Measuring the Cosmic X-ray Background accurately

TL;DR

This paper proposes a new detector design for more accurate measurement of the Cosmic X-ray Background, aiming to reduce uncertainties and improve data quality compared to existing instruments.

Contribution

It introduces a novel CXB detector with collimated spectrometers and a rotating obturator, offering higher effective area and better energy coverage than previous designs.

Findings

Design achieves ~10 times higher CXB count rate

Statistical uncertainty on CXB flux reduced to ~0.5%

Prototype construction is underway for future deployment

Abstract

Measuring the Cosmic X-ray Background (CXB) is a key to understand the Active Galactic Nuclei population, their absorption distribution and their average spectra. However, hard X-ray instruments suffer from time-dependent backgrounds and cross-calibration issues. The uncertainty of the CXB normalization remain of the order of 20%. To obtain a more accurate measurement, the Monitor Vsego Neba (MVN) instrument was built in Russia but not yet launched to the ISS (arXiv:1410.3284). We follow the same ideas to develop a CXB detector made of four collimated spectrometers with a rotating obturator on top. The collimators block off-axis photons below 100 keV and the obturator modulates on-axis photons allowing to separate the CXB from the instrumental background. Our spectrometers are made of 20 mm thick CeBr$_{3}$ crystals on top of a SiPM array. One tube features a $\sim$20 cm$^2$ effective area and more energy coverage than MVN, leading to a CXB count rate improved by a factor of $\sim$10 and a statistical uncertainty $\sim$0.5% on the CXB flux. A prototype is being built and we are seeking for a launch opportunity.