Colleen A. Wilson-Hodge, NASA Marshall Space Flight Center
Michael L. Cherry, Louisiana State University
Gary L. Case, Louisiana State University
Wayne H. Baumgartner, National Aeronautics and Space Administration
Elif Beklen, Middle East Technical University (METU)
P. Narayana Bhat, The University of Alabama in Huntsville
Michael S. Briggs, The University of Alabama in Huntsville
Ascension Camero-Arranz, National Space Science and Technology Center
Vandiver Chaplin, The University of Alabama in Huntsville
Valerie Connaughton, The University of Alabama in Huntsville
Mark H. Finger, Huntsville Program Office
Neil Gehrels, NASA Goddard Space Flight Center
Jochen Greiner, Max Planck Institute for Extraterrestrial Physics
Keith Jahoda, NASA Goddard Space Flight Center
Peter Jenke, NASA Marshall Space Flight Center
R. Marc Kippen, Los Alamos National Laboratory
Chryssa Kouveliotou, NASA Marshall Space Flight Center
Hans A. Krimm, National Aeronautics and Space Administration
Erik Kuulkers, European Space Astronomy Centre
Niels Lund, Danmarks Tekniske Universitet
Charles A. Meegan, Huntsville Program Office
Lorenzo Natalucci, INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Rome
William S. Paciesas, The University of Alabama in Huntsville
Robert Preece, The University of Alabama in Huntsville
James C. Rodi, Louisiana State University
Nikolai Shaposhnikov, National Aeronautics and Space Administration
Gerald K. Skinner, National Aeronautics and Space Administration
Doug Swartz, Huntsville Program Office
Andreas Von Kienlin, Max Planck Institute for Extraterrestrial Physics
Roland Diehl, Max Planck Institute for Extraterrestrial Physics
Xiao Ling Zhang, Max Planck Institute for Extraterrestrial Physics

Document Type

Article

Publication Date

2-1-2011

Abstract

The 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 Fe r m i 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. © 2011. The American Astronomical Society.

Publication Source (Journal or Book title)

Astrophysical Journal Letters

Download

Share

COinS