The cosmic X-ray background (CXB) was first discovered in the early 1960s, several years before the detection of the more famous cosmic microwave background (the remnant radiation dating from a few hundred thousand years after the big bang). Unlike the microwave background, which is truly diffuse in origin, the CXB is dominated by the emission from distant, high-energy point sources: active galactic nuclei, sites of intense black-hole growth at the centers of galaxies. A key goal of high-energy astrophysics has been to determine the detailed composition of the CXB in order to understand the evolution of these active nuclei.
Huge strides in revealing the composition of the CXB have been made over the past decade, with sensitive surveys undertaken especially by the Chandra X-ray Observatory and the XMM-Newton observatory. These surveys are so deep that they have resolved perhaps 90% of the CXB sources, at least at low X-ray energies where they operate. The results reveal a plethora AGNs out to distances so great that their light has been traveling toward us for about 95% of the age of the Universe. These results, however, applied only to sources detected at the lower X-ray energies, and far from the peak energy of the CXB which is four times larger.
The Nuclear Spectroscopic Telescope Array (NuSTAR) space observatory represents a breakthrough in this regard. NuSTAR was successfully launched one year ago, on 2012 June 13, and is the first high energy orbiting observatory with focusing optics, and is capable of providing ten times more precise angular information and one hundred times the sensitivity.
CfA astronomers Francesca Civano and Martin Elvis, along with a large team of colleagues, used NuSTAR to probe the cosmic X-ray background. The satellite detected sources about one hundred times fainter than previous missions. Analysis shows that these objects are mostly quasars, they emit X-rays with a luminosity over ten billion times that of the Sun, and their active nuclei reside in galaxies that seem to be more massive by a factor of five than the hosts of typical nearby active nuclei. The new results, a first step in probing the high-energy X-ray universe with NuSTAR, suggest that these newly discovered objects are broadly similar to those already known but scaled up in both mass and luminosity.
“The NuSTAR Extragalactic Survey: A First Sensitive Look at the High-Energy Cosmic X-ray Background Population," D. M. Alexander, D. Stern, A. Del Moro, G. B. Lansbury, R. J. Assef, J. Aird, and collaborators, ApJ 773, 125, 2013.