It used to be that the Crab nebula's x-ray and gamma ray brightness was so constant that astronomers used it as a "standard candle" to assess the brightness of other sources in the sky. However, continued observation now shows that its brightness varies over the years. In 2011, astronomers reported finding that the hard x-ray and gamma ray flux from the Crab Nebula was declining by 3.5% per year.
New evidence has come out that definitively supports the solar theory of the Pleistocene megafaunal extinction and puts to rest the idea of a comet impact. Before addressing that, let's review the various extraterrestrial cause theories that are out there that attempt to explain this. One theory, first proposed by myself in 1983 (PhD dissertation) suggests that the Sun as the cause, where the solar activity was aggravated by the incursion into the solar system of cosmic dust.