A Flickering X-ray Candle

Released Wednesday, January 12th, 2011
Updated Wednesday, May 3rd, 2023 at 1:53PM

ID: 10708

- Written by:
- Francis Reddy
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The Crab Nebula, created by a supernova seen nearly a thousand years ago, is one of the sky's most famous "star wrecks." For decades, most astronomers have regarded it as the steadiest beacon at X-ray energies, but data from orbiting observatories show unexpected variations, showing astronomers their hard X-ray "standard candle" isn't as steady as they once thought. From 1999 to 2008, the Crab brightened and faded by as much as 3.5 percent a year, and since 2008, it has faded by 7 percent. The Gamma-ray Burst Monitor on NASA's Fermi satellite first detected the decline, and Fermi's Large Area Telescope also spotted two gamma-ray flares at even higher energies. Scientists think the X-rays reveal processes deep within the nebula, in a region powered by a rapidly spinning neutron star — the core of the star that blew up. But figuring out exactly where the Crab's X-rays are changing over the long term will require a new generation of X-ray telescopes.

A short narrated video about the Crab Nebula's variability.

Credit: NASA/Goddard Space Flight Center

Watch this video on the NASAexplorer YouTube channel.

For complete transcript, click here.

NASA's Chandra X-ray Observatory reveals the complex X-ray-emitting central region of the Crab Nebula. This image is 9.8 light-years across. Chandra observations were not compatible with the study of the nebula's hard X-ray variations.

Credit: NASA/CXC/SAO/F. Seward et al.

The Crab Nebula, created by a supernova seen nearly a thousand years ago, is one of the sky's most famous "star wrecks." For decades, most astronomers have regarded it as the steadiest beacon at X-ray energies, but data from orbiting observatories show unexpected variations, showing astronomers their hard X-ray "standard candle" isn't as steady as they once thought. From 1999 to 2008, the Crab brightened and faded by as much as 3.5 percent a year, and since 2008, it has faded by 7 percent. The Gamma-ray Burst Monitor on NASA's Fermi satellite first detected the decline, and Fermi's Large Area Telescope also spotted two gamma-ray flares at even higher energies. Scientists think the X-rays reveal processes deep within the nebula, in a region powered by a rapidly spinning neutron star — the core of the star that blew up. But figuring out exactly where the Crab's X-rays are changing over the long term will require a new generation of X-ray telescopes.

X-ray data from NASA's Fermi, RXTE, and Swift satellites and the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) confirm that the Crab Nebula's output has declined about 7 percent in two years at energies from 15,000 to 50,000 electron volts. They also show that the Crab has brightened or faded by as much as 3.5 percent a year since 1999. Fermi's Large Area Telescope (LAT) has detected powerful gamma-ray flares (magenta lines) as well.

Credit: NASA/Goddard Space Flight Center

This view of the Crab Nebula in visible light comes from the Hubble Space Telescope and spans 12 light-years. The supernova remnant, located 6,500 light-years away in the constellation Taurus, is among the best-studied objects in the sky.

Credit: NASA/ESA/ASU/J. Hester