1,808 research outputs found
On characterising the variability properties of X-ray light curves from active galaxies
We review some practical aspects of measuring the amplitude of variability in
`red noise' light curves typical of those from Active Galactic Nuclei (AGN).
The quantities commonly used to estimate the variability amplitude in AGN light
curves, such as the fractional rms variability amplitude, F_var, and excess
variance, sigma_XS^2, are examined. Their statistical properties, relationship
to the power spectrum, and uses for investigating the nature of the variability
processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var)
shows large changes from one part of the light curve to the next, even when the
variability is produced by a stationary process. This limits the usefulness of
these estimators for quantifying differences in variability amplitude between
different sources or from epoch to epoch in one source. Some examples of the
expected scatter in the variance are tabulated for various typical power
spectral shapes, based on Monte Carlo simulations. The excess variance can be
useful for comparing the variability amplitudes of light curves in different
energy bands from the same observation. Monte Carlo simulations are used to
derive a description of the uncertainty in the amplitude expected between
different energy bands (due to measurement errors). Finally, these estimators
are used to demonstrate some variability properties of the bright Seyfert 1
galaxy Markarian 766. The source is found to show a strong, linear correlation
between rms amplitude and flux, and to show significant spectral variability.Comment: 14 pages. 12 figures. Accepted for publication in MNRA
X-ray spectral complexity in narrow-line Seyfert 1 galaxies
We present a systematic analysis of the X-ray spectral properties of a sample
of 22 ``narrow-line'' Seyfert 1 galaxies for which data are available from the
ASCA public archive. Many of these sources, which were selected on the basis of
their relatively narrow H-beta line width (FWHM <= 2000 km/s), show significant
spectral complexity in the X-ray band. Their measured hard power-law continua
have photon indices spanning the range 1.6 - 2.5 with a mean of 2.1, which is
only slightly steeper than the norm for ``broad-line'' Seyfert 1s. All but four
of the sources exhibit a soft excess, which can be modelled as blackbody
emission (T_{bb} ~ 100 - 300 eV) superposed on the underlying power-law. This
soft component is often so strong that, even in the relatively hard bandpass of
ASCA, it contains a significant fraction, if not the bulk, of the X-ray
luminosity, apparently ruling out models in which the soft excess is produced
entirely through reprocessing of the hard continuum.
Most notably, 6 of the 22 objects show evidence for a broad absorption
feature centred in the energy range 1.1 - 1.4 keV, which could be the signature
of resonance absorption in highly ionized material. A further 3 sources exhibit
``warm absorption'' edges in the 0.7 - 0.9 keV bandpass. Remarkably, all 9
``absorbed'' sources have H-beta line widths below 1000 km/s, which is less
than the median value for the sample taken as a whole. This tendency for very
narrow line widths to correlate with the presence of ionized absorption
features in the soft X-ray spectra of NLS1s, if confirmed in larger samples,
may provide a further clue in the puzzle of active galactic nuclei.Comment: 13 pages, 5 figures, accepted for publication in MNRA
The search for extraterrestrial intelligence: Telecommunications technology
Efforts to discover evidence of intelligent extraterrestrial life have become not only feasible, but respectable. Fledgling observational projects have begun that will use state-of-the-art hardware to develop sophisticated receiving and data processing systems. The rationale behind the Search for Extraterrestrial Intelligence, the manner in which the program is taking shape, and the implications for telecommunications are described. It is concluded that the breadth of technological development required for the detection of signals from galactic brethren has particular relevance for the future of telecommunications in Earth oriented uses
Migrant Farmworkers\u27 Perceptions of Pesticide Risk Exposure in Adams County, Pennsylvania: A Cultural Risk Assessment
Agricultural exceptionalism, a system in which regular labor laws and standards do not apply to farm labor, makes migrant farmworkers particularly vulnerable populations—economically, socially, and in terms of environmental health. To address inequities inherent in migrant farmworker marginÂalizaÂtion, studies advocate for actively engaging the migrant farmworker population in the conversation surrounding these issues. We conducted 40 semiÂstructured interviews with migrant farmworkers in Adams County, Pennsylvania, to understand pestiÂcide risk exposure perceptions and practices. We employed the Health Belief Model as our cultural risk assessment frame, using it in combination with technical risk assessment, which uses government calculations (from the Environmental Protection Agency) to quantify pesticide risk exposure. We used mixed methods analyses (quantitative and qualitative) to compare and understand farmworker demographics, perceived risk, perceived control, and risk behavior. Results show that demoÂgraphics —e.g., age, education, visa status—are important factors in risk perception. They also confirm observations present in many earlier studies. While trainings and educational materials are valuable to help build awareness of risk, a systemic lack of control over their circumstances make it hard for migrant farmworkers to engage in safe behavior. Results also highlight the limitations of technical risk assessment. Such calculations, however, rarely account for risk perceptions and experiences of farmÂworkers themselves. Acknowledging the voices of migrant farmworkers is an essential first step in rebalancing inequities of power in our food systems, and cultural risk assessment can help frame recommendations that target different stakeÂholders across the pesticide regulatory spectrum to ensure migrant farmworker needs and safety
MapScholar: A Web Tool for Publishing Interactive Cartographic Collections
MapScholar is an interactive visualization tool for historic map collections. It offers an open-source portal that gives individual scholars the independent means of gathering high-resolution images, analyzing them in rich geospatial contexts, and using them to illustrate new interpretations in the history of cartography and related humanities fields. It joins together data in industry-standard file formats with free and effective data-serving sites such as Flickr and Google Docs to display its on-the-fly visualizations. MapScholar enhances traditional books and articles by making it possible -- at no cost to publishers -- to mount stunning web displays of map collections assembled from libraries around the world. MapScholar’s key innovation is how it brings maps together -- regardless of the archive in which they sit -- for the purpose of generating new knowledge about human perceptions of geographic space
Potential for cogeneration of heat and electricity in California industry, phase 2
The nontechnical issues of industrial cogeneration for 12 California firms were analyzed under three categories of institutional settings: (1) industrial ownership without firm sales of power; (2) industrial ownership with firm sales of power; and (3) utility or third party ownership. Institutional issues were analyzed from the independent viewpoints of the primary parties of interest: the industrial firms, the electric utilities and the California Public utilities Commission. Air quality regulations and the agencies responsible for their promulgation were examined, and a life cycle costing model was used to evaluate the economic merits of representative conceptual cogeneration systems at these sites. Specific recommendations were made for mitigating measures and regulatory action relevant to industrial cogeneration in California
- …