Astrophysicists have long thought of the Crab Nebula as a steady “standard candle” in X-ray wavelengths. But on January 12th, at the American Astronomical Society meeting, Colleen Wilson-Hodge of NASA’s Marshall Space Flight Center presented findings from a team of researchers that showed fluctuations in the nebula’s output.
"If only one satellite instrument had reported this, no one would have believed it," said Wilson-Hodge. But when a team including instrument experts for four different X-ray satellites compared observations and found the same variations, they became convinced that the variations stemmed from “the one thing all of the observations had in common – the Crab Nebula.”
Mike Cherry and Gary Case at Louisiana State University (LSU) first noticed a dimming in the Crab’s output with the Gamma-ray Burst Monitor (GBM) on NASA’s Fermi Gamma-ray Space Telescope. To check that the dimming was real and not an artifact of instrument error, the team took past GBM data, from August 2008 to July 2010, and combined it with corresponding data from three other space-based X-ray observatories. The results showed that radiation from the Crab Nebula saw a steady decline totaling 7% over the past two years at four high-energy X-ray bands (in the range of 12,000 to 500,000 electron-volts). Looking even further back, overall output from the Crab has varied – brightened and dimmed – by as much as 3.5% a year since 1999.
“Other instrument teams saw this variability in their instruments, but didn't believe it,” said Wilson-Hodge. “Conventional wisdom (i.e. the Crab is constant) can be hard to overcome.”
The Crab Nebula is the brightest source of reasonably steady high-energy radiation (X-rays and gamma rays) in the sky. So for decades, astronomers have used it as a “standard candle” in the field of high-energy astrophysics. It’s routinely used for instrument calibration and even as a unit of measure, the “millicrab.” An unexpectedly varying Crab Nebula might have affected any research that used the Crab as a standard or a calibration tool.
“Since we haven't gone back yet and re-analyzed previous results, we can't say for sure what the impact will be,” said Wilson-Hodge. “For some instruments and observations the effect would be slight, but there may be instruments for which re-analysis could change conclusions of some studies.”
Case draws a practical lesson from the discovery: “Hopefully our results regarding a non-constant Crab will help X-ray astronomers realize the importance of cross-checking calibrations between instruments.”
The Crab Nebula, one of the most-studied celestial objects, is the remnant of a supernova that was observed by Chinese astronomers in 1054. The exploding star’s core became a pulsar, a hyperdense neutron star rotating about 30 times per second, while its outer layers formed the nebula, an expanding cloud of gas glowing across the entire electromagnetic spectrum from the pulsar's energy. Since the pulsar’s regular radio and X-ray pulses have not altered, researchers suspect that the fluctuations originate in the inner portion of the nebula.
"This environment is dominated by the pulsar's magnetic field, which we suspect is organized precariously," said Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University. "The X-ray changes may involve some rearrangement of the magnetic field, but just where this happens is a mystery."
The original article, When A Standard Candle Flickers, was published on January 12th in the Astrophysical Journal Letters. You can read the preprint at arxiv.org/PS_cache/arxiv/pdf/1010/1010.2679v1.pdf.