First XMM-Newton observations of strongly magnetic cataclysmic variables - II. Timing studies of DP Leo and WW Hor

XMM-Newton was used to observe two eclipsing, magnetic cataclysmic variables, DP Leo and WW Hor, continuously for three orbital cycles each. Both systems were in an intermediate state of accretion. For WW Hor we also obtained optical light curves with the XMM-Newton Optical Monitor and from ground-based observations. Our analysis of the X-ray and optical light curves allows us to constrain physical and geometrical parameters of the accretion regions and derive orbital parameters and eclipse ephemerides of the systems. For WW Hor we directly measure horizontal and vertical temperature variations in the accretion column. From comparisons with previous observations we find that changes in the accretion spot longitude are correlated with the accretion rate. For DP Leo the shape of the hard X-ray light curve is not as expected for optically thin emission, showing the importance of optical depth effects in the post-shock region. We find that the spin period of the white dwarf is slightly shorter than the orbital period and that the orbital period is decreasing faster than expected for energy loss by gravitational radiation alone.Comment: Accepted for publication in MNRAS, 12 pages, 6 figure

First XMM-Newton observations of strongly magnetic cataclysmic variables I: spectral studies of DP Leo and WW Hor

We present an analysis of the X-ray spectra of two strongly magnetic cataclysmic variables, DP Leo and WW Hor, made using XMM-Newton. Both systems were in intermediate levels of accretion. Hard optically thin X-ray emission from the shocked accreting gas was detected from both systems, while a soft blackbody X-ray component from the heated surface was detected only in DP Leo. We suggest that the lack of a soft X-ray component in WW Hor is due to the fact that the accretion area is larger than in previous observations with a resulting lower temperature for the re-processed hard X-rays. Using a multi-temperature model of the post-shock flow, we estimate that the white dwarf in both systems has a mass greater than 1 Msun. The implications of this result are discussed. We demonstrate that the `soft X-ray excess' observed in many magnetic cataclysmic variables can be partially attributed to using an inappropriate model for the hard X-ray emission.Comment: Accepted by MNRAS as a letter, 5 pages, 2 figure

The ACES GNSS subsystem and its applications

The ESA mission Atomic Clock Ensemble in Space (ACES) will operate a new generation of atomic clocks on board the International Space Station (ISS) in 2013-2015 timeframe. The ACES payload will be attached externally to the European Columbus module. The ACES clock signal will reach fractional frequency stability and accuracy of 1 part in 10-16. A GNSS receiver will be connected to the ACES clock signal. Primarily, the GNSS receiver will ensure orbit determination of the ACES clocks using GPS, GALILEO/GIOVE, and possibly GLONASS satellite signals in the L1, L2, and L5/E5a bands. Orbit determination is important for the correct evaluation of relativistic corrections in the space-to-ground comparison of clocks. Secondarily, the receiver offers the potential to support additional functionality for remote sensing applications in the field of GNSS radio-occultation and GNSS reflectometry, exploiting opportunities arising from the new GPS and GALILEO/GIOVE signals. The ACES GNSS instrument consists of a state-of-the-art commercial-of-the-shelf JAVAD GNSS Triumph TRE-G3T receiver board. The receiver is connected to a GNSS antenna which will be directly mounted at the corner of the ACES payload. Antenna boresight is pointing +50° off the ISS flight direction and is tilted 30° toward the zenith direction. This offers ideal conditions to receive coherent reflected GNSS signals and improves radio occultation measurements. Within the ACES project the receiver will be ruggedized and tested for space environment. Initial tests performed by DLR with the Co-60 source in Euskirchen, Germany, indicate a high tolerance to total ionizing dose. The receiver sensitivity to harmful single event effects of ionizing radiation including single event upset (SEU) and latch-up (LU) has been characterized in SEE testing using the radiation test facility of Groningen, NL. The results will be used to design the protection system counteracting these effects. In addition the receiver will be accommodated in a double redundant architecture. Under simulated low Earth orbit (LEO) conditions the JAVAD Triumph receiver firmware demonstrated fast acquisition of GPS signals and respectable orbit accuracy/ performance. Current status and test results of the ACES GNSS instrument will be presented in this paper