Posted by: P. LaViolette, February 19, 2019
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. Between 1999 and 2008, its brightness was rising and falling by about ±4%. But now it has entered a period of steady decline which has perplexed astronomers; see diagram below.
The other thing that has puzzled astronomers is that on several occasions the Crab Nebula has had an x-ray/gamma ray flare, the most recent being in 2013 (https://www.wired.com/2010/12/crab-nebulas-violent-outbursts-shock-astronomers/, https://iopscience.iop.org/article/10.1088/2041-8205/775/2/L37/meta). The current standard theory is that the Crab is powered by cosmic rays from its pulsar. However, during these flares and during the progressive decline in high energy brightness, the Crab pulsar remained constant. This has greatly stressed the astronomical community. Since the luminosity of its pulsar has not changed, this indicates that the emission cannot be coming from the pulsar. The results instead support the superwave theory which predicts that the Crab Nebula is being energized by cosmic rays from a superwave that currently is passing through that region (LaViolette, 1983, LaViolette, 1987). The superwave theory also accounts for why brightness is declining faster at higher energies, indicating spectral softening. In a superwave, lower energy electrons lag behind the higher energy electrons because they are not traveling as fast. So the decline would first be noticed at higher frequencies.