Osmi Vilhu

The D-CIXS X-ray mapping spectrometer on SMART-1

The D-CIXS Compact X-ray Spectrometer will provide high quality spectroscopic mapping of the Moon, the primary science target of the ESA SMART-1 mission. D-CIXS consists of a high throughput spectrometer, which will perform spatially localised X-ray fluorescence spectroscopy. It will also carry a solar monitor, to provide the direct calibration needed to produce a global map of absolute lunar elemental abundances, the first time this has been done. Thus it will achieve ground breaking science within a resource envelope far smaller than previously thought possible for this type of instrument, by exploiting two new technologies, swept charge devices and micro-structure collimators. The new technology does not require cold running, with its associated overheads to the spacecraft. At the same time it will demonstrate a radically novel approach to building a type of instrument essential for the BepiColombo mission and potential future planetary science targets.

The SMART-1 X-ray solar monitor (XSM): calibrations for D-CIXS and independent coronal science

Abstract The X-ray solar monitor (XSM) is a calibration instrument of the demonstration of compact imaging X-ray spectrometer (D-CIXS) experiment, with a separate Silicon detector unit on the SMART-1 spacecraft. The non-imaging HPSi PIN sensor has a wide field-of-view (FOV) to enable Sun visibility during a significant fraction of the mission lifetime, which is essential for obtaining calibration spectra for the X-ray fluorescence measurements by the imaging D-CIXS spectrometer. The energy range (1– 20 keV ), spectral resolution (about 250 eV at 6 keV ), and sensitivity (about 7000 cps at flux level of 10 −4 W m −2 in the range 1– 8 A ) are tuned to provide optimal knowledge about the Solar X-ray flux on the Lunar surface, matching well with the activating energy range for the fluorescence measured by D-CIXS. The independent science of the XSM will also be valuable, since the XSM energy range is very sensitive to solar flares. The countrate during the top of an X1 flare will be about 35 times higher than the average quiescent countrate at solar maximum. The relative increase will be the same for an M1 flare during the SMART-1 mission, which will be closer to the next solar minimum. Since the XSM will observe the Sun as a star, and the energy range and spectral resolution are close to those of present astronomical X-ray satellites (e.g., XMM-Newton, ASCA, Chandra), we will obtain an X-ray database of the Sun which can be related with the stellar X-ray observations more easily than the data from present solar X-ray instruments. In this publication we give a detailed description of the design, performance, and tasks of the XSM instrument, and view the science perspectives.

Problems of low-mass binary evolution

Some problems connected with low-mass binary evolution (from contact binaries to cataclysmic variables and origin of bursters) are considered. Most attention is given to contact W UMa-stars and to (still unclear) scenarios where the angular momentum loss by magnetic braking may, at least partly, control the contact binary evolution.

Two-Phase Modeling of the Rings in the RXTE Two-Color Diagram of GRS 1915+105

The Galactic superluminal source GRS 1915+105 was found to experience a peculiar X-ray variability in a narrow count rate range (9300-12,100 counts s-1, 5 PCUs) of the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. This can be seen as a ring-shaped pattern in the two-color diagram of count rates, where the hard hardness F(13-40 keV)/F(2-13 keV) is plotted against the soft hardness F(5-13 keV)/F(2-5 keV). The system runs one cycle with periods ranging between 50 and 100 s for different observations, one rotation in the two-color diagram corresponding to the time between two contiguous maxima in the light curve. We model this behavior successfully with the help of a self-consistent two-phase thermal model in which seed photons from an optically thick classical disk are Comptonized in a hot spherical corona surrounding the inner disk (Poutanen & Svensson; Vilhu et al.; Nevalainen et al.). In the model, changes of two parameters regulate the paths in the two-color diagram: the blackbody temperature Tin of the inner disk and the Thomson optical depth multiplied by the electron temperature of the hot phase τTe. These parameters oscillate with time but with a phase shift between each other, causing the ring-shaped pattern. During the observation studied in more detail (20402-01-30-00), the inner disk radius varied with a 97 s period between 20 and 35 km with an anticorrelation between the coronal τTe and the mass accretion rate through the disk, possibly indicating a coupling between the disk and coronal accretion. During a typical cycle, the inner disk radius rapidly shrank and returned more slowly back to the original larger value. In the rings we may see phenomena close to the black hole horizon under near Eddington accretion rates.

Scientific rationale for the D-CIXS X-ray spectrometer on board ESA's SMART-1 mission to the Moon

The D-CIXS X-ray spectrometer on ESAs SMART-1 mission will provide the first global coverage of the lunar surface in X-rays, providing absolute measurements of elemental abundances. The instrument will be able to detect elemental Fe, Mg, Al and Si under normal solar conditions and several other elements during solar flare events. These data will allow for advances in several areas of lunar science, including an improved estimate of the bulk composition of the Moon, detailed observations of the lateral and vertical nature of the crust, chemical observations of the maria, investigations into the lunar regolith, and mapping of potential lunar resources. In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, this information will allow for a more detailed look at some of the fundamental questions that remain regarding the origin and evolution of the Moon.

Heating of stellar chromospheres and coronae observational constraints and evidence for saturation

Chromospheric-coronal emission of cool stars has a clear observed (spectral type dependent) upper limit (saturation). This boundary is seen in the surface flux vs. color (and flux vs. period) diagrams for different types of cool stars, excluding T Tauri-stars. The limit is occupied by rapid rotators (τ c /P≥3), either very young stars or components in close binaries with tidally forced rotation. Saturated stars seem to be almost totally filled with equipartition magnetic fields. This is one of the reason for the saturation: super-active regions with B= 1–2 kG fill the stellar surface, and no more magnetic flux can get out. This is due to the feed-backs between magnetic fields, convection and differential rotation.

Thermal Comptonization in GRS 1915+105

The Rossi X-ray Timing Explorer (RXTE) data of GRS 1915+105 from severalobserving periods were modelled with a thermal Comptonization model. Inthe model (Figure 1), seed soft photons from an optically thick cool disk are Comptonized in a hot spherical corona surrounding the inner disk. Althoughthe coronal temperature and optical depth can not be determinedsimultaneously, a qualitative trend was observed: at high soft states theinner disk just touches the outer edge of the central corona, while duringlow hard states the corona is larger and denser and surrounds the inner disk.

XMM–Newton observations of UW CrB: detection of X‐ray bursts and evidence for accretion disc evolution

UW CrB (MS 1603+2600) is a peculiar short-period X-ray binary that exhibits extraordinary optical behaviour. The shape of the optical light curve of the system changes drastically from night to night, without any changes in overall brightness. Here we report X-ray observations of UW CrB obtained with XMM-Newton. We find evidence for several X-ray bursts, confirming a neutron star primary. This considerably strengthens the case that UW CrB is an accretion disc corona system located at a distance of at least 5-7 kpc (3-5 kpc above the Galactic plane). The X-ray and Optical Monitor (ultraviolet-optical) light curves show remarkable shape variation from one observing run to another, which we suggest are due to large-scale variations in the accretion disc shape resulting from a warp that periodically obscures the optical and soft X-ray emission. This is also supported by the changes in phase-resolved X-ray spectra.

Simultaneous EXOSAT and VLA observations of the contact binaries VW Cephei and XY Leonis - Quiescent emission and a flare on VW Cephei

Two W UMa-type contact binaries, XY Leo and VW Cep, were observed simultaneously with Exosat, the VLA, and, in the case of XY Leo, optically. The temporal coverage of each star was sufficient to monitor them throughout two orbital revolutions (P about 0.27 days); however, no orbital modulation of either the X-ray or 6 cm data was seen for either star, indicating large emitting regions. A large flare from VW Cep was detected, the best such simultaneous flare data ever obtained from a star other than the sun. Its behavior before, during, and after the flare was remarkably similar to that found in solar flares, although at 6 cm this flare on VW Cep was about 10,000 times more luminous than typical strong solar flares. For both stars, it is demonstrated that the 6 cm emission cannot be the result of bremsstrahlung radiation of the X-ray emitting plasma; in fact, it is shown, in the case of the VW Cep flare, that the 6 cm emission is consistent with gyrosynchrotron radiation from a source region of order of the system separation. 20 references.

Phase-resolved optical and X-ray spectroscopy of low-mass X-ray binary X1822–371

Context. X1822–371 is the prototypical accretion disc corona X-ray source, a low-mass X-ray binary viewed at very high inclination, thereby allowing the disc structure and extended disc coronal regions to be visible. As the brightest (closest) such source, X1822–371 is ideal for studying the shape and edge structure of an accretion disc, and comparing with detailed models. Aims. We study the structure of the accretion disc in X1822–371 by modelling the phase-resolved spectra both in optical and X-ray regime. Methods. We analyse high time resolution optical ESO/VLT spectra of X1822–371 to study the variability in the emission line profiles. In addition, we use data from XMM-Newton space observatory to study phase-resolved as well as high resolution X-ray spectra. We apply the Doppler tomography technique to reconstruct a map of the optical emission distribution in the system. We fit multi-component models to the X-ray spectra. Results. We find that our results from both the optical and X-ray analysis can be explained with a model where the accretion disc has a thick rim in the region where the accretion stream impacts the disc. The behaviour of the Hβ line complex implies that some of the accreting matter creates an outburst around the accretion stream impact location and that the resulting outflow of matter moves both away from the accretion disc and towards the centre of the disc. Such behaviour can be explained by an almost isotropic outflow of matter from the accretion stream impact region. The optical emission lines of He ii λ4686 and 5411 show double peaked profiles, typical for an accretion disc at high inclination. However, their velocities are slower than expected for an accretion disc in a system like X1822–371. This, combined with the fact that the He ii emission lines do not get eclipsed during the partial eclipse in the continuum, suggests that the line emission does not originate in the orbital plane and is more likely to come from above the accretion disc, for example the accretion disc wind.