Vikram Dwarkadas , Lewis Ball , James Caswell , Anne Green , Simon Johnston , Brian Schmidt , Mark Wardle, PASA, 17 (1), 83.
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Title/Abstract Page: SUPERNOVA REMNANTS, PULSARS AND
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Exotics I
Much of this session was devoted to a discussion of Gamma-Ray bursts (GRBs) and their relationship to supernovae. Ron Ekers (ATNF) set the pace with an overview of GRB 980425 and its relationship to SN 1998bw. The 20s outburst was detected by BATSE and localised using Beppo Sax. Information was quickly distributed on the GRB Coordinates Network (GCN). Like most well-localised bursts, follow-up radio observations were carried out using the VLA/ATCA telescopes. About 30% of GRBs have been detected in the radio, and about 50% in the optical. It is interesting that detection in the radio is always accompanied by the detection of the optical transient. The detection of a SN within the BeppoSax error box, which has a chance probability of one in 104, led to the suspicion that the GRB and SN were related. If so the SN is the most luminous radio SN known, and the light curve is quite peculiar. The
-ray luminosity was about 1041 J. Ron remarked that there had been a suggestion by Paczynski a few years ago postulating that GRBs could be the result of hypernovae, so this was one case where theory might have predicted the observation. Ray Stathakis (AAO) presented the results of a cooperative spectral monitoring campaign of SN 1998bw, carried out on the AAT, UKST and MSSSO 2.3m, between 11 and 106 days after the Gamma-Ray Burst (Stathakis et al. 1999). The spectra showed no H, He or Si lines, thus making it a Type Ic SN. They consisted of broad emission and absorption features which slowly evolved over the period. SN 1998bw had entered the supernebular phase by day 106 with the appearance of nebular emission lines. In comparison to a typical Ic supernova, SN 1994ai, SN 1998bw was much bluer, and the features were broader and more distinct at early times. However, transitions and spectral evolution seen appeared similar, confirming SN 1998bw as a peculiar type Ic supernova. While the broader lines ( 45% broader than classical supernovae at similar epochs) explain much of the peculiarities of the spectra of SN 1998bw, there is some indication that additional contribution from line species such as nitrogen, carbon or titanium may be needed to reproduce the observations.
Following this, the GRB 990123 was reviewed by Brian Boyle (AAO). This GRB was first detected by BATSE, and the burst was of 90s duration. Its brightness was in the top 0.3% of all BATSE sources. Optical observations were carried out within 22 secs of the burst by the ROTSE telescope. Followup Keck spectra were featureless, apart from some absorption lines, arising perhaps from a foreground galaxy at z=1.6. The peak V Mag was 8.6, and the total estimated energy in rays was about 3.4 x 1047 J. The luminosity of the optical transient was about 3.3 x 1016 L
. The host galaxy appears to be a blue star-forming galaxy, in common with many any other GRB hosts. Brian also summarised briefly some of the theory of GRBs and the afterglows. The optical decay can be approximated by three different power laws, due perhaps to the reverse shock and the forward shocks. The second break may be a signature of beaming effects.
GX 1+4, a low-mass X-ray pulsar toward the galactic centre, was observed by Duncan Galloway (UTas/SRCfTA) with the Rossi X-ray Timing Explorer (RXTE) satellite during July 1996, 
10 days before a short-lived ``torque reversal'' event. Persistent pulsars such as GX 1+4 typically exhibit no correlation between luminosity (and hence mass accretion) and spin-up or spin-down rates, contrary to predictions of existing models. These sources are often found in ``torque states'', where the spin-up or spin-down rate is almost constant over time-scales of up to 10 years, with torque reversals occurring irregularly between states. Often the spin-up and spin-down torques are similar in magnitude.During the RXTE observation significant variations in the mean spectrum and the pulse profile were observed over time-scales of a few hours. Variations of this type have not previously been observed on such short time-scales, and it is suggested that these phenomena may be related to the (as yet unknown) mechanism causing the torque reversals (Galloway et al. 1999; Giles et al. 1999).
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