Black hole magnetosphere with small scale flux tubes--II. Stability and dynamics

In some Seyfert Galaxies, the hard X-rays that produce fluorescent emission lines are thought to be generated in a hot corona that is compact and located at only a few gravitational radii above the supermassive black hole. We consider the possibility that this X-ray source may be powered by small scale magnetic flux tubes attached to the accretion disk near the black hole. We use three dimensional, time dependent force-free simulations in a simplified setting to study the dynamics of such flux tubes as they get continuously twisted by the central compact star/black hole. We find that, the dynamical evolution of the flux tubes connecting the central compact object and the accretion disk is strongly influenced by the confinement of the surrounding field. Although differential rotation between the central object and the disk tends to inflate the flux tubes, strong confinement from surrounding field quenches the formation of a jet-like outflow, as the inflated flux tube becomes kink unstable and dissipates most of the extracted rotational energy relatively close to the central object. Such a process may be able to heat up the plasma and produce strong X-ray emission. We estimate the energy dissipation rate and discuss its astrophysical implications.Comment: 16 pages, 17 figures. Accepted for publication in MNRA

Tests of a Deferred Tax Explanation of the Negative Association between the LIFO Reserve and Firm Value

Guenther and Trombley (1994) and Jennings, Simko, and Thompson (1996) document a negative association between a firm\u27s last-in, first-out (LIFO) reserve and the market value of its equity. In this paper, we test a deferred tax explanation of this negative association. Specifically, we argue that investors, conditional on adjusting inventory to as-if first-in, first-out (FIFO), estimate a firm\u27s future LIFO liquidation tax burden as its LIFO reserve multiplied by the appropriate corporate tax rate and include this tax-adjusted LIFO reserve in the valuation of a LIFO firm\u27s net assets. On the basis of this argument, the tax-adjusted LIFO reserve is in effect an estimate of an off-balance-sheet deferred tax liability and, as a result, we predict a negative association between the tax-adjusted LIFO reserve and market value of equity. We test our deferred tax explanation by estimating a valuation model in which a firm\u27s market value of equity is expressed as a function of the firm\u27s assets, liabilities, deferred tax liability, and tax-adjusted LIFO reserve; the model is estimated separately in years preceding and following the reduction of tax rates mandated by the US Tax Reform Act of 1986. Test results provide strong support for the deferred tax explanation of the negative association between a firm\u27s LIFO reserve and the market value of its equity

Portfolios in Stochastic Local Search: Efficiently Computing Most Probable Explanations in Bayesian Networks

Portfolio methods support the combination of different algorithms and heuristics, including stochastic local search (SLS) heuristics, and have been identified as a promising approach to solve computationally hard problems. While successful in experiments, theoretical foundations and analytical results for portfolio-based SLS heuristics are less developed. This article aims to improve the understanding of the role of portfolios of heuristics in SLS. We emphasize the problem of computing most probable explanations (MPEs) in Bayesian networks (BNs). Algorithmically, we discuss a portfolio-based SLS algorithm for MPE computation, Stochastic Greedy Search (SGS). SGS supports the integration of different initialization operators (or initialization heuristics) and different search operators (greedy and noisy heuristics), thereby enabling new analytical and experimental results. Analytically, we introduce a novel Markov chain model tailored to portfolio-based SLS algorithms including SGS, thereby enabling us to analytically form expected hitting time results that explain empirical run time results. For a specific BN, we show the benefit of using a homogenous initialization portfolio. To further illustrate the portfolio approach, we consider novel additive search heuristics for handling determinism in the form of zero entries in conditional probability tables in BNs. Our additive approach adds rather than multiplies probabilities when computing the utility of an explanation. We motivate the additive measure by studying the dramatic impact of zero entries in conditional probability tables on the number of zero-probability explanations, which again complicates the search process. We consider the relationship between MAXSAT and MPE, and show that additive utility (or gain) is a generalization, to the probabilistic setting, of MAXSAT utility (or gain) used in the celebrated GSAT and WalkSAT algorithms and their descendants. Utilizing our Markov chain framework, we show that expected hitting time is a rational function - i.e. a ratio of two polynomials - of the probability of applying an additive search operator. Experimentally, we report on synthetically generated BNs as well as BNs from applications, and compare SGSs performance to that of Hugin, which performs BN inference by compilation to and propagation in clique trees. On synthetic networks, SGS speeds up computation by approximately two orders of magnitude compared to Hugin. In application networks, our approach is highly competitive in Bayesian networks with a high degree of determinism. In addition to showing that stochastic local search can be competitive with clique tree clustering, our empirical results provide an improved understanding of the circumstances under which portfolio-based SLS outperforms clique tree clustering and vice versa

Initialization and Restart in Stochastic Local Search: Computing a Most Probable Explanation in Bayesian Networks

For hard computational problems, stochastic local search has proven to be a competitive approach to finding optimal or approximately optimal problem solutions. Two key research questions for stochastic local search algorithms are: Which algorithms are effective for initialization? When should the search process be restarted? In the present work we investigate these research questions in the context of approximate computation of most probable explanations (MPEs) in Bayesian networks (BNs). We introduce a novel approach, based on the Viterbi algorithm, to explanation initialization in BNs. While the Viterbi algorithm works on sequences and trees, our approach works on BNs with arbitrary topologies. We also give a novel formalization of stochastic local search, with focus on initialization and restart, using probability theory and mixture models. Experimentally, we apply our methods to the problem of MPE computation, using a stochastic local search algorithm known as Stochastic Greedy Search. By carefully optimizing both initialization and restart, we reduce the MPE search time for application BNs by several orders of magnitude compared to using uniform at random initialization without restart. On several BNs from applications, the performance of Stochastic Greedy Search is competitive with clique tree clustering, a state-of-the-art exact algorithm used for MPE computation in BNs

Internal Control Disclosures, Monitoring, and the Cost of Debt

We test the relationship between the change in a firm\u27s cost of debt and the disclosure of a material weakness in an initial Section 404 report. We find that, on average, a firm\u27s credit spread on its publicly traded debt marginally increases if it discloses a material weakness. We also examine the impact of monitoring by credit rating agencies and/or banks on this result and find that the result is more pronounced for firms that are not monitored. Additional analysis indicates that the effect of bank monitoring appears to be the primary driver of these monitoring results. This finding is consistent with the argument that banks are effective delegated monitors for the debt market. The results of this study suggest the need for future research, particularly to test the differential effects of monitoring on the cost of debt compared to the cost of equity

The multi-epoch X-ray tale of I Zwicky 1 outflows

The narrow-line Seyfert 1 galaxy I Zwicky 1 shows a unique and complex system of ionised gas in outflow, which consists of an ultra-fast wind and a two-component warm absorber. In the last two decades, XMM-Newton monitored the source multiple times enabling the study of the long-term variability of the various outflows. Plasma in photoionisation equilibrium with the ionising source responds and varies accordingly to any change of the ionising luminosity. However, detailed modelling of the past RGS data has shown no correlation between the plasma ionisation state and the ionising continuum, revealing a complex long-term variability of the multi-phase warm absorber. Here, we present a new observation of I Zwicky 1 by XMM-Newton taken in early 2020 characterised by a lower X-ray flux state. The soft X-ray spectrum from the RGS reveals the two components of the warm absorber with $\log \xi \sim -1.0$ and $\log \xi \sim 1.7$. Comparing our results with the previous observations, the ionisation state of the two absorbing gas components is continuously changing, following the same unpredictable behaviour. The new results strengthen the scenario in which the ionisation state of the warm absorber is driven by the density of the gas rather than the ionising luminosity. In particular, the presence of a radiation driven, inhomogeneous clumpy outflow may explain both the variability in ionisation throughout the years and the line-locked N V system observed in the UV band. Finally, the EPIC-pn spectrum reveals an ultra-fast wind with an outflow velocity of $\sim 0.26c$ and ionisation parameter of $\log \xi \sim 3.8$.Comment: 16 pages, 11 figures, 3 tables, accepted for publication in MNRA

X-ray/UVOIR Frequency-resolved Time Lag Analysis of Mrk 335 Reveals Accretion Disk Reprocessing

UV and optical continuum reverberation mapping is powerful for probing the accretion disk and inner broad-line region. However, recent reverberation mapping campaigns in the X-ray, UV, and optical have found lags consistently longer than those expected from the standard disk reprocessing picture. The largest discrepancy to-date was recently reported in Mrk 335, where UV/optical lags are up to 12 times longer than expected. Here, we perform a frequency-resolved time lag analysis of Mrk 335, using Gaussian processes to account for irregular sampling. For the first time, we compare the Fourier frequency-resolved lags directly to those computed using the popular Interpolated Cross-Correlation Function (ICCF) method applied to both the original and detrended light curves. We show that the anticipated disk reverberation lags are recovered by the Fourier lags when zeroing in on the short-timescale variability. This suggests that a separate variability component is present on long timescales. If this separate component is modeled as reverberation from another region beyond the accretion disk, we constrain a size-scale of roughly 15 light-days from the central black hole. This is consistent with the size of the broad line region inferred from H$\beta$ reverberation lags. We also find tentative evidence for a soft X-ray lag, which we propose may be due to light travel time delays between the hard X-ray corona and distant photoionized gas that dominates the soft X-ray spectrum below 2 keV.Comment: Accepted for publication in ApJ, 8 figure

A Century on the Northern Plains: the University of North Dakota at 100

This is the Centennial History of the University of North Dakota, published in 1983.https://commons.und.edu/und-books/1001/thumbnail.jp

The Influences of Ambidexterity, New Public Management and Innovation on the Public Service Quality of Government Organizations

This study assesses the influences of organizational ambidexterity and new public management (NPM) innovations on public service quality in the under-researched context of government organizations in the Arabian Peninsula Gulf States. The data used were obtained from government service organizations in the Sultanate of Oman and were analyzed using structural equation modelling. The findings indicate that exploitation is associated with public service quality and this relationship is partially mediated by public service innovation. The results show an indirect relationship between NPM and service quality mediated by service innovation. Moreover, the relationship between exploration and service quality is fully mediated by service innovation. This study extends the existing research on ambidexterity in government organizations, NPM and public service innovation and contributes to the international public management reform, organizational ambidexterity and public sector innovation literature by examining the applicability of Western practices in changing societal and politico-administrative contexts