ACTIVE GALAXIES AND QUASISTELLAR OBJECTS, X-RAY EMISSION RICHARD MUSHOTZKY X-ray emission from active galactic nuclei (AGN) is one of the major signatures of these objects and thus it is necessary to understand how it is created in order to study the physical conditions in these objects. It is thought that the x-ray emission comes from those regions that are very close to the source of energy, frequently presumed to be a massive black hole. This conclusion is based on general theoretical grounds and observations that indicate that the fastest time scale for variability most often occurs in the x-ray band. Most known forms of active galaxies (e.g., quasars, Seyfert galaxies, broad-line radio galaxies, and BL Lac objects) have been well studied in the x-ray band. located in the centers of "normal" galaxies, AGN often outshine them, radiating from 10x6 - 10x15 times the bolometric luminosity of the Sun from a region smaller than a light-month in radius. Their spectra can be roughly characterized by a power law distribution with equal energy per decade of frequency from the mid-infrared (v=10x12 Hz or ^-3cm) to the hard (v-5x10x19Hz or E-200 keV) x-ray (where ^ is the wavelength, V is the frequency of the radiation, and E is its energy). This entry concentrates on the x-ray emission from quasars and Seyfert 1 galaxies and excludes BL Lac objects, Seyfert 2 nuclei, and other types. (See Additonal Reading). X-RAY SPECTRA The observed spectrum is a signature of the mechanism via which the energy is radiated. Although x-ray spectroscopy of AGN is in its infancy (past instruments having had spectral resolution of only E/DE>10), some general results are clear. The x-ray spectra of AGN that have broad optical emission lines, in particular Seyfert 1 galaxies, are dominated in the 2-20 keV band by a simple power law continuum, F(V)- vø erg cm2 s-1Hz-1. The spectral index * is most commonly -0.7, a value that does not seen, to be a function of luminosity and, in a given object, does not seen, to change much when the source intensity varies by a factor of -10. More than 10% of the bolometric luminosity of an AGN is emitted in x-rays with energies greater than 2 keV. This seems to be the only spectral domain where the radiation is dominated by "nonthermal" processes. That is, the physical mechanism for generation of the x-ray photons is not due to "thermal" processes such as recombination or blackbody radiation; instead the effects of Compton scattering and/or electron-positron pairs must be very important. However, recent observations indicate that there may be a substantial contribution to the 20-50 keV band radiation from "thermal" reprocessing of the "nonthermal" radiation. At energies less than 2 keV, the x-ray spectra of many AGN deviate from the simple power law description. About one-third of all Seyfert galaxies show a deficit of photons at low energies. This deficit occurs most often in low-luminosity [L(x) < 5 X 10x43 erg cm - 2 s - 1] objects and has the characteristic signature of absorption of x-rays by cold material in the line of sight. The inferred column densities are on the order of 10x22 - 10x23 atoms cm - 2, consistent with the absorbing material being the same clouds responsible for the emission of the broad optical lines characteristic of AGN. However, recent observations have indicated that in some objects the absorber is not a simple "cold" (T < I0~ K) slab but must be either partially ionized or have "holes" in it, which some of the x-rays leak through. There are several possible locations for the absorbing material with the prime candidates being the clouds that are responsible for the broad optical emission lines and the outer parts of the accretion disk itself. In approximately one-half of the AGN that do not show strong absorption, there is a "soft x-ray excess." That is, the flux at energies less than 2 keV exceeds what would be predicted by an extension of the higher-energy continuum. The poor spectral resolution of past experiments was inadequate to determine the nature of this excess and there are several viable explanations for this excess emission. It may be: (1) radiation from an accretion disk, (2) the extension into the x-ray band of a "synchrotron" emission observed in the infrared band, (3) a thermal photoionized plasma associated with the gas confining the broad-line clouds, or (4) the signature of reprocessing of the radiation from the central engine of the AGN by a thermal accretion disk or "cold" accreting matter. The next generation of x-ray experiments, to be launched in the early 1990s, should have sufficient spectral resolution and sensitivity to determine the nature of these "soft x-ray excesses" and resolve these outstanding problems. TIME VARIABILTIY The general condition of causality states that the fastest time scale over which an object can vary (in the absence of special and general relativistic effects) is the light travel time, at *R*, where R is the "size" of the object and c is the speed of light. If the energy for the x-ray radiation comes from the innermost regions around a massive black hole, R 10GMBH /c2, where MBH is the mass of the black hole in solar masses. Defining M7=10x7 M*, one finds that at **-500(MBH/M7)s. Another fundamental consideration is the efficiency with which matter can be converted into energy. The physics of such conversion requires that at **pal(x)/(5X l0~~) where * is the efficiency with which matter is converted into energy via the release of gravitational energy (nominally l 0-40%) and a *(x) is the change in the x-ray luminosity of the source in a time *t. So far AGN have only been observed to vary strongly (> 50,) on time scales of less than 1 day in the x-ray domain. Such "rapid" variability has not been seen in the radio, infrared, optical, or ultraviolet bands. This seems to verify that x-rays do indeed come from the smallest regions in AGN. Analysis of a large number of Seyfert 1 galaxies with data from the Exosat satellite indicates that whereas "rapid" variability (At <1/2 day) seems to be a common property of these objects, there is a wide variety in the nature of the variability. For example, compare the properties of three low-luminosity Seyfert 1 galaxies: NGC 4051, which is continuously and always variable and is well described by a process with no characteristic time scale; NGC 4151, which shows quiescent periods but also flares with a one-day time scale; and NGC 6814, which shows "quasiperiodic" behavior with a -3-4 h time scale. The origin of these phenomena is not understood at present and we do not know if these are fundamentally different behaviors or just different aspects of the same phenomenon. Frequently, the x-ray variability is rather dramatic, such as a reduction in flux in NGC 6814 by a factor of > 5 in less than 300 s, or an increase in the flux in MCG 6-30-15 (a Seyfert 1 galaxy) by more than 50% in less than 2000s and its fall by more than a factor of 2 in less than 1000s. With a few exceptions there has been no detection of spectral variability during these events. This lack of significant spectral changes rules out a "thermal" origin for the x-ray flux and confirms the spectral result that nonthermal processes dominate. There seems to be a global pattern to the variability time scales with more luminous objects varying on rather longer time scales than less luminous objects; however, this conclusion is currently controversial. The data are consistent with most objects having variability time scales that scale linearly with the luminosity, which might be expected if they were accreting at a similar ratio of the Eddington limit. This limit is the maximum luminosity an accreting object with a steady-state spherical flow can have for a given mass L-.4XI035ergs1 M*. CORRELATIONS OF THE X-RAY WITH OTHER WAVELENGTHS Because most AGN detected in the x-ray band have a very low flux (0.005 photons cm2 s-1 in the 2-10 keV range), their spectral and temporal properties are poorly determined. However, one can use the large numbers of x-ray detections and the large numbers of upper limits for other objects to correlate with their optical and radio properties, to determine the evolution of AGN with luminosity and redshift to derive scaling laws between the different spectral bands, and to obtain the properties of AGN which are at very high redshift or which are very faint. The distribution of the number, N, of x-ray-detected AGN versus x-ray flux, S, called the log N-log S diagram, shows that AGN have evolved in either number or luminosity with cosmic epoch. Luminosity evolution seems to be more likely. However, comparison of the evolution of AGN in the x-ray and optical bands indicates that the x-ray luminosity has changed less rapidly with cosmic time than the optical. Detailed comparison of the soft x-ray with the optical luminosity shows that the ratio of the two is a function of the optical luminosity with Lx************** Thus the more optically luminous an object, the smaller is the soft x-ray fraction of the total luminosity. A similar study involving x-ray and radio luminosity shows that the two are strongly correlated with the relationship being linear. These two correlations seem to be additive with the best fit of the form log Lx l0g(AL3n1pt4 # BLrad,o), where A and B are constants. This may indicate that the x-ray emission from radio bright AGN is due to two physically separate components even though the spectral data do not require it. Additional Reading Maraschi, L., Maccacaro, T., and Ulrich, M.-H., eds.(1989). workshop on BL Lac objects. Springer-Verlag, Berlin. Tanaka, Y., ed.(1988).International Symposium on the physics of Neutron Stars and Black Holes, see section 5, extragalactic sources: Active galactic nuclei and cosmic x-ray background. Universal Academy press, Tokyo. Turner, T.J and pounds, K.A. (1989)The EXOSAT spectral survey of AGN. Monthly Not. Roy. Acad. Sci 240 833. See also Active Ga1axies Seyfert types; Active Galaxies and Quasistellar Objects, Accretion.