In 2011 NASA HQ initiated an X-ray mission concepts study through the NASA Physics of the Cosmos program office (http://pcos.gsfc.nasa.gov/studies/x-ray-mission.php). The primary charge to the committee was to evaluate a range of possible missions that could complete all or part of the IXO science case. In a strong response to NASA, the X-ray community submitted 30 white papers to the RFI regarding mission concepts and instrumentation for the next X-ray astronomy mission. A community-based study team reviewed the responses; their final report was issued August 10th.
Amongst its many findings regarding possible near-term and longer-term missions and technologies, the report found "that the extraordinary capability of a large-area X-ray calorimeter mission will address the greatest number of IXO science themes." With that in mind, members of what was the IXO science team have turned their efforts towards updating the science requirements for a large-area X-ray spectroscopic imaging mission. The study team report, by NASA request, focused specifically on the IXO science goals which in turn were based on the 2000 Decadal report. With the cancellation of IXO, our next goal is to refine the requirements on the next mission in light of high-priority science identified by the 2010 Decadal Survey of Astronomy & Astrophysics report, New Worlds and New Horizons (NWNH).
Two potential mission candidates have been identified for continued study, although neither has been optimized either from an engineering perspective or in view of the science requirements review described above. In 2011, immediately after receiving the Decadal recommendations to reduce cost while maintaining core capabilities, the IXO core team developed the Advanced X-ray Spectroscopy and Imaging Observatory (AXSIO). AXSIO reduces IXO's six instruments to two detectors - the imaging X-ray Microcalorimeter Spectrometer and the X-ray Grating Spectrometer. Specifically AXSIO would have a 10 arcsec PSF (goal of 5 arcsec) and ~ 1 m^2 of effective area at 1 keV for the microcalorimeter with 2 eV resolution in its central array and better than 3 eV resolution over its 4' field of view. The grating spectrometer would have 1000 cm^2 of effective area with a resolution E/dE > 1000 over its 0.3-1 keV bandpass. These instruments allow AXSIO to accomplish most of the IXO science goals at a significantly reduced complexity and cost ($1.5B life-cycle costs for a 3 year mission). However, the team recognizes that budgets may not support even this level of mission. Therefore, in addition to studying AXSIO, we are also studying the less capable calorimeter-only mission as developed by the CST (the so-called , the 'Notional Calorimeter' (N-CAL) mission). N-CAL has similar characteristics as AXSIO but omits the grating spectrometer and has only 0.5 m^2 of area at 1 keV.
When completed, our review of the science requirements on the mission will then drive the optimization of the mission designs of AXSIO & N-CAL over the next few years, with the goal of providing a robust designs with well-understood costs to NASA HQ, allowing for consideration of an AXSIO-like (calorimeter) mission for a start before the end of this decade.