Analyzing X-ray variability by State Space Models

In recent years, autoregressive models have had a profound impact on the description of astronomical time series as the observation of a stochastic process. These methods have advantages compared with common Fourier techniques concerning their inherent stationarity and physical background. If autoregressive models are used, however, it has to be taken into account that real data always contain observational noise often obscuring the intrinsic time series of the object. We apply the technique of a Linear State Space Model which explicitly models the noise of astronomical data and allows to estimate the hidden autoregressive process. As an example, we have analysed a sample of Active Galactic Nuclei (AGN) observed with EXOSAT and found evidence for a relationship between the relaxation timescale and the spectral hardness.Comment: 4 pages, Latex, uses Kluwer Style file crckapb.cls To appear in Proc. of Astronomical Time Series, Tel Aviv, 199

Analysis and interpretation of X-ray pulsars

By careful measurements of the fluctuations in pulsar pulse periods on time scales of days and longer, researchers determined that these fluctuations are caused by changes in the rotation rate of the stellar crust apparently arising from matter accretion. The study of these fluctuations is a particularly promising way to determine the properties of accreting pulsars, because stellar rotation is relatively simple in comparison to much other X-ray source physics and can be investigated in detail. Rotation rates can be determined precisely

Updating the orbital ephemeris of Her X-1; rate of decay and eccentricity of the orbit

We present an update of the orbital ephemeris of the binary X-ray pulsar Her X-1 and determine an improved value for the rate of orbital decay. In addition, we report the first measurement of the orbital eccentricity. We have analyzed pulse timing data of Her X-1 from X-ray observations by RXTE (Rossi X-Ray Timing Explorer) and INTEGRAL over the period 1996-2007. Accurate pulse arrival times were determined from solar system bary-centered photon arrival times by generating pulse profiles averaged over appropriately short integration times. Applying pulse phase connection techniques, it was possible to determine sufficiently accurate local ephemeris data for seven observation periods distributed over 12 years. Combining the new local T90 values with historical values from the literature we update the orbital ephemeris of Her X-1 to T90 = MJD 46359.871940(6) and Porb = 1.700167590(2) d and measure a continuous change of the orbital period of dPorb/dt = -(4.85 +/- 0.13) x 10-11 s/s. For the first time, a value for the eccentricity of the orbit of Her X-1 is measured to be e = (4.2 +/- 0.8) x 10-4.Comment: 7 pages, 4 figures, accepted by A&A on 30.03.200

Human reliability analysis in healthcare: Application of the cognitive reliability and error analysis method (CREAM) in a hospital setting

Patient safety is a concern within the healthcare domain as it is estimated that tens of thousands of people die annually from preventable medical errors. For over ten years, traditional Human Reliability Analysis (HRA) techniques (e.g., Root Cause Analysis and Failure Mode and Effect Analysis) have been used in hospitals nationwide in an attempt to explain why these errors occur and what can be done to prevent them. Still, patient safety has not improved significantly. Traditional HRA techniques are limited as analysis tools. They do not consider the context in which workers operate. They are also not based on a valid psychological model that could explain human cognitive function. The Cognitive Reliability and Error Analysis Method (CREAM) is an HRA technique that allows analysts to examine worker actions through the context of performance-shaping factors. The CREAM also employs a cognitive model to explain cognitive failures. This research used the CREAM to re-analyze events containing identifiable error modes that were previously analyzed by hospital team members using the RCA technique. The results of the re-analyses using the CREAM were compared with the previous analyses from RCA events. Additionally, several RCA events were observed and detailed written narratives of the observations were used to perform further independent analyses by three independent analysts in an effort to calculate inter-rater agreement. The results exposed a gap within categories of causal factors between the two techniques. The CREAM identified organizational factors as contributing to error in the events whereas those factors were either minimized or ignored in the RCA. The results also failed to demonstrate any significant inter-rater agreement among independent analysts performing the CREAM analyses. Due to serious data limitations, detailed analyses using the CREAM were not possible

Quantifying Rapid Variability in Accreting Compact Objects

I discuss some practical aspects of the analysis of millisecond time variability X-ray data obtained from accreting neutron stars and black holes. First I give an account of the statistical methods that are at present commonly applied in this field. These are mostly based on Fourier techniques. To a large extent these methods work well: they give astronomers the answers they need. Then I discuss a number of statistical questions that astronomers don't really know how to solve properly and that statisticians may have ideas about. These questions have to do with the highest and the lowest frequency ranges accessible in the Fourier analysis: how do you determine the shortest time scale present in the variability, how do you measure steep low-frequency noise. The point is stressed that in order for any method that resolves these issues to become popular, it is necessary to retain the capabilities the current methods already have in quantifying the complex, concurrent variability processes characteristic of accreting neutron stars and black holes.Comment: To be published in the Proceedings of "Statistical Challenges in Modern Astronomy II", University Park PA, USA, June 199

Transformations of Lines and Conics in The Z-Plane

The purpose of the problem was to investigate the behavior of curves under some simple complex transformations. The transformations used were limited tow,= z 2 w = z2 1 , , and w = -z • The curves considered were limited to straight lines and conic sections. However, the general cases of the conics were usually too complicated to be dealt with in the thesis. Therefore, most of the conics considered were special cases which were simpler and from which some indication of t he behavior of more general cases might be found. Some interesting special cases of the more complicated transformations were treated briefly, as were practical applications of complex transformations. Sketches were included showing the results of the transformations in graphic form. It was noted that, in general, subjection of a curve to a transformation complicated that curve. Cases in which the curve was simplified were less numerous, but usually had gr eater chance of application