Timing accuracy of the Swift X-Ray Telescope in WT mode

The X-Ray Telescope (XRT) on board Swift was mainly designed to provide detailed position, timing and spectroscopic information on Gamma-Ray Burst (GRB) afterglows. During the mission lifetime the fraction of observing time allocated to other types of source has been steadily increased. In this paper, we report on the results of the in-flight calibration of the timing capabilities of the XRT in Windowed Timing read-out mode. We use observations of the Crab pulsar to evaluate the accuracy of the pulse period determination by comparing the values obtained by the XRT timing analysis with the values derived from radio monitoring. We also check the absolute time reconstruction measuring the phase position of the main peak in the Crab profile and comparing it both with the value reported in literature and with the result that we obtain from a simultaneous Rossi X-Ray Timing Explorer (RXTE) observation. We find that the accuracy in period determination for the Crab pulsar is of the order of a few picoseconds for the observation with the largest data time span. The absolute time reconstruction, measured using the position of the Crab main peak, shows that the main peak anticipates the phase of the position reported in literature for RXTE by ~270 microseconds on average (~150 microseconds when data are reduced with the attitude file corrected with the UVOT data). The analysis of the simultaneous Swift-XRT and RXTE Proportional Counter Array (PCA) observations confirms that the XRT Crab profile leads the PCA profile by ~200 microseconds. The analysis of XRT Photodiode mode data and BAT event data shows a main peak position in good agreement with the RXTE, suggesting the discrepancy observed in XRT data in Windowed Timing mode is likely due to a systematic offset in the time assignment for this XRT read out mode.Comment: 6 pages, 4 figures. Accepted for publication on Astronomy&Astrophysic

Remarks on Non-Abelian Duality

A class of two-dimensional globally scale-invariant, but not conformally invariant, theories is obtained. These systems are identified in the process of discussing global and local scaling properties of models related by duality transformations, based on non-semisimple isometry groups. The construction of the dual partner of a given model is followed through; non-local as well as local versions of the former are discussed.Comment: 33 pages, CERN-TH.7414/94, RI-9-94, WIS-7-9

Simultaneous UBVRI observations of the cataclysmic variable AE Aquarii: temperature and mass of fireballs

We report simultaneous multicolour observations in 5 bands (UBVRI) of the flickering variability of the cataclysmic variable AE Aqr. Our aim is to estimate the parameters (colours, temperature, size) of the fireballs that produce the optical flares. The observed rise time of the optical flares is in the interval 220 - 440 sec. We estimate the dereddened colours of the fireballs: (U-B)_0 in the range 0.8-1.4, (B-V)_0 ~ 0.03-0.24, (V-I)_0 ~ 0.26-0.78. We find for the fireballs a temperature in the range 10000 - 25000 K, mass (7-90).10^{19} g, size (3-7).10^9 cm (using a distance of d=86 pc). These values refer to the peak of the flares observed in UBVRI bands. The data are available upon request from the authors.Comment: 8 pages, accepted in A

Maximum entropy niche-based modelling of seasonal changes in little bustard (Tetrax tetrax) distribution

P. 17-29The effects of habitat fragmentation on species may change seasonally mainly due to variations in resource availability and biotic interactions. In critical periods, such as winter, when the importance of intraspecific competition diminish, species may relax their environmental requirements widening their ecological niche to exploit the scarcer trophic resources more efficiently in comparison with spring. Those variations in niche width may implicate seasonal expansions/retractions in species distribution. In this sense, an integrated knowledge on the spatial arrangement of breeding and wintering suitable patches is essential to infer seasonal movements (migratory connectivity). This paper shows that little bustard environmental preferences were more predictable and complex (controlled by a larger number of environmental factors) in spring than in winter, when potential distribution and ecological niche width were slightly larger. In spring, habitat variables (i.e. percentage of dry crops and pasturelands and altitude) ruled species’ distribution; while, winter pattern was driven by mixed criteria, based on both habitat and climate (i.e. percentage of dry crops and wastelands and winter rainfall). Suitable patches were more connected across spatial scales in winter than in spring, i.e. landscape was perceived as less fragmented. The overlap between potential breeding and wintering distribution areas was high. In fact, most of the predicted wintering areas coincided or showed high connectedness with predicted breeding patches. Conversely, there were significant breeding patches that were predicted with low suitability, showing little connectedness with potential winter areas. Spring habitat was a better predictor of little bustard’s wintering range than vice versa, which has clear management implications (preserving breeding sites closer to wintering areas ensures the conservation of a larger proportion of the total distribution range). This is an example of how predictive large-scale modeling procedures can contribute to the optimization of land management aimed at species conservation.S