Journal article ยท Preprint article
When a Standard Candle Flickers
NASA Marshall Space Flight Center1
European Space Astronomy Centre2
Astrophysics, National Space Institute, Technical University of Denmark3
National Space Institute, Technical University of Denmark4
National Institute for Astrophysics5
Louisiana State University6
NASA Goddard Space Flight Center7
Middle East Technical University8
University of Alabama9
National Space Science and Technology Center10
Universities Space Research Association11
Max Planck Institute12
Los Alamos National Laboratory13
...and 3 moreThe Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in 2008 August, a ~ 7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique.
This decline is independently confirmed in the ~ 15-50 keV band with three other instruments: the Swift Burst Alert Telescope ( Swift /BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE /PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ~ 3-15 keV data from the RXTE /PCA and in the 50-100 keV band with GBM, Swift /BAT, and INTEGRAL /IBIS.
The pulsed flux measured with RXTE /PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~ 3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April.
As of 2010 August, the current flux has declined below the 2007 minimum.
Language: | English |
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Year: | 2011 |
Pages: | L40 |
ISSN: | 20418213 and 20418205 |
Types: | Journal article and Preprint article |
DOI: | 10.1088/2041-8205/727/2/L40 |
ORCIDs: | Lund, Niels |