Spin-resolved spectroscopy of the intermediate polar DQ Her

We present high-speed spectroscopic observations of the intermediate polar (IP) DQ Herculis. Doppler tomography of two He I lines reveals a spiral density structure in the accretion disc around the white dwarf (WD) primary. The spirals look very similar to the spirals seen in dwarf novae during outburst. DQ Her is the first well-established IP in which spirals are seen, which are in addition likely persistent because of the system's high mass transfer rate. Spiral structures give an alternative explanation for sidebands of the WD spin frequency that are found in IP light curves. The Doppler tomogram of He II lambda 4686 indicates that a large part of the emission is not disc-like. Spin trails of spectra reveal a pulsation in the He II lambda 4686 emission that is believed to result from reprocessing of X-rays from the WD's magnetic poles in the accretion flow close to the WD. We confirm the previous finding that the pulsation is only visible in the redshifted part of the line when the beam points to the back side of the disc. The absence of reprocessed light from the front side of the disc can be explained by obscuration by the front rim of the disc, but the absence of extra emission from the blueshifted back side of the disc is puzzling. Reprocessing in accretion curtains can be an answer to the problem and can also explain the highly non-Keplerian velocity components that are found in the He II lambda 4686 line. Our spin trails can form a strong test for future accretion curtain models, with the possibility of distinguishing between a spin period of 71 or 142 s. Spin trails of data taken at selected orbital phases show little evidence for a significant contribution of the bright spot to the pulsations and allow us to exclude a recent suggestion that 71 s is the beat period and 70.8 s the spin period

First principles studies of modulated Co/Cu superlattices with strongly and weakly exchange biased Co-monolayers

First-principles calculations have been performed in order to determine effective exchange integrals between {\it strongly} and {\it weakly} exchange-coupled Co monolayers in certain modulated periodic $CoCu_2/CoCu_n$-type superlattices with three non-equivalent Co planes, which have not yet been studied hitherto. For $3\le n\le 6$ we find that the two non-equivalent exchange integrals have opposite signs, i.e.~the strong coupling is antiferromagnetic and the weak coupling ferromagnetic, and differ for $n\ne 4$ from each other by one order of magnitude. It is shown that the results depend on the system as a whole and could not be obtained from separate parts. Finally we suggest that ''spin valve'' systems of such kind should be considered when trying to obtain good magneto-resistance together with low switching-fields.Comment: LaTex, 9 pages, including two .eps-figure

Investigating magnetic activity of F stars with the it Kepler mission

The dynamo process is believed to drive the magnetic activity of stars like the Sun that have an outer convection zone. Large spectroscopic surveys showed that there is a relation between the rotation periods and the cycle periods: the longer the rotation period is, the longer the magnetic activity cycle period will be. We present the analysis of F stars observed by Kepler for which individual p modes have been measure and with surface rotation periods shorter than 12 days. We defined magnetic indicators and proxies based on photometric observations to help characterise the activity levels of the stars. With the Kepler data, we investigate the existence of stars with cycles (regular or not), stars with a modulation that could be related to magnetic activity, and stars that seem to show a flat behaviour.Comment: 2 pages, 1 figure, proceedings of IAU Symposium 302 'Magnetic fields through stellar evolution', 25-30 August 2013, Biarritz, Franc

The COMPTEL instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. The identification of instrumental lines with specific isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the event rate due to a specific isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma-ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure

Physics of Eclipsing Binaries: Modelling in the new era of ultra-high precision photometry

Recent ultra-high precision observations of eclipsing binaries, especially data acquired by the Kepler satellite, have made accurate light curve modelling increasingly challenging but also more rewarding. In this contribution, we discuss low-amplitude signals in light curves that can now be used to derive physical information about eclipsing binaries but that were unaccessible before the Kepler era. A notable example is the detection of Doppler beaming, which leads to an increase in flux when a star moves towards the satellite and a decrease in flux when it moves away. Similarly, Rømer delays, or light travel time effects, also have to taken into account when modelling the supreme quality data that is now available. The detection of offsets between primary and secondary eclipse phases in binaries with extreme mass ratios, and the observation of Rømer delays in the signals of pulsators in binary stars, have allowed us to determine the orbits of several binaries without the need for spectroscopy. A third example of a small-scale effect that has to be taken into account when modelling specific binary systems, are lensing effects. A new binary light curve modelling code, PHOEBE 2.0, that takes all these effect into account is currently being developed

COMPTEL observations of the blazars 3C 454.3 and CTA 102

We have analyzed the two blazars of 3C 454.3 and CTA 102 using all available COMPTEL data from 1991 to 1999. In the 10–30 MeV band, emission from the general direction of the sources is found at the 4σ-level, being consistent with contributions from both sources. Below 10 MeV only 3C 454.3 is significantly detected, with the strongest evidence (5.6 σ) in the 3–10 MeV band. Significant flux variability is not observed for both sources, while a low emission is seen most of the years in the 3–10 MeV light curve for 3C 454.3. Its time-averaged MeV spectrum suggests a power maximum between 3 to 10 MeV

The blue-edge problem of the V1093 Her instability strip revisited using evolutionary models with atomic diffusion

We have computed a new grid of evolutionary subdwarf B star (sdB) models from the start of central He burning, taking into account atomic diffusion due to radiative levitation, gravitational settling, concentration diffusion, and thermal diffusion. We have computed the non-adiabatic pulsation properties of the models and present the predicted p-mode and g-mode instability strips. In previous studies of the sdB instability strips, artificial abundance enhancements of Fe and Ni were introduced in the pulsation driving layers. In our models, the abundance enhancements of Fe and Ni occur naturally, eradicating the need to use artificial enhancements. We find that the abundance increases of Fe and Ni were previously underestimated and show that the instability strip predicted by our simulations solves the so-called blue edge problem of the subdwarf B star g-mode instability strip. The hottest known g-mode pulsator, KIC 10139564, now resides well within the instability strip {even when only modes with low spherical degrees (l<=2) are considered.Comment: 7 pages, 7 figures. Accepted for publication in Astronomy & Astrophysic

Relativistic Winds from Compact Gamma-ray Sources: I. Radiative Acceleration in the Klein-Nishina Regime

We consider the radiative acceleration to relativistic bulk velocities of a cold, optically thin plasma which is exposed to an external source of gamma-rays. The flow is driven by radiative momentum input to the gas, the accelerating force being due to Compton scattering in the relativistic Klein-Nishina limit. The bulk Lorentz factor of the plasma, Gamma, derived as a function of distance from the radiating source, is compared with the corresponding result in the Thomson limit. Depending on the geometry and spectrum of the radiation field, we find that particles are accelerated to the asymptotic Lorentz factor at infinity much more rapidly in the relativistic regime; and the radiation drag is reduced as blueshifted, aberrated photons experience a decreased relativistic cross section and scatter preferentially in the forward direction. The random energy imparted to the plasma by gamma-rays can be converted into bulk motion if the hot particles execute many Larmor orbits before cooling. This `Compton afterburn' may be a supplementary source of momentum if energetic leptons are injected by pair creation, but can be neglected in the case of pure Klein-Nishina scattering. Compton drag by side-scattered radiation is shown to be more important in limiting the bulk Lorentz factor than the finite inertia of the accelerating medium. The processes discussed here may be relevant to a variety of astrophysical situations where luminous compact sources of hard X- and gamma-ray photons are observed, including active galactic nuclei, galactic black hole candidates, and gamma-ray bursts.Comment: LateX, 20 pages, 5 figures, revised version accepted for publication in the Ap