Newly Discovered Gamma-Ray Effect Is Not Predicted By Theories

Artist's impression of a gamma-ray burst. ESO/A. Roquette

Astronomers have observed a curious light phenomenon taking place after the emission of gamma-ray bursts (GRBs), the brightest known explosions in the universe. According to the new study, we might have missed this effect due to the limitations of our instruments.

Although extremely bright, GRBs are not easy to observe. Gamma rays are incredibly powerful and penetrative, and the telescopes we use need to be able to detect and record them. Our instruments have limited sensitivity and, according to the researchers, this has caused us to miss some features of GRB emissions.

To get around this, the team studied the light from six very bright GRBs and, as reported in The Astrophysical Journal, they spotted a peculiar behavior. The emission of the GRBs had a so-called complex time-reversible wavelike structure. It looked like the light from the explosion was repeating itself but backward.

While the phrase "time-reversal" is always exciting, there's no weird physics at work here, say the scientists. The curious phenomenon is created by either the blast wave itself or by some ejected material interacting with the surrounding medium. These interactions radiate light and as the GRB burst expands (or if it interacts with symmetric clouds) this light is reflected, making it appear as if the reflection is going the wrong way in time.

The team could only see these effects in the brightest GRBs they looked at, which appeared as three-peak light curves. For the ones that couldn’t be looked at with enough sensitivity, the researchers could only see a single bump.

The researchers were intrigued because in the theoretical framework for gamma-ray bursts, this light repetition isn't expected. The team believes that this feature is likely common in most GRBs, so it will be interesting to see if new observations confirm this. The researchers believe that much can be understood by studying these new features. There’s a lot left to find out about GRBs.

GRBs are believed to form in extreme events that lead to the creation of a black hole, such as particularly powerful supernovae or neutron star collisions. They can last from just a few seconds to a few minutes. The emission is in the form of a narrow beam, and while it can easily travel for billions of light-years, we can only spot it if it is directed right at us – yet another reason why GRBs are so difficult to study.

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