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Radio Supernova SN 1998bw and Its Relation to GRB 980425  [PDF]
Zhi-Yun Li,Roger A. Chevalier
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.
The Trans-Relativistic Blast Wave Model for SN 1998bw and GRB 980425  [PDF]
Jonathan C. Tan,Christopher D. Matzner,Christopher F. McKee
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.
The Ultimate Light Curve of SN 1998bw/GRB 980425  [PDF]
Alejandro Clocchiatti,Nicholas B. Suntzeff,Ricardo Covarrubias,Pablo Candia
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.
SN 1998bw--GRB 980425: A supernova explosion with a close massive companion?  [PDF]
R. J. Protheroe,W. Bednarek
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.
SN 1998bw/GRB 980425: Hypernova or Aspherical Explosion?  [PDF]
P. Hoeflich,J. C. Wheeler,L. Wang
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.
Aspherical Explosion Models for SN 1998bw/GRB 980425  [PDF]
P. Hoeflich,J. C. Wheeler,L. Wang
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.
The Late Time Light Curve of SN 1998bw Associated with GRB980425  [PDF]
Eric H. McKenzie,Bradley E. Schaefer
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.
Light Curve and Spectral Models for the Hypernova SN 1998bw associated with GRB980425  [PDF]
Takayoshi Nakamura,Paolo A. Mazzali,Ken'ichi Nomoto,Koichi Iwamoto
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.
SN1998bw/GRB980425 and Radio Supernovae  [PDF]
Kurt W. Weiler,Nino Panagia,Marcos J. Montes
Physics , 2001, DOI: 10.1086/322359
Abstract: The unusual supernova SN1998bw, which is thought to be related to the gamma-ray burster GRB980425, is a possible link between the two classes of objects. Analyzing the extensive radio emission data avaliable for SN1998bw, we are able to describe its time evolution within the well established framework available for the analysis of radio emission from supernovae. This then allows description of a number of physical properties of the object. The radio emission can best be explained as interaction of a mildly relativistic (Gamma about 1.6) shock with a dense pre-explosion stellar wind established circumstellar medium (CSM) which is highly structured both azimuthally, in clumps or filaments, and radially, with two observed density enhancements separated by about 3e17 cm. With assumptions as to pre-explosion stellar wind conditions, it is possible to estimate that the progenitor to SN 1998bw had a mass loss rate of about 3.5e-5 solar masses per yr with at least two approximately 30% increases in mass-loss rate; the most recent extending from about 1,600 - 4,700 yr before explosion and the oldest known having occurred, with possibly comparable length, about 12,000 yr before explosion. Because of its unusual characteristics for a Type Ib/c SN, the relation of SN1998bw to GRB980425 is strengthened with consequent improvement in our understanding of these poorly understood objects.
Radio Supernovae and GRB 980425  [PDF]
Kurt W. Weiler,Nino Panagia,Richard A. Sramek,Schuyler D. Van Dyk,Marcos J. Montes,Christina K. Lacey
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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