When A Standard Candle Flickers
Colleen A. Wilson-Hodge (NASA/MSFC), Michael L. Cherry (LSU), Wayne H., Baumgartner (NASA/CRESST) Elif Beklen (METU/SDU), P. Narayana Bhat, Michael, S. Briggs (UAH), Ascension Camero-Arranz (NSSTC), Gary L. Case (LSU),, Vandiver Chaplin, Valerie Connaughton (UAH)

TL;DR
The Crab Nebula, traditionally used as a standard candle in X-ray astronomy, has exhibited a significant and persistent decline in flux over several years across multiple instruments and energy bands, challenging its role as a standard candle.
Contribution
This study provides the first multi-instrument, multi-band evidence of a long-term decline in the Crab Nebula's flux, questioning its reliability as a standard candle in high-energy astrophysics.
Findings
A ~7% decline in Crab flux since 2008 observed across multiple instruments.
Correlated flux variations on a ~3-year timescale from 2005 to 2008.
Flux decline has continued beyond the 2007 minimum, indicating a persistent change.
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…
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