Analysis of the new INTEGRAL Earth observations to measure the cosmic X-ray background

TL;DR

The paper analyzes recent Earth occultation observations by INTEGRAL, revealing that variability in detector signals is influenced by spacecraft activation and solar activity, complicating measurements of the cosmic X-ray background.

Contribution

It demonstrates that solar maximum and spacecraft activation significantly affect Earth occultation data, highlighting challenges in accurately measuring the cosmic X-ray background.

Findings

2012 data shows unexpected variability not caused by Earth passage

Radioactive decay from spacecraft activation influences detector signals

Solar maximum and auroral emissions impact X-ray background measurements

Abstract

A new series of Earth occultation observations has been started in 2012 to refine the determination of the cosmic X-ray background by the INTEGRAL mission. We show here that the new detector lightcurves in the 3 to 160 keV range differ from the ones obtained in 2006. Instead of the expected modulation induced by the passage of the Earth through the field of view of the JEM-X, IBIS/ISGRI and SPI instruments, we record unrelated variability on shorter timescales. We discuss the differences obtained with the datasets of 2006 and 2012 in view of the changes in pointing direction, spacecraft orbit and solar cycle phase. We conclude that the Earth occultation signal in 2012 is likely blended by radioactive decay resulting from the activation of the spacecraft when crossing the proton radiation belt at perigee passage. The observed variability, on the other hand, results most likely from the current solar maximum. In addition to a variable particle environment from inhomogeneities of the solar wind, we also find evidence for hard X-ray auroral emission. While the former can be traced by SPI/ACS counts, the latter - by enhancing unpredictably the Earth emission - is a major disturbance for measuring the diffuse X-ray background through occultation by the Earth.