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Register a new userInfluence of a carbon over-coat on the X-ray reflectance of XEUS mirrors
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We describe measurements of the X-ray reflectance in the range 2 to 10 keV of samples representative of coated silicon wafers that are proposed for the fabrication of the XEUS (X-ray Evolving Universe Spectrometer) mission. We compare the reflectance of silicon samples coated with bare Pt, with that for samples with an additional 10nm thick carbon over-coating. We demonstrate a significant improvement in reflectance in the energy range ~1 to 4 keV, and at a grazing incidence angle of 10 mrad (0.57 degrees). We consider the resulting effective area that could be attained with an optimized design of the XEUS telescope. Typically an improvement of 10 to 60 % in effective area, depending on photon energy, can be achieved using the carbon overcoat.
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Fast X-ray timing can be used to probe strong gravity fields around collapsed objects and constrain the equation of state of dense matter in neutron stars. These studies require extremely good photon statistics. In view of the huge collecting area of its mirrors, XEUS could make a unique contribution to this field. For this reason, we propose to include a fast X-ray timing capability in the focal plane of the XEUS mirrors. We briefly outline the scientific motivation for such a capability. We compute some sensitivity estimates, which indicate that XEUS could provide better than an order of magnitude sensitivity improvement over the Rossi X-ray Timing Explorer. Finally, we present a possible detector implementation, which could be an array of small size silicon drift detectors operated out of focus.
The high chemical reactivity of strontium, which can opacify a viewport exposed to a strontium atomic source, is a concern for some atomic physics experiments where it is sometimes necessary to send a laser beam counter-propagating relative to the atomic beam. While a number of experiments use heated sapphire windows to reduce strontium deposition and increase the viewport lifetime, here we study another possibility, consisting of sending the laser beam into the atomic flux by reflecting it off a mirror at 45$^{circ}$ exposed to the strontium flux. We present our attempt to find a substrate that can be exposed to strontium and maintain high reflectivity. We first present the formation of a strontium metallic mirror under high flux ($> 10^{13}$ at/s/cm$^2$) on a sapphire substrate, and measure its reflectivity at 45$^circ$ to be 0.65 (S) and 0.51 (P). On two other substrates, initially reflective metallic mirrors, we show for slightly lower fluxes (i.e., a factor of 3) that some reaction - most probably oxidation - is able to prevent the formation of the metallic layer even in high vacuum conditions. Instead, we observe the growth of a dielectric transparent medium. Despite the continuous deposition of strontium, the back surface reflectivity continues to dominate. We show the unusual evolution of reflectivity on these substrates, and emphasize two observations: i) a sharp threshold in the strontium flux separating transparent material growth from lossy material growth; ii) strontiums highly efficient capture of oxygen, even from rarefied sources: here mostly the residual high vacuum pressure (10$^{-7}$mbar full pressure) and possibly a protective SiO$_2$ surface on one of the substrates.
X-ray Polarimetry is almost as old as X-ray Astronomy. Since the first discovery of X-ray sources theoretical analysis suggested that a high degree of linear polarization could be expected due either to the, extremely non thermal, emission mechanism or to the transfer of radiation in highly asymmetric systems. The actual implementation of this subtopic was, conversely, relatively deceiving. This is mainly due to the limitation of the conventional techniques based on the Bragg diffraction at 45deg, or on Thomson scattering around 90deg. Acually no X-ray Polarimeter has been launched since 25 years. Nevertheless the expectations from such measurement on several astrophysical targets including High and Low Mass X-Ray Binaries, isolated neutron Stars, Galactic and Extragalactic Black Holes is extremely attractive. We developed a new technique to measure the linear polarization of X-ray sources. It is based on the visualization of photoelectron tracks in a, finely subdivided, gas filled detector (micropattern). The initial direction of the photoelectron is derived and from the angular distribution of the tracks the amount and angle of polarization is computed. This technique can find an optimal exploitation in the focus of XEUS-1. Even in a very conservative configuration (basically the already existing prototype) the photoelectric polarimeter could perform polarimetry at % level on many AGNs. Further significant improvements can be expected from a technological development on the detector and with the use of XEUS-2 telescope.
XEUS is a large area telescope aiming to rise X-ray Astronomy to the level of Optical Astronomy in terms of collecting areas. It will be based on two satellites, locked on a formation flight, one with the optics, one with the focal plane. The present design of the focal plane foresees, as an auxiliary instrument, the inclusion of a Polarimeter based on a Micropattern Chamber. We show how such a device is capable to solve open problems on many classes of High Energy Astrophysics objects and to use X-ray sources as a laboratory for a substantial progress on Fundamental Physics.
We present the result of a study of the X-ray emission from the Galactic Centre Molecular Clouds (MC), within 15 arcmin from Sgr A*. We use XMM-Newton data spanning about 8 years. We observe an apparent super-luminal motion of a light front illuminating a MC. This might be due to a source outside the MC (such as Sgr A* or a bright and long outburst of a X-ray binary), while it can not be due to low energy cosmic rays or a source located inside the cloud. We also observe a decrease of the X-ray emission from G0.11-0.11, behaviour similar to the one of Sgr B2. The line intensities, clouds dimensions, columns densities and positions with respect to Sgr A*, are consistent with being produced by the same Sgr A* flare. The required high luminosity (about 1.5 10^39 erg s-1) can hardly be produced by a binary system, while it is in agreement with a flare of Sgr A* fading about 100 years ago.
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