Why gamma ray bursts are not standard candles

GRB noncandles

A revolution is currently underway in our knowledge of gamma ray bursts thanks to NASA’s Swift telescope which has been looking out for them from its perch in orbit since 2004.

But in the wild enthusiasm to embrace the firehose of data that Swift is sending back, it looks as if astronomers have made a basic mistake in their analysis, says Li-Xin Li, a theorist at the Max-Plank Institute for Astrophysics in Garching, Germany.

Gamma ray bursts emit a huge amount of energy in a very short period of time. One of the peculiar discoveries in the Swift data is that the total energy emitted in a burst has a very narrow distribution of around 10^44 Joules.

A number of astrophysicists say this is evidence that GRBs are standard candles. In other words, that GRBs all emit the same amount of energy so measuring their brightness gives you a good idea of their distance.

Cosmological distances are notoriously tricky to measure so finding a new standard candle is a major discovery for astronomers.

Sadly, there is a mistake in this reasoning, say Li. He reckons that astronomers have failed to account for a serious problem in the GRB data known as Malmquist-type selection bias that is well known in other types of astronomical surveys.

The bias comes about because observers see only the brightest GRBs from the most distant parts of the universe because dimmer ones are undetectable at present. So the number of faint GRBs is significantly underestimated.

Re-examine the data with this in mind and the notion of GRBs as standard candles rapidly begins to look untenable.

Still, it was a nice idea while it lasted.

Ref: arxiv.org/abs/0806.2770: Are Gamma-Ray Bursts a Standard Energy Reservoir?

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