When A Standard Candle Flickers

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 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.