Matteo Guainazzi

Caveats on X-ray data analysis and calibration

In the last decade, performances of X-ray detectors have improved over all parameter spaces (throughput, energy and spatial resolution, timing). Measurements of X-ray observables in celestial sources are increasingly limited by systematic rather then by statistical errors. Unfortunately, all attempts at defining X-ray "standard candles" have proven unsuccessful so far. The energy scale can be still absolutely calibrated through emission lines produced by atomic transitions in on-board or astrophysical calibration sources. Likewise, timing accuracy can be estimated using fast rotators such as the Crab pulsar. On the other hand, uncertainties at the level of the order of 10% (and more) affect the absolute flux calibration. These "cross-calibration" uncertainties are energy-dependent, thus implying uncertainties on spectral measurements. I present in this paper the cross-calibration status among historical and operational X-ray detectors, focusing in particular on the suite of CCD detectors currently operational in the ~0.1-10 keV energy band. Efforts to monitor, document and improve the cross-calibration status are carried out primarily by the IACHEC (International Astronomical Consortium for High Energy Calibration: I will also briefly discuss the impact that these uncertainties (may) have on fields as diverse as the measurements of spin in accreting black holes, and the determination of cosmological parameters through surveys of galaxy clusters.