The Metamorphosis of SN 1998bw

Abstract

We present and discuss the photometric and spectroscopic evolution of the peculiar SN 1998bw, associated with GRB 980425, through an analysis of optical and near-IR data collected at ESOÈLa Silla. The spectroscopic data, spanning the period from day [9 to day ]376 (relative to B maximum), have shown that this supernova (SN) was unprecedented, although somewhat similar to SN 1997ef. Maximum expansion velocities as high as 3 ] 104 km s~1 to some extent mask its resemblance to other Type Ic SNe. At intermediate phases, between photospheric and fully nebular, the expansion velocities (D104 km s~1) remained exceptionally high compared to those of other recorded core-collapse SNe at a similar phase. The mild linear polarization detected at early epochs suggests the presence of asymmetry in the emitting material. The degree of asymmetry, however, cannot be decoded from these measurements alone. The He I 1.083 and 2.058 km lines are identiÐed, and He is suggested to lie in an outer region of the envelope. The temporal behavior of the Ñuxes and proÐles of emission lines of Mg I] j4571, [O I] jj6300, 6364, and a feature ascribed to Fe are traced to stimulate future modeling work. The uniqueness of SN 1998bw became less obvious once it entered the fully nebular phase (after 1 yr), when it was very similar to other Type Ib/cÈIIb objects, such as the Type Ib SN 1996N and the Type IIb SN 1993J, even though SN 1998bw was 1.4 mag brighter than SN 1993J and 3 mag brighter than SN 1996N at a comparable phase. The late-phase optical photometry, which extends up to 403 days after B maximum, shows that the SN luminosity declined exponentially but substantially faster than the decay rate of 56Co. The ultraviolet-optical-infrared bolometric light curve, constructed using all available optical data and the early JHK photometry presented in this work, shows a slight Ñattening starting on about day ]300. Since no clear evidence of ejecta-wind interaction was found in the late-time spectroscopy (see also the work of Sollerman and coworkers), this may be due to the contribution of the positrons since most c-rays escape thermalization at this phase. A contribution from the superposed H II region cannot, however, be excluded.

Classification of Supernovae

“…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]). The complete dataset of optical and IR observations spanning over one year after the explosion has recently appeared [96]. The analysis of the late time spectra [84] seems to confirm that the explosion was asymmetric as suggested by polarimetry [65].…”

Evidence for Magnetar Formation in Broad-lined Type Ic Supernovae1998bw and 2002ap

“…Sollerman et al (2002) attributed this difference to different background subtraction strategy. Third, Patat et al (2001) adopted a larger distance to SN 1998bw (37.8 Mpc) than Sollerman et al (2002) (35 Mpc). Finally, because of the missing of measurements in IR band, these two groups of authors assumed different contribution from IR band to the bolometric luminosities.…”

“…Thanks to its proximity, only 9.4 Mpc in distance, high quality observational data were acquired until 580 days after explosion, which is prerequisite for the identification of late excess over the exponential 56 Co decay. SN 1998bw (Galama et al 1998;McKenzie & Schaefer 1999;Sollerman et al 2000;Patat et al 2001;Clocchiatti et al 2011), on the other hand, is the nearest SN associated with a GRB (Cano et al 2016). Although at a distance greater than SN 2002ap, its brighter luminosity qualifies SN 1998bw as an ideal observational target and its light curve was measured to ∼ 1000 days post explosion (Sollerman et al 2002).…”

“…In view of this fact, we adopt the measurement error in photospheric velocity as half of the measured value. 6 The data points for SN 1998bw up to 506 days after explosion, including the UVOIR bolometric luminosity data and photospheric velocity data, are taken from Patat et al (2001). 7 (2002), we do not try to fit the data from Sollerman et al (2002).…”

“…The fact that the data points during this period were well sampled by Patat et al (2001) on the one hand, and Clocchiatti et al (2011) only provided UBV(RI) C band observations up to 85 days, on the other, makes us decide to choose the data of Patat et al (2001) in this work.…”