Shemar et al. (2016) presented results based on the output of a feasibility study for the European Space Agency (ESA) on the use of X-ray pulsars for deep space navigation, a concept often referred to as 'XNAV'. Here we describe some of the key results as well as providing additional information which includes navigation uncertainties and the potential X-ray technology that could be used. For a conventional deep space mission, an X-ray navigation system must be practical to implement as a spacecraft subsystem and to this end it must meet restrictive mass, volume and power consumption requirements. The implementation of an X-ray observatory sized instrument is unrealistic in this case. The Mercury Imaging X-ray Spectrometer (MIXS) instrument, due to be launched on the ESA/JAXA BepiColombo mission to Mercury in 2018, is an example of an instrument that may be further developed as a practical telescope for XNAV. Simulations involving different pulsar combinations and navigation strategies are used to estimate the navigation uncertainties that may be achievable using such an instrument. Possible options for future developments in terms of simpler, lower-cost Kirkpatrick-Baez optics are discussed, in addition to the principal design and development challenges that must be addressed in order to realise an operational XNAV system.

We investigate the feasibility of deep-space navigation using the highly stable periodic signals from X-ray pulsars in combination with dedicated instrumentation on the spacecraft: a technique often referred to as ‘XNAV’. The results presented are based on the outputs from a study undertaken for the European Space Agency. The potential advantages of this technique include increased spacecraft autonomy and lower mission operating costs. Estimations of navigation uncertainties have been obtained using simulations of different pulsar combinations and navigation strategies. We find that the pulsar PSR B1937 + 21 has potential to allow spacecraft positioning uncertainties of ~2 and ~5 km in the direction of the pulsar after observation times of 10 and 1 h respectively, for ranges up to 30 AU. This could be achieved autonomously on the spacecraft using a focussing X-ray instrument of effective area ~50 cm 2 together with a high performance atomic clock. The Mercury Imaging X-ray Spectrometer (MIXS) instrument, due to be launched on the ESA/JAXA BepiColombo mission to Mercury in 2018, is an example of an instrument that may be further developed as a practical telescope for XNAV. For a manned mission to Mars, where an XNAV system could provide valuable redundancy, observations of the three pulsars PSR B1937 + 21, B1821-24 and J0437-4715 would enable a three-dimensional positioning uncertainty of ~30 km for up to 3 months without the need to contact Earth-based systems. A lower uncertainty may be achieved, for example, by use of extended observations or, if feasible, by use of a larger instrument. X-ray instrumentation suitable for use in an operational XNAV subsystem must be designed to require only modest resources, especially in terms of size, mass and power. A system with a focussing optic is required in order to reduce the sky and particle background against which the source must be measured. We examine possible options for future developments in terms of simpler, lower-cost Kirkpatrick-Baez optics. We also discuss the principal design and development challenges that must be addressed in order to realise an operational XNAV system.

This paper describes the next generation X-ray observatory XEUS which has been submitted to the European Space Agency in the framework of the Cosmic Vision 2015–2025 competition and has been selected for an assessment study. The paper summarizes the scientific goals and instrumental concepts of the proposed X-ray telescope with 5 m2 effective area and angular resolution better than 5 arc sec.

We use recent XMM-Newton observations to study the ''diffuse'' X-ray emission seen in the Galactic Centre Region. Spectrally, the emission can be separated into three major components, each characterised by a prominent spectral line. Using these lines as tracers, we investigate the underlying spatial distribution of the various components. Specifbally, we find the 6.7-keV line of helium-like iron, has a relatively smooth, circularly symmetric distribution centred on Sgr A* and a surface brightness which falls off with radius as r−0.87±0.06 over the range r = 3' − 12'. This mirrors the distribution of the underlying stellar population and adds strong support to the hypothesis that the 6.7-keV line and the associated hard thermal continuum (with kT ≈ 8 keV) originates in the summed emission of faint point sources.

The bright, soft X-ray spectrum Seyfert 1 galaxies Akn 564 and Ton S180 were monitored for 35 days and 12 days respectively with ASCA and RXTE (and EUVE for Ton S180). These represent the most intensive X-ray monitoring of any such soft spectrum Seyfert 1 to date. Light curves were constructed for Ton S180 in six bands spanning 0.1–10 keV and for Akn 564 in five bands spanning 0.7–10 keV. The short time scale (hours–days) variability patterns were very similar across energy bands, with no evidence of lags between any of the energy bands studied. The fractional variability amplitude was almost independent of energy band, unlike hard spectrum Seyfert 1s, which show stronger variations in the softer bands. It is difficult to simultaneously explain soft Seyferts stronger variability, softer spectra, and weaker energy-dependence of the variability relative to hard Seyferts. There was a trend for soft and hard band light curves of both objects to diverge on the longest time scales probed (∼weeks), with the hardness ratio showing a secular change throughout the observations. This is consistent with the fluctuation power density spectra that showed relatively greater power on long time scales in the softest bands. The simplest explanation of all of these is that two continuum emission components are visible in the X-rays: a relatively hard, rapidly-variable component that dominates the total spectrum and a slowly-variable soft excess that only shows up in the lowest energy channels of ASCA. Although it would be natural to identify the latter component with an accretion disk and the former with a corona surrounding it, a standard thin disk Astronomy Department; University of California; Los Angeles, CA 90095-1562; USA X-ray Astronomy Group; Leicester University; Leicester LE1 7RH; United Kingdom Laboratory for High Energy Astrophysics; Code 660; NASA/Goddard Space Flight Center; Greenbelt, MD 20771; USA Joint Center for Astrophysics; Physics Department; University of Maryland Baltimore County; 1000 Hilltop Circle; Baltimore, MD 21250; USA Institute of Astronomy; Madingley Road; Cambridge CB3 0HA; United Kingdom Center for Space Research; Massachusetts Institute of Technology; 77 Massachusetts Ave.; NE80; Cambridge, MA 02139; USA