2003-2019 Monitoring of the Crab Emission through INTEGRAL SPI, or Vice Versa

The Crab Nebula is used by many instruments as a calibration source, in particular at high energy, where it is one of the brightest celestial objects. The spectrometer INTEGRAL SPI (20 keV-8 MeV), in operation since 2002 October, offers a large data set dedicated to this source, with regular campaigns planned twice per year. We have analyzed the available data to quantify the source behavior on a long-term scale and examine the stability level on timescales from hours to years. As a result, the source flux variability appears to be contained within less than 5% around an ∼20 yr mean value for broad bands covering the 20-400 keV energy domain, above which statistics limits any firm conclusion. In terms of spectral shape, the Band model provides a good description of the observed emission between 20 keV and 2.2 MeV. The averaged spectrum best-fit parameters correspond to a low-energy slope of 1.99 0.01, a high-energy slope of −2.32 0.02, and a characteristic energy Ec of 531 50 keV to describe the curvature joining both power laws. The spectral parameters have then been determined on the revolution timescale (∼1-2 days), and their steadiness confirms the source emission stability. As a complementary result, this study demonstrates that the SPI instrument efficiency remains within 5% of its initial value after 17 yr of operation.