When A Standard Candle Flickers
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 Fermi Gamma-ray Burst Monitor (GBM) in August 2008, 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 with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the INTErnational Gamma-Ray Astrophysics Laboratory Imager on Board INTEGRAL (IBIS). A similar decline is also observed in the ~3 - 15 keV data from the RXTE/PCA and INTEGRAL Joint European Monitor (JEM-X) and in the 50 - 100 keV band with GBM and INTEGRAL/IBIS. Observations from 100 to 500 keV with GBM suggest that the decline may be larger at higher energies. 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 April 2007. As of August 2010, the current flux has declined below the 2007 minimum.
Keywords
Cite
@article{arxiv.1010.2679,
title = {When A Standard Candle Flickers},
author = {Colleen A. Wilson-Hodge and Michael L. Cherry and Wayne H. Baumgartner and Elif Beklen and P. Narayana Bhat and Michael S. Briggs and Ascension Camero-Arranz and Gary L. Case and Vandiver Chaplin and Valerie Connaughton and Mark H. Finger and Neil Gehrels and Jochen Greiner and Keith Jahoda and Peter Jenke and R. Marc Kippen and Chryssa Kouveliotou and Hans A. Krimm and Erik Kuulkers and Charles A. Meegan and Lorenzo Natalucci and William S. Paciesas and Robert Preece and James C. Rodi and Nikolai Shaposhnikov and Gerald K. Skinner and Doug Swartz and Andreas von Kienlin and Roland Diehl and Xiao-Ling Zhang},
journal= {arXiv preprint arXiv:1010.2679},
year = {2011}
}
Comments
accepted version 22 pages, 5 figures; published in Astrophysical Journal Letters; 50-100 keV panel added to figure 5, text changes from submitted version