4.4. X-rays from AGN

One June 18, 1962, an Aerobee sounding rockets blasted skyward from White Sands proving ground in New Mexico. It carried a Geiger counter designed to detect astronomical sources of X-rays. The experiment, carried out by Giacconi et al. (1962), discovered an X-ray background and a "large peak" in a 10 degree error box near the Galactic center and the constellation Scorpius. A rocket experiment by Bowyer et al. (1964) also found an isotropic background, confirmed the Scorpius source, and detected X-rays from the Crab nebula. Friedman and Byram (1967) identified X-rays from the active galaxy M 87. A rocket carrying collimated proportional counters sensitive in the 1 to 10 keV energy range, found sources coincident with 3C 273, NGC 5128 (Cen A), and M 87 (Bowyer, Lampton, and Mack 1970). The positional error box for 3C 273 was small enough to give a probability of less that 10^-3 of a chance coincidence. The X-ray luminosity, quoted as ~ 10⁴⁶ erg s^-1, was comparable with quasar's optical luminosity.

The first dedicated X-ray astronomy satellite, Uhuru, was launched in 1970. Operating until 1973, it made X-ray work a major branch of astronomy. X-rays were reported from the Seyfert galaxies NGC 1275 and NGC 4151 (Gursky et al. 1971). The spectrum of NGC 5128 was consistent with a power law of energy index = - 0.7, where L ; and there was low energy absorption corresponding to a column density of 9 × 10²² atoms cm^-2, possibly caused by gas in the nucleus (Tucker et al. 1973). Early variability studies were hampered by the need to compare results from different experiments, but Winkler and White (1975) found a large change in the flux from Cen A in only 6 days from OSO-7 data. Using Ariel V observations of NGC 4151, Ives et al. (1976) found a significant increase in flux from earlier Uhuru measurements. Marshall et al. (1981), using Ariel V data on AGN gathered over a 5 year period, found that roughly half of the sources varied by up to a factor of 2 on times less than or equal to a year. A number of sources varied in times of 0.5 to 5 days. Marshall et al. articulated the importance of X-ray variability observations, which show that the X-rays "arise deep in the nucleus" and "relate therefore to the most fundamental aspect of active galaxies, the nature of the central `power house'."

Strong X-ray emission as a characteristic of Sy 1 galaxies was established by Martin Elvis and his coworkers from Ariel V data (Elvis et al. 1978). This work increased to 15 the number of known Seyfert X-ray sources, of which at least three were variable. Typical luminosities were ~ 10^42.5 to 10^44.5 erg s^-1. The X-ray power correlated with the infrared and optical continuum and H line. Seyfert galaxies evidently made a significant contribution to the X-ray background, and limits could be set on the evolution of Seyfert galaxy number densities and X-ray luminosities in order that they not exceed the observed background. Elvis et al. considered thermal bremsstrahlung (10⁷ K), synchrotron, and synchrotron self-Compton models of the X-ray emission.

HEAO-1, the first of the High Energy Astronomy Observatories, was an X-ray facility that operated from 1977 to 1979. It gathered data on a sufficient sample of objects to allow comparisons of different classes of AGN and to construct a log N-log S diagram and improved luminosity function. HEAO-1 provided broad-band X-ray spectral information for a substantial set of AGN, showing spectral indices - 0.7, with rather little scatter, and absorbing columns < 5 × 10²² cm^-2 (Mushotzky et al. 1980).

The Einstein Observatory (HEAO-2) featured grazing incidence focusing optics allowing detection of sources as faint as ~ 10^-7 the intensity of the Crab nebula. Tananbaum et al. (1979) used Einstein data to study QSOs as a class of X-ray emitters. Luminosities of 10⁴³ to 10⁴⁷ erg s^-1 (0.5 to 4.5 keV) were found. OX169 varied substantially in under 10,000 s, indicating a small source size. This suggested a black hole mass not greater than 2 × 10⁸ M, if the X-rays came from the inner portion of an accretion flow. By this time, strong X-ray emission was established as a characteristic of all types of AGN and a valuable diagnostic of their innermost workings.