INTEGRAL Picture Of the Month
March 2021

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New measurements of the cosmic X-ray background

With the development of X-ray astronomy, the understanding of the nature of the cosmic X-ray background, i.e., the isotropic X-ray radiation discovered in the early 1960s, has become firmly established. As it turned out, this background consists of the radiation of many millions of individual objects: supermassive black holes, absorbing the surrounding matter in the nuclei of distant galaxies. Despite the relatively long history of X-ray astronomy, accurate measurements of the cosmic X-ray background are difficult to realize, mainly due to the need to separate cosmic radiation from instrumental background arising in X-ray detectors.

Using a unique technique of spatial modulation of the X-ray background on the detectors (i.e., two Focal Plane Modules, FPMA and FPMB) of NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) orbiting telescope, it was possible to achieve an accuracy of less than one percent in measuring the cosmic X-ray background. The new method is sensitive to different background intensities in different sky fields, which is mainly explained by cosmic variance. The image demonstrates the new measurement of the cosmic X-ray background intensity (expressed in units of 10−11 erg s−1 cm−2 deg−2) with NuSTAR. It is compared with the canonical HEAO-1 result, as well as with the more recent INTEGRAL measurements which were made via Earth occultation manoeuvre (see INTEGRAL POM March 2006 and November 2013). The new cosmic X-ray background measurement (combined over the fields) with NuSTAR is consistent with the INTEGRAL result, but about 8% higher than that measured with HEAO-1.

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