The Crab Nebula: An 'Unreliable' Calibration Source ?
The Crab Nebula, shown in the thumbnail picture, is being used
for decades as a standard candle to calibrate most X-ray and
gamma-ray telescopes once they have been launched. This is
because its flux is both, intense and steady.
However, INTEGRAL has made a significant contribution to demonstrating
that the Crab does not entirely radiate like a standard candle. Based
on observations made by NASA's Fermi Gamma-Ray Burst Monitor (GBM), the
Swift Burst Alert Telescope, the Rossi X-Ray Timing Explorer (RXTE), and
INTEGRAL, a real, intrinsic decline of the Crab Nebula's flux can be
discerned from the year 2008 of about 7% in the 15 - 50 keV energy band;
a similar decline can be seen in the 50 - 100 keV band. Moreover the Crab
had brightened and faded by as much as 3.5% a year since 1999. The
flickering arises from the nebula, and not from the pulsar located inside,
as no unexpected variations are detected in the pulsed flux. Unlike the
NASA spacecraft involved in the study, which are on relatively low orbits,
INTEGRAL is operating in a highly eccentric orbit. Therefore, the INTEGRAL
orbital environment has a different level of background radiation with
respect to the other spacecraft, and this allows to exclude any orbital
induced background effect in the Crab Nebula's flickering.
