of Southampton

New discovery answers long-standing astronomical puzzle

6 June 2008

Astronomers have discovered a previously unexpected astronomical phenomenon – a radio jet in a star system where none had been expected – which suggests that all stars and black holes feed in the same way.

Astronomers from Southampton and Manchester Universities and the National Radio Astronomy Observatory in Virginia, USA, in collaboration with the American Association of Variable Star Observers and other amateur astronomers (including those of the British Astronomical Association), were monitoring the outburst of the dwarf nova SS Cygnus when they made their discovery. The research paper will be published by the journal Science on 6 June.

When compact stars and black holes swallow up new material that is brought close to them, they eject a significant fraction of it in the form of narrow, high-speed plasma streams called jets. Only one type of object - dwarf novae – did not emit jets. Their uniqueness had been a long-standing puzzle and caused much speculation about what ingredients are truly necessary for jet production.

The team of astronomers have now found evidence for jets also in dwarf novae. The discovery means that jet production may be common to all mass-gaining astronomical objects and that a single physical mechanism may be responsible.

While white dwarfs, neutron stars and black holes are physically very different, they all interact with their surrounding gas mainly via gravity. On earth, astronomers can only observe these sources because some gas is gravitationally attracted by the central mass and falls towards the object. This process, called accretion, releases energy that is partially radiated in the form of observable light. Accretion of matter onto a compact object is a common and extremely efficient source of energy. It is the source of power of black hole X-ray binaries, neutron star X-ray binaries (XRBs) and active galactic nuclei (AGN, the supermassive black holes in the centres of galaxies).

For most accreting objects, jets can be observed originating from the source. The only major exceptions were thought to be dwarf novae.

For the first time, researchers were able to obtain a detailed radio light-curve of a dwarf nova in outburst. The source showed the strongest radio emission, used as a tracer for the jet, during the beginning of the outburst, as in XRBs. Additionally, the inferred jet power is in agreement with the predictions based on XRBs.

Dr Elmar Koerding, from the University of Southampton, comments: “This has large implications for the physical jet launching mechanism. It also opens the possibility to construct a unified model for all accretion powered jet sources. In addition to XRBs, AGN and accreting white dwarfs, this would then also include young stellar objects and gamma ray bursts.”

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