SUPERNOVA REMNANTS, PULSARS AND THE INTERSTELLAR MEDIUM - SUMMARY OF A WORKSHOP HELD AT U SYDNEY, MARCH 1999
Vikram Dwarkadas , Lewis Ball , James Caswell , Anne Green , Simon Johnston , Brian Schmidt , Mark Wardle, PASA, 17 (1), 83.
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Supernova Remnants and the Surrounding Medium II

Miroslav Filipovic (U Western Sydney) presented evidence for a young, nearby SN remnant, RX J0852.0-4622, initially identified by its X-ray and $\gamma$-ray emission. He showed that the X-ray image obtained in the ROSAT all-sky survey shows a disk-like, partially limb-brightened emission region, which is the typical appearance of a shell-like SNR. The object's high temperature of > 3 x 10⁷K indicates that RXJ0852.0-4622 is a young object which must also be relatively nearby (because of its large angular diameter of 2$^\circ$). Comparison with historical SNRs limits the age to about $\sim$1,500 years and the distance to <1 kpc. Miroslav showed that any doubt of the identification of RX J0852.0-4622 as a SNR should be erased by the detection of $\gamma$-ray line emission from ⁴⁴Ti, which is produced almost exclusively in supernovae. Using the mean lifetime of ⁴⁴Ti (90.4 yrs), the angular diameter, adopting a mean expansion velocity of 5000 km/s, and a ⁴⁴Ti yield of 5 x 10 ^-5 M$_{\odot}$, Iyudin et al. (1998) derived an age of $\sim 680$ years and a distance of $\sim$ 200 pc, which argues that RX J0852.0-4622 (GRO J0852-4642) is the closest supernova to Earth to have occurred during recent human history. However, these observations are in apparent conflict with historical records. Miroslav also reported a positive radio-continuum detection at 4.75 GHz (PMN) which shows similarities to the X-ray emission. Further studies of this SNR will compare a mosaic radio-continuum survey to observations at other wavelengths such as the ROSAT and ASCA X-ray images and spectra (already observed) and UKST H$\alpha$, [SII] and [OIII] plates.

Vikram Dwarkadas (SRCfTA) presented work he, along with Roger Chevalier (UVa), is carrying out on SN-circumstellar interaction, motivated by the presence of a circumstellar bubble surrounding SN 1987A. The evolution of supernova remnants in circumstellar bubbles depends mainly on a single parameter - the ratio of the mass of the circumstellar shell to the mass of the ejecta (Franco et al. 1991). For low values, the supernova remnant, over many doubling times, eventually `forgets' about the existence of this shell, and the resulting density profile looks as it would have in the absence of the shell. Vikram showed that analytical approximations and numerical models indicate that the evolution becomes more rapid as this ratio increases, and that the amount of energy transmitted from the shock to the shell also increases. Unless the shell mass substantially exceeds the ejecta mass, reflected and transmitted shocks are formed when the SN shock hits the circumstellar shell. Vikram demonstrated that the shock-shell interface is hydrodynamically unstable. The reflected shock moves towards the center, and may rebound off the center. Eventually several shocks may be found criss-crossing the remnant, leading to a highly complicated interior structure, with more than one hydrodynamically unstable region possible (Dwarkadas 2000). A rise in X-ray emission accompanies each shock-shell collision. When applied to the observations of SN 1987A, the SN-circumstellar shell model, with appropriate modifications, confirms the prediction of the outgoing shock colliding with the circumstellar ring in about 2005 (Chevalier & Dwarkadas 1995).

Chris Wright (ADFA) presented work on ISO observations of the SN remnant RCW 103. This supernova remnant has been studied extensively in the past in the near-infrared (NIR) by Oliva et al. (1989, 1990 and 1999) who showed that the remnant blast wave is interacting with the interstellar medium and producing very bright emission in lines of [FeII] and H₂. The [FeII] emission coincides with the optical, radio and x-ray emission, but the H₂ emission occurs 20-30 arcseconds outside (i.e. in front) of it. This poses a problem in that standard shock excitation of H₂ predicts that the H₂ would reside either behind or coincident with the optical emission. Extinction arguments cannot be applied since the extinction to all of the optical, [FeII] and H₂ emission is independently observed to be the same. Further, the H₂ spectrum ``looks'' thermal. Therefore, x-rays have been proposed as a possible excitation mechanism. Chris presented ISO observations which covered a large suite of pure rotational and ro-vibrational H₂ lines, out to 28 microns, as well as lines of H, [NeII], [OIV] and [FeII] and the x-ray sensitive molecules H₃+ and HeH+. The latter two were not detected, and their upper limits may imply interesting constraints on the amount of x-ray heating. Many H₂ lines were detected, and the spectrum still appears to be shock (i.e. thermally) excited, although more modelling is required to determine the type of shock. However, there are several cases where the line appears to have a non-thermal component to it.

Amy Mioduszewski (SRCfTA) discussed simulating Radio Images from Numerical Hydrodynamic Models (Mioduszewski et al. 1997). She motivated her discussion by emphasising that while hydrodynamic simulations are widely used to understand objects such as supernovae or jets, the calculated pressure, density, and velocity must be linked to what is observed, the synchrotron radiation from the material. Assuming minimum energy, Amy demonstrated that the synchrotron emissivity and opacity can be related to the hydrodynamical pressure and the number density of the particles. Using these, she calculated the total synchrotron flux and created an "image" of the source. Amy also pointed out that in case of relativistic jets it is important to consider light travel time effects, because they significantly influence the appearance of the jets. In addition she showed that the simulated total intensity light curves, even of non-evolving jets, are not easily related to the relatively simple and regular shock structure in the underlying flow.

John Patterson (U Adelaide) discussed the potential for using very high energy gamma rays to understand the high energy astrophysical processes which occur in objects such as supernova remnants, gamma ray pulsars and AGN (BL Lacs), as well as the many unidentified EGRET ($\sim$1 GeV) sources. See Ong (1998) for a review of the field. As a leading member of the joint Australian-Japanese CANGAROO Project at Woomera, John is pushing the frontier of this ground-based observational area of photons with energies around 100 GeV. These high energy photons are produced in a variety of places by relativistic processes such as inverse Compton effect and shock acceleration. A new 10 m Cangaroo II telescope has been commissioned, and John warmly welcomes co-operation with other Australian facilities and universities.

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