Supernova Features

There is now general consensus that the long/soft GRBs are intimately connected to the death of massive stars. Massive stars undergo core-collapse, and about 70% of core-collapse supernovae are those of type II. One of the peculiar sub-classes that form part of the other 30% are type Ib/c supernovae.

While the supernova-GRB connection has been proposed some years ago [45], the unambiguous spectroscopic identification of the lowest redshift GRBs as supernovae during the last 2 yrs has provided convincing evidence for this picture [40, 87] (Fig. 24.7). The supernovae in the four spectroscopically confirmed gamma-ray bursts GRB 980425/SN 1998bw, 030329/2003dh, 031203/2003lw, and 060218/2006aj are all of type Ib/c, with unusually large kinetic energy (evidenced by very large expansion velocities of order 10-30 thousand kilometers per second after 10 days) and ejected mass of radioactive 56Ni (subsequently called hypernovae [68]). In particular this last property suggests progenitors with masses >20-30 Mq [65]. Theoretically, supernovae Ib/c are favored over type II because Ib/c have typically smaller envelope masses, and are thus thought to allow easier breakout of the jet, and subsequent GRB production. In fact, the lack of hydrogen lines in the spectra is consistent with model expectations that the progenitor star lost its hydrogen envelope to become a Wolf-Rayet star before collapsing.

Rest Wavelength (nm)

Fig. 24.7 Evolution of the optical spectrum of GRB 030329 showing the obvious signatures of a SN Ic, and compared to the spectra of SN1998bw (dotted lines). The age of the supernova is given on the right side (from [40])

Rest Wavelength (nm)

Fig. 24.7 Evolution of the optical spectrum of GRB 030329 showing the obvious signatures of a SN Ic, and compared to the spectra of SN1998bw (dotted lines). The age of the supernova is given on the right side (from [40])

980703

■■/"•■'- ■ "m. V •

■ Sí. ■ ?

- ■ 4 . 1 -

■v.- - i: . > . ;•

.■■ f . , .' : . ' 4 . * ' , .

990712

991208

> 991216

000418

000926

&

®

: t, & -: r ^

t

' « , r-,.1,- . ;

010921

011030

*

'M

Fig. 24.8 Selection of HST images of GRB host galaxies, adapted from [17]. The circles mark the center of the respective galaxy and not the position of the GRB in that galaxy. GRB host galaxies are generally very compact, even for HST's high spatial resolution (image size is ~4"x4")

Fig. 24.8 Selection of HST images of GRB host galaxies, adapted from [17]. The circles mark the center of the respective galaxy and not the position of the GRB in that galaxy. GRB host galaxies are generally very compact, even for HST's high spatial resolution (image size is ~4"x4")

In contrast to these similar spectroscopic properties, the y-ray emission properties of these events are quite different with respect to total emitted energy, temporal profile, and spectral shape, suggesting that the y-ray properties are governed by other properties than the progenitor mass.

Rebrightenings in the light curve at (8-20)(1 + z) days after the GRB are highly suggestive, though not unequivocal evidence for an associated supernova also in several GRBs at higher redshift. An estimate of GRB and hypernova rates in the Universe shows that they are statistically equal [69].

Was this article helpful?

0 0

Post a comment