Supernova 1987A is the only supernova visible to the naked eye to explode nearby in the 400 years since the invention of the telescope. Lewis Ball and John Kirk have continued modelling the radio emission from this fascinating supernova.
Stars spend most of their lifetime burning hydrogen into helium. When the hydrogen fuel in the centre of a star is exhausted, the star dies leaving behind either a white dwarf or a neutron star - known collectively as compact stars. Relatively light stars whose mass is less than about six times that of our Sun, go out with a whimper, throwing off excess mass to become white dwarfs. More massive stars go out with a bang in a huge explosion called a supernova (SN). A neutron star - or perhaps a black hole - slightly more massive than our Sun is left behind by the explosion, but most of the mass is thrown off at very high speeds (up to at least 30,000 km per sec). The material thrown off by the explosion drives a strong pressure wave, or shock, out through the material which surrounded the star before it exploded. The shock accelerates particles to relativistic energies. These relativistic particles emit radio waves through synchrotron emission as they spiral around the magnetic field lines. Supernova remnants (SNRs) - the products of a supernova seen long after (100-100,000 years) the explosion itself - are most easily detected as a result of their radio emission. Many SNRs also emit in the optical and X-ray bands due to the heating of gas by the shock.
Supernova 1987A first appeared in the southern sky on 23 February 1987. It was produced by the explosion of the star Sanduleak -69deg202 in one of our nearest neighbour galaxies, the Large Magellanic Cloud. It captured the interest of the public as well as that of professional and amateur astronomers. It is the only visible supernova to explode nearby in the 400 years since the invention of the telescope. It was visible to the naked eye, even against Sydney's bright night sky.
Due to the position of SN1987A in the southern sky, Australia has essentially the only world-class radio telescopes which can observe it. Observations with these telescopes showed a prompt phase of radio emission which began almost immediately after the explosion, peaked about four days later, then decayed and became undetectable after just a few weeks. A second phase of radio emission from SN1987A, started in June 1990, some 3 and a half years after the explosion, and continues to brighten steadily.
Lewis Ball and Dr John Kirk (Max-Planck-Institute for Nuclear Physics, MPIK, Heidelberg) have continued their work on the modelling of SN1987A. In 1991 they suggested that the second phase of radio emission is due to electrons accelerated at the supernova shock via the mechanism called `diffusive shock acceleration'.
The modelling of the radio emission suggests that the electrons are accelerated by a shock which is very much weaker than expected from a supernova explosion. Lewis and John have suggested that this may be due to the modification of the shock by other particles (mainly protons) which are also accelerated. The accelerated protons contribute significantly to the pressure in front of the shock, smoothing out the pressure jump seen by the electrons. This process is being investigated in collaboration with Dr Peter Duffy (also from MPIK), and it may prove important for shock acceleration in this and many other environments.
High-resolution images of the radio emission obtained during 1993 with the Australia Telescope Compact Array by Dr Lister Staveley-Smith (Australia Telescope National Facility) have shown details of the structure of the radio source for the first time. These images have prompted a collaboration between Lewis Ball, Lister Staveley-Smith and Duncan Campbell-Wilson (Astrophysics Department, Sydney University) to attempt to model the radio emission using a specific source geometry which is consistent with the high-resolution radio images. Once the geometry is chosen, the effects of delays due to travel time can be included in the calculations of the synchrotron emission. Radio emission from a distant part of the radio source is delayed in the sense that it reaches the Earth later than radiation emitted at the same time from less distant parts of the source. An example of a possible model geometry is shown in the left panel of Figure 2. The model consists of two components: a shell and a ring. A preliminary attempt to account for the observed increase of the radio emission with time, including the travel time in this geometry, are shown in the right panel of Figure 2. The good fit between theory and observation is very encouraging.
Figure: A possible source geometry (left) for the radio emission from SN1987A,
and (right) a corresponding tentative model fit (solid line)
to the observed radio emission (crosses).
A related aspect of the research in the RCfTA is into the fundamental theory of acceleration processes at shocks. Three specific aspects of the theory were considered in 1993. One involved Professor Bram Achterberg (University of Utrecht) and Lewis Ball, who considered diffusive shock acceleration in the case where the shock is propagating nearly perpendicular to the magnetic field lines, so that the intersection of a field line with the shock is moving faster than the speed of light. This condition should apply when a spherical shock propagates into the spiral magnetic structure of the wind of the progenitor star. Such a model is thought relevant for SN1987A, and for the initial phase of radio emission from SN1978K, another unusual radio supernova which was the subject of an RCfTA workshop in 1992. The second project, which involved Bram Achterberg, Colin Norman and Don Melrose, concerned the acceleration of the highest energy cosmic rays. Limits on the maximum energy possible for diffusive shock acceleration were explored. It was concluded that the maximum energy achievable for shocks associated with quasars fail to account for the highest energy cosmic rays if these are protons, as the latest data imply. The third project concerned shock drift acceleration which was investigated by Honours year student Arthur Street, supervised by Lewis Ball and Don Melrose. Arthur explored the application of the shock drift mechanism, due to the electric field in the shock, to the acceleration of the energetic electrons that produce so-called herringbone radio bursts from shocks in the solar wind.