I don’t want to leave the subject of gamma ray bursts (GRBs) without considering findings that seem to reduce the potential threat from these events. And the revision of a significant GRB paper that I meant to discuss earlier gives me the chance to circle back around to it. The subject is intriguing because it bears on the spread of life in the cosmos. If gamma ray bursts — powerful flashes of energy emitted in narrow jets — are nearby, an evolving species might be destroyed before it could ever achieve sentience, much less technology.
Krzysztof Stanek (Ohio State University) and collaborators approach the GRB question assuming that long gamma ray bursts (two seconds or more in duration) result from the death of massive stars. They also note two further facts about the unusual events. GRBs are highly beamed, and the supernovae remnants they leave behind are deficient in both hydrogen and helium in their spectra. And then we add this: Compared to average galaxies, those hosting GRBs appear to be much less luminous.
Using these findings, Stanek studies five low redshift GRBs, so called ‘local’ bursts, all of which were followed by well documented supernovae. The word ‘local’ is, of course, by comparison only. As opposed to far more distant GRBs, the highest redshift in this sample corresponds to a look back time of about 2/3 the age of the Earth. “With five well-studied events at hand,” the authors write, “for the first time there are enough data in this interesting redshift range to make a direct and statistically significant empirical study.”
The findings: Gamma ray bursts occur only in metal-poor environments. Large spiral galaxies like the Milky Way have been too metal-rich to host a GRB for several billion years. That seems to rule out the scenario of a nearby GRB (a few kiloparsecs away) causing mass extinctions on Earth. From the paper:
Our results make this scenario most unlikely — by the time the Earth formed, the Milky Way disk was already too metal-rich to host a long GRB. SN 1998bw/GRB 980425, the only local event to happen in a fairly metal-enriched galaxy, was also by far the weakest localized GRB ever, with at least 10,000 times lower energy than a typical z ~ 1 GRB [the value z is the redshift parameter]. As such, it would not cause mass extinction at several kpc from Earth. The same can be said about short GRBs, which are not only less frequent than long GRBs…but also less energetic and less beamed… Short GRBs are also not concentrated to star-forming regions, thus on average they are much further away from any life-hosting planets…
Something else to consider is that planet-forming stars tend to be even more metal rich than our Sun, making long GRBs an unlikely factor in life extinction events anywhere near us. “So to finish with a bit of good news,” write the authors, “we can probably cross GRBs off the rather long list of things that could cause humankind to ‘join the dinosaurs’ on the extinct species list.”
Sharp-eyed readers will remember Kris Stanek’s earlier work on the Hubble Constant, suggesting that it be revised based on his team’s studies of the Triangulum Galaxy (M33). The current paper is Stanek et al., “Protecting Life in the Milky Way: Metals Keep the GRBs Away,” slated for publication in Acta Astronautica and available in preprint form online. It adds to the growing consensus about GRBs as a threat, though as we’ve recently seen, just what kind of supernova causes long-duration GRBs is still controversial.