Some galaxies contain compact nuclei, which emit vast amounts of energy over the entireelectromagnetic spectrum from radio through gamma-rays, thus called active galactic nuclei(AGN). The powerful activity originates from the release of gravitational energy through accretionof material on to a supermassive black hole with a typical mass in the range of 106−9M⊙.Some of the gravitational energy is thermalized in the accretion disk to radiate ultraviolet andsoft X-ray emission. Some is channeled into the generation of energetic particles (or jets) andproduces X-ray through gamma-ray emission via inverse Compton scattering and radio throughX-rays via synchrotron emission. This radiation from the vicinity of the black hole may heatcircumnuclear dust in the obscuring torus of clouds, which is believed to exist on parsec scalesoutside the accretion disk. The warm torus can then be a source of a strong infrared emission.AGN tend to show large variety in their observational properties, such as luminosities, opticalline properties, radio brightness, variability, and so on. In spite of such apparent diversity, itis widely believed that the various AGN are intrinsically the same. Just a few parameters -inclination, accretion rate, and presence/absence of jets - may be responsible for the apparentdifferences. However, details of the geometry of the nuclei, especially of the torus, are poorlyknown because their angular scales are very small and difficult to resolve spatially.The main goal of this thesis is to study the geometry of the nuclear emission region basedupon unbiased survey data gathered from multiple wavebands. For this purpose, I take twoapproaches; one is to explore the correlation between hard X-ray and infrared luminosities usingunbiased samples systematically for each AGN type. The other is to analyze variability of midinfraredemission for individual sources and for each AGN type statistically using two all-skysurvey catalogs. This is the first large-scale systematic study of mid-infrared variability in AGN,probing timescales of several years separately for different types of AGN.For the first approach, I used all-sky surveys conducted by Swift in the hard X-ray (> 10 keV)band and by AKARI in the infrared band. The Swift/Burst Alert Telescope all-sky surveyprovides an unbiased, flux-limited selection of hard X-ray detected AGN. The hard X-ray bandis rather insensitive to the photo-electric absorption due to the intervening clouds up to mildlyCompton-thick (NH ∼1024 cm−2) column densities. In other words, the hard X-ray flux obtainedby the survey reflects the intrinsic luminosity for all Compton-thin AGN and also for mildlyCompton-thick ones, thus providing samples largely unbiased by obscuration. In the case ofinfrared observations, high angular resolution is crucial in order to properly separate AGN fromstellar emission in the host galaxy. However, this was not possible until the advent of recenttelescopes. The AKARI satellite completed an all-sky survey whose catalog was released justbefore the beginning of this research. This survey is several times more sensitive than previousones, and was carried out at a much higher angular resolution of the order of arcseconds. Crosscorrelatingthe 22-month hard X-ray survey with the AKARI all-sky survey, I studied 158 AGNdetected by both instruments. I find a strong correlation for most AGN between the infrared(9, 18, and 90 μm) and hard X-ray (14–195 keV) luminosities, and quantified the correlation forvarious complete subsamples of AGN. Partial correlation analysis confirms that the correlationis intrinsic; that is, the correlation between the luminosities remains significant after removingthe contribution of redshift. Under the unification scheme of AGN, this result may be viewedas supporting clumpy torus models. The good one-to-one correlation between mid-infraredbolometric luminosities and hard X-ray ones for over four orders of magnitude indicates thatthe covering factor of torus will decrease with the increase of the intrinsic luminosity. Thecorrelation for radio galaxies has a slope and normalization identical to that for Seyfert 1s,where we have a direct view of the nuclear regions in both hard X-rays and infrared, implyingsimilar hard X-ray/infrared emission processes in both. In contrast, sources with large Comptonthickcolumn densities show a large deficit in the hard X-ray band, because high gas columndensities in the torus diminish their apparent luminosities even in the hard X-ray band.On the other hand, a few radio-loud sources (radio galaxies and blazars) show systematic deviationstoward higher X-ray luminosities in the correlations as compared to radio-quiet sources.Origin of the broadband emission of radio galaxies is a matter of considerable debate. One possibleexplanation of this deviation (i.e., excess X-rays) is the contribution of jets to the hardX-ray emission. Observations of flux variability can be useful for isolating the jet contribution,because strong and rapid fluctuations are characteristic of beamed jet emission. Useful hardX-ray variability data are not yet available, but such data have recently become available inthe mid-infrared band. Furthermore, mid-infrared variations can also potentially constrain thegeometry of the dusty torus by measuring the response of the torus to changes of the nucleusemission. This leads to the second topic of my thesis to study mid-infrared variability of AGNsystematically. I combine two mid-infrared all-sky surveys, i.e., the data released by AKARIand Wide-field Infrared Survey Explorer (WISE). WISE was launched about four years afterAKARI and accomplished all-sky surveys with high sensitivity in several mid-infrared bands(particularly relevant for my work are the 12 and 22 μm bands). Because the bands observedby the two telescopes are slightly different, I calculated the flux ratio of WISE and AKARIafter subtracting the contribution of band differences, for which the spectral slope calculatedfor each source was used. In addition, cross-calibration errors of the two telescopes are carefullyexamined. I find significant mid-infrared variations in 3 sources, 2 blazars and 1 radio galaxy, ineither or both of the 9 and 18 μm bands. Although no significant variations are detected fromthe rest of the sources, low level variations may be hidden in statistical errors. Therefore, I triedto constrain the average sample variability for different AGN types, which was actually detectedfrom Seyfert 1 in the 9 μm band. This is the first detection of variability from Seyfert 1 by usingtwo mid-infrared all-sky surveys. I quantified the amplitudes of sample variability and foundthat the amplitude reaches ∼10% in 4 years for Seyfert 1 in the 9 μm band. If this variability isexplained by the torus emission only, dust distribution in the torus should be compact, althoughother possibilities, such as jet contribution, cannot be excluded. Combining the results of thehard X-ray vs. mid-infrared correlations and mid-infrared variability, geometry and structureof the obscuring torus are discussed
