When you’re calculating the odds on life in any region of the galaxy, the rate of supernova explosions comes into play. As we saw yesterday, one factor Nikos Prantzos examined in his recent work on the galactic habitable zone was the effect that hard radiation could have on exposed land life. But what about gamma ray bursts (GRBs)? They’re more powerful and, although rarer than supernovae, can create beamed energy that makes them lethal from larger distances.
One theory is that because gamma ray bursts are associated with regions of low metallicity outside our galaxy, their frequency in the Milky Way is now close to zero. But a reminder of how little we actually know comes in the December 21 Nature, where four papers discuss GRB activity, and in particular a burst picked up by NASA’s Swift satellite last June 14. It’s a cosmic oddity, a kind of hybrid that probably marks the birth of a black hole. But, as Derek Fox (Penn State) says, “This burst — unlike all other long gamma-ray bursts we have seen at close distance — was not accompanied by a supernova, for reasons we do not yet fully understand.”
The current dichotomy for gamma ray bursts runs something like this: long GRBs last more than two seconds and appear to mark the birth of a black hole resulting from a supernova explosion. Short GRBs may be as brief as several milliseconds, and apparently mark the merger of two neutron stars, or a neutron star and a black hole.
The trick with the recent GRB 060614 burst is that it was a long one — 102 seconds — from a place (some 1.6 billion light years away in the constellation Indus) where star formation rates are low and few stars are massive enough to produce supernovae. Moreover, no trace of a supernova can be found there. “This burst was close enough to detect a supernova if it existed,” said Avishay Gal-Yam (Caltech), lead author on one of the Nature papers, “but even Hubble didn’t see anything.”
Equally interesting is GRB 060505, a burst detected last May. That one, originating in a galaxy 1 billion light years away in the direction of the constellation Piscis Austrinus, also left no supernova remnant.
Maybe the merger model needs an overhaul. Or if supernovae were involved in both events, then they may have formed black holes that allowed absolutely no matter to escape, the usual supernova ejecta being completely consumed. In any case, some new process seems to be at work in the two bursts, one that calls into question our understanding of GRBs and therefore our ability to predict their future frequency with accuracy. And it underscores the difficulty in estimating the long-term effects of GRBs on possible extraterrestrial life.
The paper referenced above is Gal-Yam et al., “A novel explosive process is required for the bold gamma-ray burst GRB 060614,” Nature 444 (21 December 2006), pp. 1053-1055 (abstract here). Three other short papers discuss the recent GRB findings in the same issue.