A hypernova model for the supernova associated with the γ-ray burst of 25 April 1998

Abstract

Nature © Macmillan Publishers Ltd 19988 amounts of 56 Ni (ϳ0.7 solar masses) have to be synthesized in the explosion 16 ; the large energy and 56 Ni mass would be unprecedented for a core-collapse supernova.If one accepts the possibility that GRB980425 and SN1998bw are associated, one must conclude that GRB980425 is a rare type of GRB, and SN1998bw is a rare type of supernova. The radio properties 8,9 of SN1998bw show the peculiar nature of this event independent of whether or not it is associated with GRB980425.The consequence of an association is that the ␥-ray peak luminosity of GRB980425 is L ␥ ¼ ð5:5 Ϯ 0:7Þ ϫ 10 46 erg s −1 (in the 24-1,820 keV band) and its total ␥-ray energy budget is (8:1 ϫ 1:0Þ ϫ 10 47 erg. These values are much smaller than those of 'normal' GRBs which have peak luminosities of up to 10 52 erg s −1 and total energies 5 up to several times 10 53 erg. This implies that very different mechanisms can produce GRBs which cannot be distinguished on the basis of their ␥-ray properties, and that models explaining GRB980425/SN1998bw are unlikely to apply to 'normal' GRBs and vice versa. Ⅺ

Spectrum formation in superluminous supernovae (Type I)

“…SN 2011kl was much more luminous than classical GRB/SNe, but was not quite as luminous as SLSNe (Greiner et al 2015). Its light curve evolved over just a few weeks, as do GRB/SNe, but its spectrum (available only near maximum) did not show any of the broad lines that characterize GRB/SNe (see, e.g., Iwamoto et al 1998) or broad-lined SNe Ic (see, e.g., Mazzali et al 2013). The spectrum was blue, turning over only at ∼ 3000 Å, like SLSNe.…”

Classification of Supernovae

“…In addition to its stringent spatial and temporal association to GRB980425 [55,67], SN1998bw was peculiar in many respects. The spectrum at early phases was unprecedented and led to different classifications (Ib [106] or peculiar Ic [46,95]).…”

“…The spectrum at early phases was unprecedented and led to different classifications (Ib [106] or peculiar Ic [46,95]). SN1998bw was as bright as an SNIa and displayed expansion velocities as high as 3 × 10 4 km s −1 , suggesting that it was the result of an extremely energetic explosion (> 10 52 erg), even if different degrees of asymmetry and beaming allow for a broad range of values [65,67,139]. Its very powerful radio emission has been interpreted as due to the presence of a mildly relativistic blast wave interacting with a clumpy, structured CSM deriving from a complex mass-loss history (see the chapter by Weiler et al in this volume and [133]).…”

Optical Observations of Core-Collapse Supernovae

“…Probably the most compelling example thus far is that of SN 1998bw and GRB 980425 (e.g., [77][78][79][80]), which were temporally and spatially coincident. SN 1998bw was, in many ways, an extraordinary SN; it was very luminous at optical and radio wavelengths, and it showed evidence for relativistic outflow.…”