Astronomers see right into the heart of exploding star

An international team of astronomers has been able to see into the heart of an exploding star by combining data from telescopes that are hundreds or even thousands of kilometres apart.

An artist’s impression of the gas ejected in the nova explosion with the binary star system at the centre. This image illustrates how fast winds are funnelled along the binary star system’s poles.
An artist's impression of the gas ejected in the nova explosion with the binary star system at the centre. This image illustrates how fast winds are funnelled along the binary star system’s poles. Credit: Bill Saxton, NRAO/AUI/NSF.

Highly-detailed images produced using radio telescopes across Europe and America have pinpointed the locations where a stellar explosion, called a nova, emitted gamma rays (the most energetic form of electromagnetic waves). The discovery revealed how the gamma-ray emissions are produced, something which mystified astronomers when they were first observed in 2012.

"We not only found where the gamma rays came from, but also got a look at a previously-unseen scenario that may be common in other nova explosions," said Laura Chomiuk, of Michigan State University, lead author of the study published today in Nature.

Tim O'Brien of The University of Manchester's Jodrell Bank Observatory, one of the international team of astronomers who worked on the study, explains, "A nova occurs when gas from a companion star falls onto the surface of a white dwarf star in a binary system. This triggers a thermonuclear explosion on the surface of the star which blasts the gas into space at speeds of millions of miles per hour".

"When it explodes it brightens hugely, leading in some cases to the appearance of a new star in the sky, hence the term nova. These explosions are unpredictable, so when one goes off the pressure is on for us to try and get as many of the world's telescopes as possible to take a look before it fades away. For this nova, our international team was primed and ready to go and we really came up trumps."

Astronomers did not expect this nova scenario to produce high-energy gamma rays. However, in June of 2012, NASA's Fermi spacecraft detected gamma rays coming from a nova called V959 Mon, some 6500 light-years from Earth.

At the same time, observations with the Karl G. Jansky Very Large Array (VLA) in the USA indicated that radio waves coming from the nova were probably the result of subatomic particles moving at nearly the speed of light interacting with magnetic fields. The high-energy gamma-ray emission, the astronomers noted, also required such fast-moving particles.

Later observations from the telescopes of the European VLBI network (EVN) and the Very Long Baseline Array (VLBA) in the USA revealed two distinct knots of radio emission. These knots then were seen to move away from each other. This observation, along with studies made with the e-MERLIN telescope array in the UK, and further VLA observations in 2014, provided the scientists with information that allowed them to put together a picture of how the radio knots, and the gamma rays, were produced.

Artist's impressions of the gas ejected in the nova explosion with the binary star system at the centre. The images show how the common envelope interaction evolves with time.

A nova does not explode like an expanding ball, but instead throws out gas in different directions at different times and different speeds. When this gas inevitably crashes together, it produces shocks and high-energy gamma-ray photons. The complex explosion and gas collisions in nova V959 Mon is illustrated here. In the first days of the nova explosion, dense, relatively slow-moving material is expelled along the binary star system's equator (yellow material in left panel). Over the next several weeks, fast winds pick up and are blown off the binary, but they are funneled along the binary star system's poles (blue material in central panel). The equatorial and polar material crashes together at their intersection, producing shocks and gamma-ray emission (red regions in central panel). Finally, at later times, the nova stops blowing a wind, and the material drifts off into space, the fireworks finished (right panel).

Credit: Bill Saxton, NRAO/AUI/NSF

Larger images: left panel; central panel; right panel.

In the first stage of this scenario, the white dwarf and its companion give up some of their orbital energy to boost some of the explosion material, making the ejected material move outward faster in the plane of their orbit. Later, the white dwarf blows off a faster wind of particles moving mostly outward along the poles of the orbital plane. When the faster-moving polar flow hits the slower-moving material, the shock accelerates particles to the speeds needed to produce the gamma rays, and the knots of radio emission.

"By watching this system over time and seeing how the pattern of radio emission changed, then tracing the movements of the knots, we saw the exact behaviour expected from this scenario," Chomiuk said.

A technique called radio interferometry, in which data from various radio telescopes are combined to obtain a sharper image, played a fundamental role in this result. By connecting together radio telescopes across tens, hundreds and even thousands of kilometres, the scientists were able to zoom in to get a much sharper view of the heart of this exploding star.

The different distributions of individual telescopes in the three arrays used in this study provide different views of the nova. When combined these lead to a more complete picture of the phenomenon.

"Using telescope arrays with very different resolving power for the observations, we could successfully locate the particular areas where particles are accelerated to relativistic speeds within the broad outflow of hot gas that was ejected during the explosion. The sharpest-view EVN and VLBA data also showed how the relativistic ejecta expanded," Zsolt Paragi from the Joint Institute for VLBI in Europe added.

Gamma rays from several nova explosions have now been detected so it may be that the phenomenon is relatively common, but perhaps seen only when the nova is sufficiently close to Earth.

Because this type of ejection seen is also seen in other binary-star (two stars orbiting each other) systems, the new insights may help astronomers understand how those systems develop. The "common envelope" phase in which matter ejected from one star engulfs its companion occurs in all close binary stars, and is poorly understood.

"We may be able to use novae as a 'testbed' for improving our understanding of this critical stage of binary evolution," Chomiuk said.

Radio imaging of V959 Mon.
Radio imaging of V959 Mon.
Click for a full-size image.

(a) The EVN images from 91 days (contour lines) and 113 days (in colour) after the gamma-ray discovery. These show the compact radio knots expanding diagonally.

(b) e-MERLIN image of the ejecta from the nova explosion (in colour) from 87 days after discovery with the EVN conturs from day 91 superimposed.

(c) The same components as (b) but one month later, this time imaged with the VLA 126 days after discovery (in colour) and EVN at day 113 (contours).

(d) Shows how the nova ejecta have expanded and how the major axis of the radio emission flips from E-W (horizontal) to N-S (vertical) in an image made with the VLA after 615 days (in colour)with contours from the day 126 image of (c) for comparison.

Scale bars assume a distance of 1.5 kpc and the white cross is the presumed location of the central binary system.

Background notes

The research is published in Nature and is available online from October 08 2014:
Binary orbits as the driver of gamma-ray emission and mass ejection in classical novae
L. Chomiuk, J. D. Linford, J. Yang, T. J. O'Brien, Z. Paragi, A. J. Mioduszewski, R. J. Beswick, C. C. Cheung, K. Mukai, T. Nelson, V. A. R. M. Ribeiro, M. P. Rupen, J. L. Sokoloski, J. Weston, Y. Zheng, M. F. Bode, S. Eyres, N. Roy, G. B. Taylor. 
dx.doi.org/10.1038/nature13773

The US National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation of the USA, operated under cooperative agreement by Associated Universities, Inc.

e-MERLIN is an array of up to seven radio telescopes in the UK. It is operated from Jodrell Bank Observatory by The University of Manchester for the Science and Technology Facilities Council (STFC).

The European VLBI Network (EVN) is a collaboration of the major radio astronomical institutes in Europe, Asia and South Africa and performs high angular resolution observations of cosmic radio sources.

The Joint Institute for VLBI in Europe (JIVE) is a scientific foundation based in the Netherlands with a mandate to support the operations of the European VLBI Network.

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