After the discovery of quasars (QSO), in the mid-1960s the X-ray astronomy started and was important to understand better the nature and the properties of the Active Galactic Nuclei (AGN), as the X-ray emission seemed to occur in the majority of them. The first X-ray detected emission of AGN was that of M87 and 3C 273 in a survey of the Virgo region with a proportional counter (Geiger counter) on board an Aerobee rocket in 1967. Later, Uhuru, the first X-ray satellite launched in 1970 was responsible for the first detection of a Seyfert galaxy, while Ariel V, a British-USA collaboration satellite launched in 1974, mended to monitor the X-ray sky. Results of Ariel V data included the confirmation of the X-ray variability in the center of AGN and strong X-ray emission in Seyfert galaxies (1978). At that time, astronomers claimed that not all QSOs have the basic properties defined by Maarten Schmidt, but the majority of them have X-ray emission. The next generation X-ray telescopes included in addition to proportional counters, scintillation detectors.

These are a) the NASA’s High Energy Astronomy Observatory (HEAO 1) that was launched in 1977 and carried the first complete X-ray sky survey, especially towards high Galactic amplitudes and b) the Einstein Observatory satellite launched in 1978 that was the first telescope with focusing optics and capable of producing images. The advanced astronomical findings of the latter led to surveys with many X-ray point-like sources and also diffuse sources with very high spatial resolution of a few arcseconds. Furthermore, it was the first time that some of the main characteristic features of AGN tha we know today were observed (the power-law in the 3-50 keV regime, the very first discovery of the soft X-ray bump in the Seyfert galaxies, a good spectral characterization of the X-ray cosmic background and studies of X-ray spectra and time variability). At that time, the X-ray emission has become basic and main characteristic of the AGN and in the decades after, many other X-ray telescopes were launched with match higher spatial resolution and with much larger collective areas. Some examples of those are:

a) the European EXOSAT and the Japanese Ginga satellites in 1980’s with results confirming the soft bump in X-ray spectra and showing the flattening of hard X-ray spectra, a strong indicator of X-ray reflection,

b) the ROSAT satellite operating in energies smaller than 2 keV that mapped the whole sky with very high spatial resolution and resulted in more than hundred thousands of AGN and ultra deep observations that resolved the soft X-ray spectrum,

c) the Japanese ASCA, the Italian BeppoSAX and the Rossi X-ray Timing Explorer (RXTE) in the late 1990’s and

d) the more recent Chandra X-ray Telescope and the XMM-Newton Telescope that provided many scientific results with their high spatial resolution and large collective areas along with the NuSTAR telescope and eRosita.

The future of the X-ray telescopes is the Athena (Advanced Telescope for High ENergy Astrophysics) mission that is the second long-range space mission (L2) of the long term program “Cosmic Vision” of the European Space Agency (ESA). This mission is dedicated to the scientific theme “The hot and violent Universe” which it plans to explore with the launch in 2033 of a large X-ray observatory designed by a consortium of European institutes.

Source: PhD Thesis – E. Pouliasis (https://zenodo.org/record/3892459#.YN7Z6DpRWut)