Simulations of Stellar Collisions Involving Pre-Main Sequence Stars

In this paper, we present the results of smoothed particle hydrodynamic (SPH) simulations of collisions between pre-main sequence stars and a variety of other kinds of stars. Simulations over a range of impact parameters and velocities were performed. We find that pre-main sequence stars tend to ``wrap themselves'' around their impactor. We discuss the probable evolutionary state of products of collisions between pre-main sequence stars and pre-main sequence, main sequence, giant branch, and compact stars. The nature of the collision product does not depend strongly on the impact parameter or the velocity of the collision.Comment: Accepted by Ap

10 Years of Object-Oriented Analysis on H1

Over a decade ago, the H1 Collaboration decided to embrace the object-oriented paradigm and completely redesign its data analysis model and data storage format. The event data model, based on the RooT framework, consists of three layers - tracks and calorimeter clusters, identified particles and finally event summary data - with a singleton class providing unified access. This original solution was then augmented with a fourth layer containing user-defined objects. This contribution will summarise the history of the solutions used, from modifications to the original design, to the evolution of the high-level end-user analysis object framework which is used by H1 today. Several important issues are addressed - the portability of expert knowledge to increase the efficiency of data analysis, the flexibility of the framework to incorporate new analyses, the performance and ease of use, and lessons learned for future projects.Comment: 14th International Workshop on Advanced Computing and Analysis Techniques in Physics Researc

A Striking Confluence Between Theory and Observations of High-Mass X-ray Binary Pulsars

We analyse the most powerful X-ray outbursts from neutron stars in ten Magellanic high-mass X-ray binaries and three pulsating ultraluminous X-ray sources. Most of the outbursts rise to $L_{max}$ which is about the level of the Eddington luminosity, while the rest and more powerful outbursts also appear to recognize that limit when their emissions are assumed to be anisotropic and beamed toward our direction. We use the measurements of pulsar spin periods $P_S$ and their derivatives $\dot{P_S}$ to calculate the X-ray luminosities $L_p$ in their faintest accreting ("propeller") states. In four cases with unknown $\dot{P_S}$, we use the lowest observed X-ray luminosities, which only adds to the heterogeneity of the sample. Then we calculate the ratios $L_p/L_{max}$ and we obtain an outstanding confluence of theory and observations from which we conclude that work done on both fronts is accurate and the results are trustworthy: sources known to reside on the lowest Magellanic propeller line are all located on/near that line, whereas other sources jump higher and reach higher-lying propeller lines. These jumps can be interpreted in only one way, higher-lying pulsars have stronger surface magnetic fields in agreement with empirical results in which $\dot{P_S}$ and $L_p$ values were not used.Comment: Added LMC X-4 and commented on the cyclotron absorption line of SMC X-2. 4 pages, 1 figure, 2 tables, submitted to MNRAS

Long-term Properties of Accretion Disks in X-ray Binaries: II. Stability of Radiation-Driven Warping

A significant number of X-ray binaries are now known to exhibit long-term ``superorbital'' periodicities on timescales of $\sim$ 10 - 100 days. Several physical mechanisms have been proposed that give rise to such periodicities, in particular warping and/or precession of the accretion disk. Recent theoretical work predicts the stability to disk warping of X-ray binaries as a function of the mass ratio, binary radius, viscosity and accretion efficiency, and here we examine the constraints that can be placed on such models by current observations. In paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray datasets (CGRO, RXTE), focusing on the remarkable, smooth variations in the superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to investigate the stability of the superorbital periodicities in the neutron star X-ray binaries Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability predictions with observation. We find that the period and nature of superorbital variations in these sources is consistent with the predictions of warping theory. We also use a dynamic lightcurve analysis to examine the behaviour of Her X-1 as it enters and leaves the 1999 Anomalous Low State (ALS). This reveals a significant phase shift some 15 cycles before the ALS, which indicates a change in the disk structure or profile leading into the ALS.Comment: 12 pages, 14 figures, Re-submitted to MNRAS after referee's comment

Competition in UK Electricity markets

This article includes information relating to competition in the UK electricity market, formerly published as part of UK Energy Sector Indicators. The article examines the two parts of the industry where there is competition for provision: generation and sales. For both markets, the article describes the number of companies operating, and the market concentrations. The Herfindahl-Hirschman measure (see explanation at the end of this article) is used to provide the market concentration as it provides extra emphasis on the contribution of participants with the largest shares. For electricity sales, this article covers the major suppliers1 surveyed by BEIS comprising approximately 96% of the market

The Great Pretenders Among the ULX Class

The recent discoveries of pulsed X-ray emission from three ultraluminous X-ray (ULX) sources have finally enabled us to recognize a subclass within the ULX class: the great pretenders, neutron stars (NSs) that appear to emit X-ray radiation at isotropic luminosities $L_X = 7\times 10^{39}$~erg~s$^{-1}-1\times 10^{41}$~erg~s$^{-1}$ only because their emissions are strongly beamed toward our direction and our sight lines are offset by only a few degrees from their magnetic-dipole axes. The three known pretenders appear to be stronger emitters than the presumed black holes of the ULX class, such as Holmberg II \& IX X-1, IC10 X-1, and NGC300 X-1. For these three NSs, we have adopted a single reasonable assumption, that their brightest observed outbursts unfold at the Eddington rate, and we have calculated both their propeller states and their surface magnetic-field magnitudes. We find that the results are not at all different from those recently obtained for the Magellanic Be/X-ray pulsars: the three NSs reveal modest magnetic fields of about 0.3-0.4~TG and beamed propeller-line X-ray luminosities of $\sim 10^{36-37}$~erg~s$^{-1}$, substantially below the Eddington limit.Comment: To appear in Research in Astronomy and Astrophysic

Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud

Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray pulsars. Together with discoveries from other satellites at least 45 SMC pulsars are now known. One of these sources, a pulsar with a period of approximately 7.8 seconds, was first detected in early 2002 and since discovery it has been found to be in outburst nine times. The outburst pattern clearly shows a period of 45.1 +/- 0.4 d which is thought to be the orbital period of this system. Candidate outburst periods have also been obtained for nine other pulsars and continued monitoring will enable us to confirm these. This large number of pulsars, all located at approximately the same distance, enables a wealth of comparative studies. In addition, the large number of pulsars found (which vastly exceeds the number expected simply by scaling the relative mass of the SMC and the Galaxy) reveals the recent star formation history of the SMC which has been influenced by encounters with both the LMC and the Galaxy.Comment: 5 pages, 4 figures, AIP conference proceedings format. Contribution to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA, November, 200