Absolute timing of the Crab pulsar with the INTEGRAL/SPI telescope

We have investigated the pulse shape evolution of the Crab pulsar emission in the hard X-ray domain of the electromagnetic spectrum. In particular, we have studied the alignment of the Crab pulsar phase profiles measured in the hard X-rays and in other wavebands. To obtain the hard X-ray pulse profiles, we have used six year (2003-2009, with a total exposure of about 4 Ms) of publicly available data of the SPI telescope on-board of the INTEGRAL observatory, folded with the pulsar time solution derived from the Jodrell Bank Crab Pulsar Monthly Ephemeris. We found that the main pulse in the hard X-ray 20-100 keV energy band is leading the radio one by $8.18\pm0.46$ milliperiods in phase, or $275\pm15 \mu s$ in time. Quoted errors represent only statistical uncertainties.Our systematic error is estimated to be $\sim 40 \mu s$ and is mainly caused by the radio measurement uncertainties. In hard X-rays, the average distance between the main pulse and interpulse on the phase plane is $0.3989\pm0.0009$. To compare our findings in hard X-rays with the soft 2-20 keV X-ray band, we have used data of quasi-simultaneous Crab observations with the PCA monitor on-board the Rossi X-Ray Timing Explorer (RXTE) mission. The time lag and the pulses separation values measured in the 3-20 keV band are $0.00933\pm0.00016$ (corresponding to $310\pm6 \mu s$) and $0.40016\pm0.00028$ parts of the cycle, respectively. While the pulse separation values measured in soft X-rays and hard X-rays agree, the time lags are statistically different. Additional analysis show that the delay between the radio and X-ray signals varies with energy in the 2 - 300 keV energy range. We explain such a behaviour as due to the superposition of two independent components responsible for the Crab pulsed emission in this energy band

Tracking granules on the Sun's surface and reconstructing horizontal velocity fields: I. the CST algorithm

Determination of horizontal velocity fields on the solar surface is crucial for understanding the dynamics of structures like mesogranulation or supergranulation or simply the distribution of magnetic fields. We pursue here the development of a method called CST for coherent structure tracking, which determines the horizontal motion of granules in the field of view. We first devise a generalization of Strous method for the segmentation of images and show that when segmentation follows the shape of granules more closely, granule tracking is less effective for large granules because of increased sensitivity to granule fragmentation. We then introduce the multi-resolution analysis on the velocity field, based on Daubechies wavelets, which provides a view of this field on different scales. An algorithm for computing the field derivatives, like the horizontal divergence and the vertical vorticity, is also devised. The effects from the lack of data or from terrestrial atmospheric distortion of the images are also briefly discussed.Comment: in press in Astronomy and Astrophysics, 9 page

Is the depressive effect of renewables on power prices contagious? A cross border econometric analysis

European power markets have become more integrated and the implementation of market coupling has reinforced the efficiency of cross-border trading. This paper investigates empirically the impact of renewables growth in Germany on German and French power price volatility. We find that renewables depress power prices on average and increase volatility not only domestically but also across borders. We also leverage market resiliency data to investigate the impact of increases in interconnection capacity. We find that power price volatility would decrease in France despite some contagion effects of volatility from German renewables production. Our findings have important policy implications as they demonstrate the need to coordinate cross-border support policies for renewables in order to mitigate the impact of volatility on power prices in coupled power markets

Kuiper Belt Object Occultations: Expected Rates, False Positives, and Survey Design

A novel method of generating artificial scintillation noise is developed and used to evaluate occultation rates and false positive rates for surveys probing the Kuiper Belt with the method of serendipitous stellar occultations. A thorough examination of survey design shows that: (1) diffraction-dominated occultations are critically (Nyquist) sampled at a rate of 2 Fsu^{-1}, corresponding to 40 s^{-1} for objects at 40 AU, (2) occultation detection rates are maximized when targets are observed at solar opposition, (3) Main Belt Asteroids will produce occultations lightcurves identical to those of Kuiper Belt Objects if target stars are observed at solar elongations of: 116 deg < epsilon < 125 deg, or 131 deg < epsilon < 141 deg, and (4) genuine KBO occultations are likely to be so rare that a detection threshold of >7-8 sigma should be adopted to ensure that viable candidate events can be disentangled from false positives.Comment: Accepted AJ, 12 pages, 12 figure

Determination of Optical Properties of a Field Emission Gun Coupled with a Linear Accelerator for High Voltage Microscopy

The electron optical properties of a field emission gun plus a twenty-stage linear accelerator system have been determined in the case where the entrance pupil of the system is defined by one of the electrode apertures. The first part of the calculation concerns the electrical and geometrical parameters of triode and tetrode field emission guns which can give a fixed location of the source and a small spherical aberration coefficient. The second part is related to the possibility of placing a tetrode field emission gun at the top of the linear accelerator of the scanning high voltage electron microscope which is being constructed in the Toulouse Laboratory. Interesting conditions can be obtained when the electrical parameters are fixed as follows: either Vi/Ve= 0 or Vi/Ve= 20 (Ve is the extracting voltage, Vi is the potential of the second anode of the gun) whatever the accelerating voltage may be

A single sub-km Kuiper Belt object from a stellar Occultation in archival data

The Kuiper belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper belt objects (KBOs). Small, sub-km sized, KBOs elude direct detection, but the signature of their occultations of background stars should be detectable. Observations at both optical and X-ray wavelengths claim to have detected such occultations, but their implied KBO abundances are inconsistent with each other and far exceed theoretical expectations. Here, we report an analysis of archival data that reveals an occultation by a body with a 500 m radius at a distance of 45 AU. The probability of this event to occur due to random statistical fluctuations within our data set is about 2%. Our survey yields a surface density of KBOs with radii larger than 250 m of 2.1^{+4.8}_{-1.7} x 10^7 deg^{-2}, ruling out inferred surface densities from previous claimed detections by more than 5 sigma. The fact that we detected only one event, firmly shows a deficit of sub-km sized KBOs compared to a population extrapolated from objects with r>50 km. This implies that sub-km sized KBOs are undergoing collisional erosion, just like debris disks observed around other stars.Comment: To appear in Nature on December 17, 2009. Under press embargo until 1800 hours London time on 16 December. 19 pages; 7 figure

The INTEGRAL/SPI response and the Crab observations

The Crab region was observed several times by INTEGRAL for calibration purposes. This paper aims at underlining the systematic interactions between (i) observations of this reference source, (ii) in-flight calibration of the instrumental response and (iii) the development and validation of the analysis tools of the SPI spectrometer. It first describes the way the response is produced and how studies of the Crab spectrum lead to improvements and corrections in the initial response. Then, we present the tools which were developed to extract spectra from the SPI observation data and finally a Crab spectrum obtained with one of these methods, to show the agreement with previous experiments. We conclude with the work still ahead to understand residual uncertainties in the response.Comment: 4 pages, 4 figures, Proc. of the 5th INTEGRAL Workshop (Feb. 16-20 2004), to be published by ES

Mid-Infrared Ethane Emission on Neptune and Uranus

We report 8- to 13-micron spectral observations of Neptune and Uranus from the NASA Infrared Telescope Facility spanning more than a decade. The spectroscopic data indicate a steady increase in Neptune's mean atmospheric 12-micron ethane emission from 1985 to 2003, followed by a slight decrease in 2004. The simplest explanation for the intensity variation is an increase in stratospheric effective temperature from 155 +/- 3 K in 1985 to 176 +/- 3 K in 2003 (an average rate of 1.2 K/year), and subsequent decrease to 165 +/- 3 K in 2004. We also detected variation of the overall spectral structure of the ethane band, specifically an apparent absorption structure in the central portion of the band; this structure arises from coarse spectral sampling coupled with a non-uniform response function within the detector elements. We also report a probable direct detection of ethane emission on Uranus. The deduced peak mole fraction is approximately an order of magnitude higher than previous upper limits for Uranus. The model fit suggests an effective temperature of 114 +/- 3 K for the globally-averaged stratosphere of Uranus, which is consistent with recent measurements indicative of seasonal variation.Comment: Accepted for publication in ApJ. 16 pages, 10 figures, 2 table

Wetting to Non-wetting Transition in Sodium-Coated C_60

Based on ab initi and density-functional theory calculations, an empirical potential is proposed to model the interaction between a fullerene molecule and many sodium atoms. This model predicts homogeneous coverage of C_60 below 8 Na atoms, and a progressive droplet formation above this size. The effects of ionization, temperature, and external electric field indicate that the various, and apparently contradictory, experimental results can indeed be put into agreement.Comment: 4 pages, 4 postscript figure