Gamma‐Ray Bursts and Type Ic Supernova SN 1998bw

Abstract

Recently a Type Ic supernova, SN 1998bw, was discovered coincident with a gamma-ray burst, GRB 980425. The supernova had unusual radio, optical, and spectroscopic properties. Among other things, it was especially bright for a Type Ic and rose quickly to maximum. When modeled in the usual way as a spherically symmetric explosion, this requires a large mass of 56 Ni, 0.45 -0.60 M ⊙ , a quite massive star, and a very large explosion energy. We explore here models based upon helium stars in the range 9 -14 M ⊙ and carbon-oxygen stars 6 -11 M ⊙ which experience unusually energetic explosions (kinetic energy 0.5 − 2.8 × 10 52 erg). Bolometric light curves and multi-band photometry are calculated and compared favorably with observations. No spectroscopic data are available at this time, but both LTE and non-LTE spectra are calculated for the model that agrees best with the light curve, a carbon-oxygen core of 6 M ⊙ exploded with a kinetic energy of 2.2 × 10 52 erg. We also examine potential mechanisms for producing the observed gamma-ray burst (GRB)shock break-out and relativistic shock deceleration in circumstellar material. For spherically symmetric models, both fail to produce a GRB of even the low luminosity inferred for GRB 980425. The high explosion energies required to understand the supernova are in contrast to what is expected for such massive stars and may indicate that a new sort of explosion has been identified, possibly the consequence of a collapsar (Woosley 1993(Woosley , 1996 whose main sequence mass was ∼ 35 M ⊙ (helium core mass 14 M ⊙ ). Indeed a more likely explanation for what was seen is a highly asymmetric explosion in which the GRB was produced by a relativistic jet, perhaps viewed obliquely, and only a fraction of the total stellar mass was ejected, the remainder accreting into a black hole. The ejected mass (but not the 56 Ni mass), explosion energy, and velocities may then be

Classification of Supernovae

“…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]).…”

“…In particular, SN1997ef was possibly associated with GRB971115 [131] and, although it was fainter than SN1998bw, its kinetic energy was close to that of SN1998bw [83]. The case of the bright SNIc 1992ar [37], which occurred at about two σ from the position of GRB920616 [139], is also very interesting.…”

“…Additional SNe possibly associated with GRBs are SN1997cy, (GRB970514 [59,139]), observationally classified as SNIIn and possibly the brightest SN ever observed [123], and its twin SN1999E [26] (GRB980910 [118]). As in the case of the type IIn SN1988Z, they show evidence for strong ejecta-CSM interaction.…”

Spectropolarimetric observations of supernovae

“…Sparking even more interest in SN morphology is the strong spatial and temporal association between some "hypernovae" (SNe with early-time spectra characterized by unusually broad line features; see K. Maeda's contribution to these Proceedings) and gamma-ray bursts (GRBs; e.g., Galama et al 1998;Iwamoto et al 1998;Woosley, Eastman, & Schmidt 1999;Stanek et al 2003;Hjorth et al 2003). These associations have fueled the proposition that some (or, perhaps all) core-collapse SNe explode due to the action of a "bipolar" jet of material (Wheeler, Meier, & Wilson 2002;Khokhlov et al 1999;MacFadyen & Woosley 1999), as opposed to the conventional neutrino-driven mechanism (Colgate & White 1966;Burrows et al 2000, and references therein).…”

Circumstellar Interaction Around Supernovae

“…SN 1998bw also had an unusual optical spectrum, with velocities in lines extending out to ∼ 60, 000 km s −1 [22]. The interpretation of the supernova light curve and spectra led to an explosion energy estimate of (2 − 4) × 10 52 ergs, extraordinarily high for a supernova [31,61]. Other Type Ic supernovae have been discovered with broad lines in their spectra, including SN 1997ef, SN 2002ap, and SN 2002bl.…”