Variability of the Unresolved X-Ray Background
The cosmic x-ray background is diffuse, uniform x-ray radiation originat- ing from outside our galaxy. Over the past several decades since its discovery, much of it has been resolved into discrete sources. However, a fraction of the x-ray background still remains unresolved. This remaining fraction is com- posed of sources from the very early universe, and high energy processes from this time period have major impacts on the modern universe. We can- not observe these sources directly, but we can observe how the unresolved x-ray background varies over time. Sources such as active galactic nuclei are known to have variable fluctuations in brightness while galaxies do not, so by measuring the variability in brightness of individual x-ray background pixels over time, we can infer whether or not it is comprised of mostly ac- tive galactic nuclei or galaxies. We analyze x-ray images of the same area of sky taken at different points in time and measure the standard deviation of individual pixels in order to measure how they vary in brightness over time. We compare this to a mock dataset generated using the average pixel value of our experimental dataset following a random Poisson distribution that simulates a non-variable x-ray background. Our results indicate that the unresolved cosmic x-ray background is not variable, thus we infer that it is most likely primarily due to galaxies from the very early universe. This has significant cosmological implications because these are likely to be x-rays radiated from the very first generation of stars. Studying the unresolved cos- mic x-ray background provides insight into what sort of high energy processes occurred in the early universe and how those processes shaped the universe that we observe today.