Browsing by Subject "variability"

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    Autogenic signals in an experimental source-to-sink system
    (2015-12) Gazzetti, Edward
    Interpretations of the sedimentary rock record typically focus on allogenic forcing like fluctuations in relative sea-level and autogenic processes like fluvial avulsions and delta-lobe switching within the depositional system. I investigated the potential for drainage-basin-derived autogenic variability in sediment discharge to be preserved in the sedimentary rock record. I constructed a source-to-sink experimental apparatus to study the development of autogenic variability ‘signals’ in a net-erosional drainage basin and to trace these autogenic signals to a connected net-depositional basin (the ‘sink’), where they are preserved in a prograding fluvo-deltaic system. By maintaining constant tectonic uplift rate, climate (precipitation rate), and rock strength, I attempted to isolate and measure how allogenic forcing generates autogenic signals in the source terrane. Experimental results suggest that the interaction of landslides with channel processes in the drainage basin generates sediment-discharge variability. The scale of experimental landsliding is set by ridge relief, and landscapes with high rock strength, high uplift rate, and low precipitation rate have the potential to generate high-relief ridges. Thus, these systems can generate relatively large autogenic variability (+/- 10% from the mean) in sediment discharge. These experiments suggest that this autogenic variability in sediment supply propagates through the net-depositional fluvial system with minor time lags and can be preserved in the deltaic strata.
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    The State of the Blazar: Investigations of Variability Patterns in the Very High Energy Gamma-ray Emission from Northern Blazars
    (2017-12) Shahinyan, Karlen
    Blazars are active galactic nuclei with jets aimed in our direction. They are one of the most energetic astrophysical phenomena and in the very-high-energy (VHE; > 100 GeV) gamma-ray regime comprise the dominant extragalactic source class. Variability in the non-thermal emission of blazars is one of the key observables for discerning the details of their emission regions. In this thesis, I explore VHE gamma-ray emission properties of blazars in a variety of states. After a detailed description of the VERITAS instrument and data analysis, I present an in-depth study of an unusually stable extreme blazar (formerly, an unidentified source), HESS J1943+213, for which no concrete evidence of variability has been detected to date. The study includes the steps towards identifying HESS J1943+213 as a blazar using VLBA observations and estimation of the source redshift with gamma-ray spectra. Models of leptonic emission from the source and hadronic emission from cosmic-ray interactions along the line of sight are presented for explaining the gamma-ray observations. I then proceed to characterize the emission of two blazars in rapidly flaring states, Mrk 421 and MS 1221.8+2452. The dataset for Mrk 421, the first blazar detected in VHE gamma rays, comes from one of the brightest VHE gamma-ray flares ever observed and offers a unique opportunity for exploring VHE gamma-ray and multi-wavelength emissions of the source on timescales of few minutes. The flare of MS 1221.8+2452, on the other hand, allows for the construction of the first VHE gamma-ray spectrum of the source, which is used along with contemporaneous multi-wavelength observations to construct and model the SED of the source during the flare. Moreover, the source exhibits a spectral hysteresis pattern in X-rays during the flaring period, which is tied to the acceleration and cooling timescales of particles in the emission zone. Finally, I investigate, PG 1553+113, a blazar exhibiting long-term periodic flux modulations in lower-energy gamma rays – due to periodic accretion flow instabilities, jet precession, or the presence of a binary supermassive black hole system – and search for a similar periodicity signal in the VHE gamma rays.