X-ray astronomers have for decades calibrated their detectors using the Crab Nebula – a supernova remnant that appeared to have an extremely steady brightness. But now an international team of astronomers has discovered that the X-ray output of the Crab has dropped by 7% in the last two years. Although astronomers may now have to look elsewhere for X-ray calibration, the discovery could help astrophysicists to gain a better understanding of how the Crab and similar structures generate vast amounts of high-energy radiation.

One of the most studied objects in the sky, the Crab Nebula is the remnant of an exploded star 6500 light-years away from Earth. At its core is a neutron star that spins 30 times per second, driving processes that are responsible for it X-ray and gamma-ray emissions. Until recently the X-ray intensity of the Crab was considered to be so stable that it is used as a "standard candle" to judge the relative brightness of other objects in the sky. Indeed, X-ray brightness is often expressed in units of "millicrab".

Steady decline

But now Colleen Wilson-Hodge at NASA's Marshall Space Flight Center and colleagues have published a painstaking study using data from five different instruments that clearly shows variability in the Crab's output. Team members first became suspicious when they analysed recent data from the Fermi space telescope's Gamma-ray Burst Monitor (GMB). It revealed a clear and steady decline in the intensity at X-ray energies between 15 and 50 keV.

Not convinced that the decline was real, the team then looked at data from four other instruments – some of which stretched back as far as 1999. The measurements suggest that the intensity increases before dropping by 7% in just two years (see figure). "We're clearly seeing how much our candle flickers," said Wilson-Hodge who was at the 217th Meeting of the American Astronomical Society in Seattle.

Giant slinky

Also at the meeting was Roger Blandford of Stanford University, who believes that the variability could be related to the rapid motion of magnetic field lines surrounding the spinning neutron star. Electrons spiral around these field lines, creating synchrotron X-rays. The high energy of this radiation suggests that these lines are moving close to the speed of light, Blandford reckons.

"The magnetic field lines resemble a giant slinky," he says, "and anyone who has played with a slinky knows how unstable they are". Blandford believes that such instabilities could lead to variations in the X-ray intensity.

Until recently, astronomers had also believed that the Crab was a very steady source of gamma rays. But last week Marco Tavani of the University of Rome and colleagues published a paper in Science showing that the nebula has produced two powerful gamma ray flares since 2007. Both flares contained radiation in the 100 MeV – 10 GeV range and were about three times the normal intensity of the Crab in that energy range.

The flares both lasted for several days and were spotted by the Italian Space Agency's AGILE probe. The gamma rays from the Crab are produced by electrons accelerated to extremely high velocities and Tavani said that the flares show that the Crab is a "very efficient" particle accelerator. "We have lost a standard candle for astrophysics but have gained insight into the acceleration process," he says.

The X-ray study is described in Astrophysical Journal Letters (727 L40).