In this thesis I study the inner-most regions of Active Galactic Nuclei (AGN) using the reverberation mapping technique, and neutron star low-mass X-ray binaries in quiescence using X-ray observations. Using the 13-year optical monitoring data for the AGN NGC 5548, the luminosity dependence of the Hβ emitting radius was modelled using a delay map, finding that the radius scales with luminosity as predicted by recent theoretical models. Time-delays between the continuum at different wavelengths in AGN can be used to probe the accretion disc. Here, continuum time-delays in a sample of 14 AGN were used to measure the radial temperature profile of the accretion discs, determine the nuclear extinction, and measure distances to the objects. However, the distances measured correspond to a value for Hubble's constant that is a factor of ~2 lower than the accepted value. The implications of this on the thermal disc reprocessing model are discussed. I present two Chandra observations of the neutron star transient in the globular cluster NGC 6440 in quiescence, where the power-law component to the spectrum is seen to be variable between the observations, suggesting that there is ongoing residual accretion. From a Chandra observation of the globular cluster Terzan 1, I have identifed the likely quiescent counterpart to a transient previously observed in outburst, and discuss the other sources within the cluster. Using Chandra and XMM-Newton monitoring observations of two neutron star transients (KS 1731-260 and MXB 1659-29) in quiescence I have found that the neutron star crusts in both sources have now returned to thermal equilibrium with the core. These observations also indicate that the crusts in both sources may have a high thermal conductivity and that enhanced neutrino emission may be occurring in the core. Finally, the discovery of an X-ray transient with XMM-Newton is presented, and the other sources in this observation discussed.

Searching for active galactic nuclei (AGNs) in dwarf galaxies is important for our understanding of the seed black holes that formed in the early universe. Here, we test infrared selection methods for AGN activity at low galaxy masses. Our parent sample consists of similar to 18,000 nearby dwarf galaxies (M-* < 3 x 10(9) M-circle dot, z < 0.055) in the Sloan Digital Sky Survey with significant detections in the first three bands of the AllWISE data release from the Wide-field Infrared Survey Explorer (WISE). First, we demonstrate that the majority of optically selected AGNs in dwarf galaxies are not selected as AGNs using WISE infrared color diagnostics and that the infrared emission is dominated by the host galaxies. We then investigate the infrared properties of optically selected star-forming dwarf galaxies, finding that the galaxies with the reddest infrared colors are the most compact, with blue optical colors, young stellar ages, and large specific star formation rates. These results indicate that great care must be taken when selecting AGNs in dwarf galaxies using infrared colors, as star-forming dwarf galaxies are capable of heating dust in such a way that mimics the infrared colors of more luminous AGNs. In particular, a simple W1 - W2 color cut alone should not be used to select AGNs in dwarf galaxies. With these complications in mind, we present a sample of 41 dwarf galaxies that fall in the. WISE infrared color space typically occupied by more luminous AGNs and that are worthy of follow-up observations.

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We present new optical and radio spectroscopic observations of the remarkable galaxy Malin 1. This galaxy has unique features that include an extremely low surface brightness disk with an enormous mass of neutral hydrogen, and a low luminosity Seyfert nucleus. Malin 1 is exceptional in its values of MHO, LB, and MHI /Ln, and modest in its surface mass density of gas and stars. Spirals with large Min /LB tend to have low mean column densities of HI, and are close to the threshold for star formation due to instabilities in a rotating gas disk. In these terms, Malin 1 has a disk with extremely inefficient star formation. The bulge spectrum is dominated by the absorption features of an old, metal rich stellar population, although there is some evidence for hot (young) stars. The emission line excitations and widths in the nucleus are typical of a Seyfert galaxy; but Malin 1 is in the lowest 5% of the luminosity function of Seyferts, despite a copious fuel supply. Malin 1 is in a low density region of the universe. We propose it as an unevolving disk galaxy, where the surface mass density is so low that the chemical composition and mass fraction in gas change very slowly over a Hubble time. Its properties are similar to those of the damped Lyman -a absorption systems seen in the spectra of high redshift quasars. We emphasize that there are strong observational selection effects against finding gas -rich galaxies that are both massive and diffuse. Finally, we suggest that large and massive HI disks may have formed as early as z - 2, and remained quiescent to the present day. Subject headings : individual (Malin 1) - galaxies : photometry - galaxies : Seyfert - galaxies : stellar content - radio sources : 21 cm radiation - stars : formation

A study of the Galactic Center stellar population is continuing with a sensitive 2μm CCD camera. Using a 64 x 64 detector array, background limited images are recorded with modest amounts of observing time (tob, 20 sec to reach K =13). Magnitudes have been extracted using DAOPHOT from repeated imaging of the central 5' x 5' to search among approximately 1500 stars for long period variables (LPV's, P > 200d), particularily Miras. Miras have a well defined period - luminosity relationship as well as one in period -mass. This program investigates the nature of highly luminous stars at the Galactic Center. Presently 12 variables have been found and have several characteristics consistant with Miras. They have a maximum bolometric luminosity of -4.4 mag which supports the case that high luminosity stars in the central 6 pc are young supergiants.

During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) UV monitoring campaign of the Seyfert 1 galaxy NGC 5548 performed from 2014 February to July, the normally highly correlated far UV continuum and broad emission line variations decorrelated for similar to 60-70 days, starting similar to 75 days after the first HST/COS observation. Following this anomalous state, the flux and variability of the broad emission lines returned to a more normal state. This transient behavior, characterized by significant deficits in flux and equivalent width of the strong broad UV emission lines, is the first of its kind to be unambiguously identified in an active galactic nucleus reverberation mapping campaign. The largest corresponding emission line flux deficits occurred for the high ionization, collisionally excited lines C IV and Si IV(+O IV]), and also He II(+O III]), while the anomaly in Ly alpha was substantially smaller. This pattern of behavior indicates a depletion in the flux of photons with E-ph > 54 eV relative to those near 13.6 eV. We suggest two plausible mechanisms for the observed behavior: (i) temporary obscuration of the ionizing continuum incident upon broad line region (BLR) clouds by a moving veil of material lying between the inner accretion disk and inner (BLR), perhaps resulting from an episodic ejection of material from the disk, or (ii) a temporary change in the intrinsic ionizing continuum spectral energy distribution resulting in a deficit of ionizing photons with energies > 54 eV, possibly due to a transient restructuring of the Comptonizing atmosphere above the disk. Current evidence appears to favor the latter explanation.

This thesis presents theoretical work on the propagation of ultra high energy cosmic rays, from their source to Earth. The different energy loss processes, resulting from cosmic ray interactions with the radiation fields, are addressed. The subsequent uncertainties in the energy loss rates and the effect produced on the arriving cosmic ray spectrum are highlighted. The question of the composition of ultra high energy cosmic rays remains unresolved, with the range of possibilities leading to quite different results in both the secondary fluxes of particles produced through cosmic ray energy loss interactions en route, and the arriving cosmic ray spectra at Earth. A large range of nuclear species are considered in this work, spanning the range of physically motivated nuclear types ejected from the cosmic ray source. The treatment of cosmic ray propagation is usually handled through Monte Carlo simulations due to the stochastic nature of some of the particle physics processes relevant. In this work, an analytic treatment for cosmic ray nuclei propagation is developed. The development of this method providing a deeper understanding of the main components relevant to cosmic ray nuclei propagation, and through its application, a clear insight into the contributing particle physics aspects of the Monte Carlo simulation. A flux of secondary neutrinos, produced as a consequence of cosmic ray energy loss through pion production during propagation, is also expected to be observed at Earth. This spectrum, however, is dependent on several loosely constrained factors such as the radiation field in the infrared region and cosmic ray composition. The range of possible neutrino fluxes obtainable with such uncertainties are discussed in this work. High energy cosmic ray interactions with the radiation fields present within the source may also occur, leading to cosmic ray energy loss before the cosmic ray has even managed to escape. The secondary spectra produced are investigated through the consideration of three candidate sources. A relationship between the degree of photo-disintegration in the source region and the neutrino flux produced through p γ interactions is found.

We report the detection of a significant infrared variability of the nearest tidal disruption event (TDE) ASASSN-14li using Wide-field Infrared Survey Explorer and newly released Near-Earth Object WISE Reactivation data. In comparison with the quiescent state, the infrared flux is brightened by 0.12 and 0.16 mag in the W1 (3.4 mu m) and W2 (4.6 mu m) bands at 36 days after the optical discovery (or similar to 110 days after the peak disruption date). The flux excess is still detectable similar to 170 days later. Assuming that the flare-like infrared emission is from the dust around the black hole, its blackbody temperature is estimated to be similar to 2.1 x 10(3) K, slightly higher than the dust sublimation temperature, indicating that the dust is likely located close to the dust sublimation radius. The equilibrium between the heating and radiation of the dust claims a bolometric luminosity of similar to 10(43) - 10(45) erg s(-1), comparable with the observed peak luminosity. This result has for the first time confirmed the detection of infrared emission from the dust echoes of TDEs.

The nucleus circularis has been linked to a role in regulating osmotic thirst but evidence has also shown that full bilateral destruction of the nucleus circularis was not necessary to achieve a deficit in drinking behavior after an osmotic challenge. The present study attempted to answer two primary research questions. The first question was whether osmoreceptive cells existed around the nucleus circularis in a homogeneous fashion or if these cells existed in a structured fashion stretching from the nucleus circularis forward. The second question was whether animals with lesions of the nucleus circularis and the surrounding areas were different in normal daily water intake than animals with no lesions. The first question was approached by lesioning the nucleus circularis, the area one millimeter anterior to the nucleus circularis, one millimeter posterior to the nucleus circularis, one half of a millimeter medial to the nucleus circularis and using a sham group which had the electrode passed through the brain to a spot one millimeter above the nucleus circularis but passing no current. All animals were then given an osmotic challenge which consisted of half of each group with an injection of hypertonic saline while the other half of each group was given isotonic saline. After a five-day recovery period, the injection procedure was reversed. Water consumption on each test day was measured at ten-minute intervals for one hour. Difference scores were then computed by subtracting the amount of water consumed after hypertonic saline injection from the amount of water consumed after isotonic saline injection. The difference scores were then used in an analysis of variance which revealed a significant difference between groups. A subsequent post hoc test showed that the nucleus circularis group was different from all other groups except for the anterior lesion group which showed a trend in the same direction as the nucleus circularis group. The second research question was approached in two ways. The first way was to simply record the amount of water consumed in each twenty-four hour period. An analysis of variance showed no significant difference between any of the groups. The second method for testing the second research question was to put the animals on a twenty-three hour water deprivation schedule and measure the amount of water consumed during the one hour when water was available. Once again, no significant differences were observed.

Swift monitoring of NGC 4151 with an similar to 6 hr sampling over a total of 69 days in early 2016 is used to construct light curves covering five bands in the X-rays (0.3-50 keV) and six in the ultraviolet (UV)/optical (1900-5500 angstrom). The three hardest X-ray bands (> 2.5 keV) are all strongly correlated with no measurable interband lag, while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging similar to 3-4 days behind the hard X-rays. The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by similar to 0.5-1 days. This combination of greater than or similar to 3 day lags between the X-rays and UV and less than or similar to 1 day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component, which illuminates the disk and drives its variability.