Over The Horizon

Gamma Rays and Supernova

Alan Gilmore

The apparent coincidence of a gamma ray burst and a peculiar supernova is intriguing astronomers. In April, a gamma ray burst was detected by two satellites and immediate follow-up images by ground-based observatories showed a supernova rising in brightness near the region of the burst.

The supernova, designated 1998bw, was in a galaxy 140 million light years away and did not fit the standard types. It had no helium or hydrogen lines in its spectrum, thus it was not a Type Ib or II supernova. Nor did it show any silicon, ruling out Type Ia, even though its brightness was similar to that class. So it was was provisionally classified as a Type Ic.

Its changing brightness and colour over the following month could be explained by the collapse of the core of a massive star. Fitting such a model to the light curve required that the core collapse occur within one day of the gamma ray burst on April 25 (GRB 980425), in excellent agreement with the observations.

In a Type Ic supernova, the massive core of the star doesn't stop collapsing when it has crushed the atoms into tightly packed neutrons. Instead the crushing continues till the core becomes a black hole. Material left outside the black hole rapidly spirals in, releasing a large amount of energy.

Whether it was the source of the gamma ray burst or not, SN 1998bw is a peculiar kind of supernova anyway. Radio observations showed it to be intrinsically the brightest supernova ever observed at radio wavelengths.

It is only in the last 18 months that a few gamma ray bursts have been optically identified. Their spectra show that they are billions of light years away. On these scales, GRB 980425 was next door, yet the intensity of the burst measured at Earth was only comparable to the distant sources. Thus it was a very weak source, only a millionth as energetic as other GRBs.

The weakness of the gamma ray burst might be explained by beaming effects. The mechanisms for gamma ray emissions are still highly speculative but many involve a rapidly spinning disk of material being sucked into a black hole. GRB energy calculations have assumed that gamma rays are emitted uniformly in all directions. If the gamma rays are instead concentrated in narrow beams, then far less energy is required.

This might help explain the extraordinary burst observed last December from a source in a galaxy 10 billion light years away. If its energy was radiated uniformly, then it was 100 times more energetic than could be accounted for by a coalescing neutron star binary. However, if the rays happened to be beamed at us, then current theories might suffice.

The coincidence in time and position of SN 1998bw with GRB 980425 is so good that the chance they are not related is as low as 1 in 100,000. There does remain one small alternative possibility. Follow-up observations showed two X-ray sources, neither of which coincides with SN 1998bw. One was constant. The other was detected about a day after the burst and decayed in 22 hours. This behaviour is similar to previously observed X-ray afterglows of GRBs so it might be a possible counterpart for GRB 980425. But earlier surveys have also shown that variable X-ray sources are common.

Neither source was seen in red-light images, but then at least two of last year's GRBs were not seen optically either, despite good satellite positions.

If some GRBs are indeed caused by the collapse of massive stars then they will likely occur in star-forming regions. Massive stars evolve quickly, becoming supernovae before they are out of the nest. Star-forming regions are also very dusty, so it would not be surprising if some GRBs are hidden in dust clouds.

Thus it seems that GRBs may have more than one kind of source. Exciting discoveries and much speculation are bound to continue.

Alan Gilmore works at the Mt John Observatory in Tekapo