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 Physics , 1999, DOI: 10.1086/308031 Abstract: SN 1998bw is an unusual Type Ic supernova that may be associated with the $\gamma$-ray burst GRB 980425. We use a synchrotron self-absorption model for its radio emission to deduce that the synchrotron-emitting gas is expanding into a circumstellar medium of approximately $r^{-2}$ density profile, at a speed comparable to the speed of light. We assume that the efficiencies of production of relativistic electrons and magnetic field are constant through the evolution. The circumstellar density is consistent with that expected around the massive star core thought to be the progenitor of SN 1998bw. The explosion energy in material moving with velocity $>0.5c$ is $\sim 10^{49}- 3\times 10^{50}$ ergs, with some preference for the high values. The rise in the radio light curves observed at days 20-40 is inferred to be the result of a rise in the energy of the blast wave by a factor $\sim 2.5$. Interaction with a jump in the ambient density is not consistent with the observed evolution. We infer that the boost in energy is from a shell of matter from the explosion that catches up with the decelerating shock front. Both the high explosion energy and the nature of the energy input to the blast wave are difficult to reconcile with energy input from the shock-accelerated high velocity ejecta from a supernova. The implication is that there is irregular energy input from a central engine, which is the type of model invoked for normal $\gamma$-ray bursts. The link between SN 1998bw and GRB 980425 is thus strengthened.
 Physics , 2001, DOI: 10.1063/1.1419631 Abstract: The spatiotemporal coincidence of supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 and this supernova's unusual optical and radio properties have prompted many theoretical models that produce GRBs from supernovae. We review the salient features of our simple, spherical model in which an energetic supernova explosion shock accelerates a small fraction of the progenitor's stellar envelope to mildly relativistic velocities. This material carries sufficient energy to produce a weak GRB and a bright radio supernova through an external shock against a dense stellar wind.
 Physics , 2011, DOI: 10.1088/0004-6256/141/5/163 Abstract: We present multicolor light curves of SN 1998bw which appeared in ESO184-G82 in close temporal and spacial association with GRB 980425. They are based on observations done at Cerro Tololo Inter-American Observatory and data from the literature. The CTIO photometry reaches ~86 days after the GRB in $U$ and ~160 days after the GRB in BV(RI)_C. The observations in U extend by about 30 days the previously known coverage, and determine the slope of the early exponential tail. We calibrate a large set of local standards in common with those of previous studies and use them to transform published observations of the SN to our realization of the standard photometric system. We show that the photometry from different sources merges smoothly and provide a unified set of 300 observations of the SN in five bands. Using the extensive set of spectra in public domain we compute extinction and K corrections, and build quasi-bolometric unreddened rest frame light curves. We provide low degree piecewise spline fits to these light curves with daily sampling. They reach ~86 rest frame days after the GRB with U band coverage, and ~498 rest frame days after the GRB without U.
 Physics , 1999, Abstract: Some gamma ray bursts may be produced by supernovae exploding in close massive binary systems (type Ib/c supernovae) as suggested by the recent observation of SN 1998bw/GRB 980425. We propose that high energy radiation observed in such gamma ray bursts may be produced by synchrotron radiation of electrons accelerated by 1st order Fermi acceleration at a quasi-stationary shock in the high velocity SN ejecta colliding with the companion star or some other nearby massive object. Nuclei would also be accelerated, and could give rise to an observable fluence of high energy neutrinos at Earth. This paper has been withdrawn by the authors following evidence that SN 1998bw and GRB 980425 are two distinct sources.
 Physics , 1998, Abstract: The recent discovery of the unusual supernova SN1998bw and its apparent correlation with the gamma-ray burst GRB 980425 has raised new issues concerning both the GRB and SNe. SN1998bw was unusually bright at maximum light and expansion velocities were large making SN1998bw a possible candidate for a "hypernova" with explosion energies exceeding 10^{52} erg. We show that the light curve of SN1998bw can be understood as the result of an aspherical explosion along the rotational axis of a basically spherical, non-degenerate C/O core of a massive star with an explosion energy of 2E51 erg, a total ejecta mass of 2 M_o, and a 56Ni mass of 0.2M_o if it is seen from high inclinations with respect to the plane of symmetry.In this model, the high expansion velocities are a direct consequence of the aspherical explosion which, in turn, produces oblate iso-density contours. This suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.
 Physics , 1998, DOI: 10.1086/307521 Abstract: The recent discovery of the unusual supernova SN1998bw and its apparent correlation with the gamma-ray burst GRB 980425 has raised new issues concerning both the GRB and supernovae. Although the spectra resemble those of TypeIc supernovae, there are distinct differences at early times and SN1998bw appeared to be unusually bright and red at maximum light. The apparent expansion velocities inferred by the Doppler shift of (unidentified) absorption features appeared to be high, making SN1998bw a possible candidate for a "hypernova" with explosion energies between 20 and 50E51 erg and ejecta masses in excess of 6 - 15 M_o. Based on light curve calculations for aspherical explosions and guided by the polarization observations of "normal" SNIc and related events, we present an alternative picture that allows SN1998bw to have an explosion energy and ejecta mass consistent with core collapse supernovae (although at the 'bright' end). We show that the LC of SN1998bw can be understood as result of an aspherical explosion along the rotational axis of a basically spherical, non-degenerate C/O core of massive star with an explosion energy of 2foe and a total ejecta mass of 2 M_o if it is seen from high inclinations with respect to the plane of symmetry. In this model, the high expansion velocities are a direct consequence of an aspherical explosion which, in turn, produces oblate iso-density contours. It suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.
 Physics , 1999, DOI: 10.1086/316404 Abstract: We report 139 photometric observations through the B, V, and I filters of the supernova SN 1998bw, an object which is associated with the Gamma-Ray Burst GRB 980425. Detailed light curves of this unique supernova can be compared to theoretical models, so we report here our light curve for 123 days between 27 June 1998 and 28 October 1998. The light curve of SN 1988bw is consistent with those of the Type Ic class. We find that the magnitude-versus-time relation for this supernova is linear to within 0.05 mags in all colors over the entire duration of our study. Our measured uniform decline rates are $0.0141 \pm 0.0002$, $0.0184 \pm 0.0003$, and $0.0181 \pm 0.0003$ magnitudes per day in the B, V, and I bands. The linear decline and the rate of that decline suggest that late time light curve is powered by the radioactive decay of cobalt with some leakage of the gamma rays.
 Physics , 2000, DOI: 10.1086/319784 Abstract: A refined model for the unusual Type Ic supernova 1998bw, discovered as the optical counterpart of GRB980425, is presented, and synthetic light curves and spectra are compared with the observations. The first 30 days of the light curve and the broad line features of the spectra can be reproduced with the hydrodynamical model of the explosion of a 14$M_\odot$ C+O star, the core of a star with initial mass 40$M_\odot$, assuming that the explosion was very energetic (kinetic energy $E_{\rm K} = 5 \times 10^{52}$ erg) and that 0.4$M_\odot$ of $^{56}$Ni were synthesized. At late times, however, the observed light curve tail declines more slowly than this energetic model, and is in better agreement with a less energetic ($E_{\rm K} = 7 \times 10^{51}$ erg) one. This shift to a less energetic model may imply that the inner part of the ejecta has higher density and lower velocities than the spherically symmetric model with $E_{\rm K} = 5 \times 10^{52}$ erg, so that $\gamma$-rays deposit more efficiently. We suggest that an aspherical explosion can produce such a structure of the ejecta. We also study detailed nucleosynthesis calculations for hyper-energetic supernova explosions and compare the yields with those of normal supernovae.
 Physics , 2000, Abstract: Study of radio supernovae (RSNe) over the past 20 years includes two dozen detected objects and more than 100 upper limits. From this work we are able to identify classes of radio properties, demonstrate conformance to and deviations from existing models, estimate the density and structure of the circumstellar material and, by inference, the evolution of the presupernova stellar wind, and reveal the last stages of stellar evolution before explosion. It is also possible to detect ionized hydrogen along the line of sight, to demonstrate binary properties of the stellar system, and to show clumpiness of the circumstellar material. More speculatively, it may be possible to provide distance estimates to radio supernovae. The interesting and unusual radio supernova SN 1998bw, which is thought to be related to the gamma-ray burst GRB 980425, is discussed in particular detail. Its radio properties are compared and contrasted with those of other known RSNe.
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